                               
                               















                           XT-8000S

                    Product Specification
                   and OEM Technical Manual























Document 1015586
Revision B
July 1989

                               




                               
                               




























































                               


                                                              


                       REVISION RECORD




         Revision   Date Published    Revised Contents




            01      April 1988        Preliminary




            02      August 1988       Preliminary




            03      September 1988    Preliminary




            A       October 1988      Formal Release

       A (Adden 1)  April 1989        Formal Release




            B       July 1989         Formal Release

     B (Adden 1)    January 1990      Formal Release




                  Document Number:  1015586

                           WARRANTY

Maxtor  warrants  the  XT-8000S  Family  disk  drives  against
defects in  materials and  workmanship for  a period of twelve
months, for  the original  purchaser.   Direct  any  questions
regarding the  warranty to  your Maxtor  Sales Representative.
Maxtor maintains  Customer Service  Centers for the repair and
reconditioning of  all Maxtor  products.   Direct all requests







                               


for repair  to the  Maxtor Service  Center in  San Jose.  This
assures you of the fastest possible service.

                     REGULATORY APPROVALS

           UL Recognition obtained:     File Number E87276 (S)
           CSA Certification obtained:  File Number LR54048-6
           VDE Recognition obtained:    File Number 37230G

                               


Address comments concerning this manual to:

Maxtor Corporation
Technical Publications
211 River Oaks Parkway
San Jose, California 95134
Telephone:  (408) 432-1700
Telex:  171074
FAX:  (408) 434-6469










Technical Data Restrictions

In case of sale to or use of units by DoD, use, duplication or
disclosure of  any software, firmware or related documentation
is subject to restrictions stated in paragraph (c) (1) (ii) of
the Rights  in Technical  Data and Computer Software clause at
DFAR 252.227-7013.   For Civilian Agencies: Use, reproduction,
or disclosure  of the  software and  related documentation  is
subject  to   restrictions  set   forth  in   FAR   52.227-19.
Unpublished rights  reserved under  the copyright  laws of the
United States.   Maxtor  Corporation, 211  River Oaks Parkway,
San Jose, CA 95134.










Copyright Notice

This manual  and all material contained in it are copyrighted.
The manual may not be copied, in whole or in part, without the
written consent  of Maxtor  Corporation.  The contents of this
manual may be revised without prior notice.

  Copyright 1988 by Maxtor Corporation, San Jose, California,
USA.  All rights reserved









                               
                               

       


























































                               


PREFACE

Maxtor reserves  the right to make changes and/or improvements
to  its   products  without   incurring  any   obligation   to
incorporate such  changes or  improvements in units previously
sold or shipped.

Maxtor publishes  descriptive brochures  and  data  sheets,  a
quick reference  guide, and  this OEM  manual for each product
line.   Changes that  affect the  content of  any  manual  are
covered by  publishing an  addendum or  revising the  affected
manual.















































    XT-8000S Product Specification & OEM Technical Manual


                      TABLE OF CONTENTS

    XT-8000S Product Specification & OEM Technical Manual


                      TABLE OF CONTENTS


PREFACE.....................................................V


1.0..............................................INTRODUCTION   
   1

  1.1.....................................General Description   
      1
  1.2.................................Key Disk Drive Features   
      2
  1.3.................................Key Controller Features   
      2
  1.4......................Summary of Supported SCSI Commands   
      4

2.0.....................................SPECIFICATION SUMMARY   
   5

  2.1..............................Performance Specifications   
      5
  2.2...............................Functional Specifications   
      6
  2.3............................Environmental Specifications   
      6
  2.4.................................Physical Specifications   
      7
  2.5..............................Reliability Specifications   
      7
  2.6.......................Error Rates at the SCSI Interface   
      8
  2.7...................................DC Power Requirements   
      8
  2.8...............................Standards and Regulations   
      9

3.0................................FUNCTIONAL CHARACTERISTICS   
   11

  3.1.............................General Theory of Operation   
      11
  3.2......Read/Write Control and SCSI Controller Electronics   
      11
  3.3....................................Disk Drive Mechanism   
      12





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                      TABLE OF CONTENTS

  3.4...................................Air Filtration System   
      12
  3.5..............................Head Positioning Mechanism   
      13
  3.6..............................Read/Write Heads and Disks   
      14
  3.7.................................Track and Sector Format   
      15
  3.8..........................................SCSI Interface   
      16
     3.8.1....................Initiator-Target Configurations   
          16
     3.8.2..............Logical/Electrical Signal Definitions   
          18
     3.8.3.......Pin Assignments and Connector (Single Ended)   
          19
     3.8.4.......Pin Assignments and Connector (Differential)   
          20
  3.9..............................Electrical Power Interface   
      23
     3.9.1..................................Power-Up Sequence   
          24
     3.9.2....................................Power Connector   
          25
     3.9.3.......................Frame (HDA) Ground Connector   
          25
  3.10..............................Mounting and Installation   
      25
  3.11...............................................Shipping   
      29

4.0...................................USER SELECTABLE OPTIONS   
   31

  4.1.......................................SCSI ID Selection   
      32
  4.2..................................Drive Power-Up Options   
      33
  4.3..............................Terminator Power Selection   
      34
  4.4....................................Write Protect Option   
      35
  4.5...........................................Parity Option   
      35
  4.6...........................................Other Jumpers   
      35
  4.7.............................................Sector Size   
      35
  4.8...................................Interface Termination   
      35





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                      TABLE OF CONTENTS

  4.9.....................................Auxiliary Connector   
      36
  4.10........................Spindle Synchronization Control   
      38

5.0...................................LOGICAL CHARACTERISTICS   
   39

  5.1..................................Power Up and BUS RESET   
      39
     5.1.1.................................Self-Test Sequence   
          39
     5.1.2............................Initialization Sequence   
          40
     5.1.3.................................Self Configuration   
          41
     5.1.4...........................UNIT ATTENTION Condition   
          41
  5.2........................................Buffering Scheme   
      41

6.0..................................SCSI PHASES AND MESSAGES   
   43

  6.1..........................................BUS FREE Phase   
      44
  6.2.......................................ARBITRATION Phase   
      45
  6.3.........................................SELECTION Phase   
      45
  6.4.......................................RESELECTION Phase   
      46
  6.5.............................Information Transfer Phases   
      47
     6.5.1..................Asynchronous Information Transfer   
          48
     6.5.2..........................Synchronous Data Transfer   
          48
     6.5.3......................................COMMAND Phase   
          49
     6.5.4........................DATA IN and DATA OUT Phases   
          50
     6.5.5.......................................STATUS Phase   
          50
     6.5.6......................................MESSAGE Phase   
          52
  6.6...........................................SCSI Messages   
      52
     6.6.1...................................MESSAGE IN Phase   
          52





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                      TABLE OF CONTENTS

     6.6.2..................................MESSAGE OUT Phase   
          53
     6.6.3.............................COMMAND COMPLETE (00h)   
          53
     6.6.4............................SAVE DATA POINTER (02h)   
          54
     6.6.5.............................RESTORE POINTERS (03h)   
          54
     6.6.6...................................DISCONNECT (04h)   
          54
     6.6.7.....................INITIATOR DETECTED ERROR (05h)   
          54
     6.6.8........................................ABORT (06h)   
          55
     6.6.9...............................MESSAGE REJECT (07h)   
          55
     6.6.10................................NO OPERATION (08h)   
          55
     6.6.11........................MESSAGE PARITY ERROR (09h)   
          55
     6.6.12.....................LINKED COMMAND COMPLETE (0Ah)   
          56
     6.6.13.........LINKED COMMAND COMPLETE (WITH FLAG) (0Bh)   
          56
     6.6.14............................BUS DEVICE RESET (0Ch)   
          56
     6.6.15................................IDENTIFY (C0h/80h)   
          56
     6.6.16...SYNCHRONOUS DATA TRANSFER REQUEST Message (01h)   
          57
  6.7........................................Error Conditions   
      59
     6.7.1.....................MESSAGE OUT Phase Parity Error   
          59
     6.7.2.........................COMMAND Phase Parity Error   
          60
     6.7.3........................DATA OUT Phase Parity Error   
          60
     6.7.4...........................Initiator Detected Error   
          60
     6.7.5...................................REJECTED Message   
          61
     6.7.6.....................Initiator MESSAGE PARITY ERROR   
          62
     6.7.7...............................RESELECTION Time-Out   
          62
     6.7.8.........................Internal Controller Errors   
          62







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                      TABLE OF CONTENTS

7.0.................................SCSI COMMAND DESCRIPTIONS   
   63

  7.1...................................SCSI Command Overview   
      63
  7.2.............................................FORMAT UNIT   
      68
     7.2.1.............................Defect List Management   
          69
     7.2.2....................................SCSI Deviations   
          70
     7.2.3.................................Command Parameters   
          70
     7.2.4.......................................Format Modes   
          72
     7.2.5..............................Initiator Defect List   
          77
       A..................................D List Block Format   
          78
       B.......................D List Bytes from Index Format   
          79
       C........................D List Physical Sector Format   
          80
     7.2.6...................................Error Conditions   
          82
  7.3.................................................INQUIRY   
      84
     7.3.1....................................SCSI Deviations   
          84
     7.3.2.................................Command Parameters   
          84
     7.3.3........................................Data Format   
          85
  7.4.............................................MODE SELECT   
      89
     7.4.1....................................SCSI Deviations   
          90
     7.4.2.................................Command Parameters   
          90
     7.4.3..............................Parameter List Format   
          91
       A.........................Parameter List Header Format   
          91
       B...............Parameter List Block Descriptor Format   
          92
       C...................................Page Header Format   
          93
     7.4.4.....................Error Recovery Parameters Page   
          94
     7.4.5..........................Disconnect-Reconnect Page   
          101




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     7.4.6.................................Format Device Page   
          102
     7.4.7..........Rigid Disk Drive Geometry Parameters Page   
          105
     7.4.8.......................................Caching Page   
          107
     7.4.9..........................Maxtor Drive Control Page   
          109
     7.4.10...........................Read-Ahead Control Page   
          110
     7.4.11..................................Error Conditions   
          111
  7.5..............................................MODE SENSE   
      114
     7.5.1....................................SCSI Deviations   
          115
     7.5.2.................................Command Parameters   
          115
     7.5.3..............................Parameter List Format   
          117
       A.........................Parameter List Header Format   
          118
       B...............Parameter List Block Descriptor Format   
          119
       C...................................Page Header Format   
          120
     7.5.4.....................Error Recovery Parameters Page   
          120
     7.5.5...............Disconnect/Reconnect Parameters Page   
          123
     7.5.6........Direct-Access Device Format Parameters Page   
          125
     7.5.7..........Rigid Disk Drive Geometry Parameters Page   
          128
     7.5.8.......................................Caching Page   
          130
     7.5.9..........................Maxtor Drive Control Page   
          132
     7.5.10................Read-Ahead Control Parameters Page   
          134
     7.5.11..................................Error Conditions   
          135
  7.6....................................................READ   
      136
     7.6.1....................................SCSI Deviations   
          136
     7.6.2.................................Command Parameters   
          136
     7.6.3...................................Error Conditions   
          137





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                      TABLE OF CONTENTS

  7.7.........................................READ (EXTENDED)   
      138
     7.7.1....................................SCSI Deviations   
          138
     7.7.2.................................Command Parameters   
          138
     7.7.3...................................Error Conditions   
          139
  7.8.............................................READ BUFFER   
      140
     7.8.1....................................SCSI Deviations   
          140
     7.8.2.................................Command Parameters   
          140
     7.8.3......................................Command Usage   
          141
     7.8.4........................................Data Format   
          141
     7.8.5...................................Error Conditions   
          142
  7.9...........................................READ CAPACITY   
      143
     7.9.1....................................SCSI Deviations   
          143
     7.9.2.................................Command Parameters   
          143
     7.9.3........................................Data Format   
          144
     7.9.4...................................Error Conditions   
          145
  7.10.......................................READ DEFECT LIST   
      146
     7.10.1...................................SCSI Deviations   
          146
     7.10.2................................Command Parameters   
          146
     7.10.3............................................Header   
          148
     7.10.4......................................Block Format   
          149
     7.10.5...........................Bytes from Index Format   
          150
     7.10.6............................Physical Sector Format   
          151
     7.10.7..................................Error Conditions   
          152
  7.11..............................................READ LONG   
      153
     7.11.1...................................SCSI Deviations   
          153





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     7.11.2................................Command Parameters   
          153
     7.11.3..................................Error Conditions   
          154
  7.12.....................................READ LONG (SCSI-2)   
      155
  7.13.........................READ MANUFACTURERS DEFECT LIST   
      157
     7.13.1...................................SCSI Deviations   
          157
     7.13.2................................Command Parameters   
          157
     7.13.3.......................................Data Format   
          158
  7.14.........................................REASSIGN BLOCK   
      160
     7.14.1...................................SCSI Deviations   
          160
     7.14.2................................Command Parameters   
          160
     7.14.3................................Defect List Format   
          161
     7.14.4..................................Error Conditions   
          162
  7.15.............................RECEIVE DIAGNOSTIC RESULTS   
      163
     7.15.1...................................SCSI Deviations   
          163
     7.15.2................................Command Parameters   
          163
     7.15.3.......................................Data Format   
          164
     7.15.4..................................Error Conditions   
          164
  7.16...........................................RELEASE UNIT   
      165
     7.16.1...................................SCSI Deviations   
          165
     7.16.2................................Command Parameters   
          165
     7.16.3..................................Error Conditions   
          166
  7.17..........................................REQUEST SENSE   
      167
     7.17.1...................................SCSI Deviations   
          167
     7.17.2................................Command Parameters   
          167
     7.17.3.......................................Data Format   
          168





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                      TABLE OF CONTENTS

       A........................Nonextended Sense Data Format   
          169
       B...........................Extended Sense Data Format   
          169
       C..........................................Sense Codes   
          175
     7.17.4..................................Error Conditions   
          179
  7.18...........................................RESERVE UNIT   
      180
     7.18.1...................................SCSI Deviations   
          180
     7.18.2................................Command Parameters   
          180
     7.18.3..................................Error Conditions   
          181
  7.19............................................REZERO UNIT   
      182
     7.19.1...................................SCSI Deviations   
          182
     7.19.2................................Command Parameters   
          182
  7.20...................................................SEEK   
      183
     7.20.1...................................SCSI Deviations   
          183
     7.20.2................................Command Parameters   
          183
     7.20.3..................................Error Conditions   
          184
  7.21........................................SEEK (EXTENDED)   
      185
     7.21.1...................................SCSI Deviations   
          185
     7.21.2................................Command Parameters   
          185
     7.21.3..................................Error Conditions   
          186
  7.22........................................SEND DIAGNOSTIC   
      187
     7.22.1...................................SCSI Deviations   
          187
     7.22.2................................Command Parameters   
          187
     7.22.3..................................Error Conditions   
          189
     7.22.4.......................................Subcommands   
          190
       A..........................GET DRIVE STATUS Subcommand   
          190





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                      TABLE OF CONTENTS

       B........................PASS DRIVE COMMAND Subcommand   
          192
       C...............................READ HEADER Subcommand   
          195
  7.23........................................START/STOP UNIT   
      198
     7.23.1...................................SCSI Deviations   
          198
     7.23.2................................Command Parameters   
          198
     7.23.3..................................Error Conditions   
          199
  7.24........................................TEST UNIT READY   
      200
     7.24.1...................................SCSI Deviations   
          200
     7.24.2................................Command Parameters   
          200
     7.24.3..................................Error Conditions   
          200
  7.25.................................................VERIFY   
      201
     7.25.1...................................SCSI Deviations   
          201
     7.25.2................................Command Parameters   
          201
     7.25.3..................................Error Conditions   
          202
  7.26..................................................WRITE   
      203
     7.26.1...................................SCSI Deviations   
          203
     7.26.2................................Command Parameters   
          203
     7.26.3..................................Error Conditions   
          204
  7.27.......................................WRITE (EXTENDED)   
      205
     7.27.1...................................SCSI Deviations   
          205
     7.27.2................................Command Parameters   
          205
     7.27.3..................................Error Conditions   
          206
  7.28.......................................WRITE AND VERIFY   
      207
     7.28.1...................................SCSI Deviations   
          207
     7.28.2................................Command Parameters   
          207





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                      TABLE OF CONTENTS

  7.29...........................................WRITE BUFFER   
      209
     7.29.1...................................SCSI Deviations   
          209
     7.29.2................................Command Parameters   
          209
     7.29.3.......................................Data Format   
          210
     7.29.4..................................Error Conditions   
          211
  7.30.............................................WRITE LONG   
      212
     7.30.1...................................SCSI Deviations   
          212
     7.30.2................................Command Parameters   
          212
     7.30.3..................................Error Conditions   
          213
  7.31....................................WRITE LONG (SCSI-2)   
      214

APPENDIX A:  THE READ-AHEAD FEATURE.......................217

  What is Read-Ahead?.....................................217
  How Does it Work?.......................................217
  Key Read-Ahead Parameters...............................219
  Controlling the Read-Ahead Feature......................220
  Read Algorithm..........................................220
  Read-Ahead Performance..................................220

APPENDIX B:  DISCONNECT/RECONNECT CONDITIONS..............223


APPENDIX C:  DEFECT MANAGEMENT............................225

  Reassignment Sequences..................................226
     Cylinder-Oriented Reassignment Sequence..............226
     Track-Oriented Reassignment Sequence.................228
     Disk Drive-Oriented Reassignment Sequence............228
     Host Defect Management Sequence......................229
  Defect Management Assumptions and Statistics............229

APPENDIX D:  CDB BIT DEFINITIONS..........................233


APPENDIX E:  UNITS OF MEASURE.............................235


APPENDIX F:  BUFFER RATIO APPLICATIONS....................237

  What are the Buffer Ratios?.............................237




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                      TABLE OF CONTENTS

  What are the Performance Benefits from Buffer Ratios?...237
  Buffer Full Ratio.......................................237
  Buffer Empty Ratio......................................239
  What value should be used for the Buffer Ratios?........239

GLOSSARY..................................................241

















































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                           FIGURES

    XT-8000S Product Specification & OEM Technical Manual


                           FIGURES

Figure 31..............................Air Filtration System   
13
Figure 32.........................Head Positioning Mechanism   
14
Figure 33.......................................Track Format   
15
Figure 34........................Typical SCSI Configurations   
17
Figure 35............Connector Locations, Rear View of Drive   
22
Figure 36...............................SCSI Cable Connector   
23
Figure 37...................Motor Start Current Requirements   
24
Figure 38.......................................J3 Connector   
25
Figure 39.............Mechanical Outline, Top and Side Views   
27
Figure 310................Mechanical Outline, Isometric View   
28
Figure 311...............................Removable Faceplate   
29
Figure 41..................PCB Layout (Single Ended Version)   
31
Figure 42..........................PCB Layout (Differential)   
32
Figure 61..............Signal Sequence Chart for SCSI Phases   
44
Figure 71...............Flow Chart of Automatic Reallocation   
97
Figure A1................Sequential Reads without Read-Ahead   
218
Figure F1.......Data Transfer Rate Without Buffer Full Ratio   
238
Figure F2..........Data Transfer Rate With Buffer Full Ratio   
239

    XT-8000S Product Specification & OEM Technical Manual


                            TABLES

    XT-8000S Product Specification & OEM Technical Manual


                            TABLES

Table 21..........................Performance Specifications   
5
Table 22...........................Functional Specifications   
6
Table 23................................Environmental Limits   
7
Table 24.................................Physical Dimensions   
7
Table 25..........................Reliability Specifications   
8
Table 26.........................................Error Rates   
8
Table 27...............................DC Power Requirements   
9
Table 31...............Data Field Size vs. Sectors per Track   
16
Table 32............Connector Pin Assignments (Single Ended)   
20
Table 33............Connector Pin Assignments (Differential)   
21
Table 41.....................................SCSI ID Jumpers   
33
Table 42.........................Summary of Power-Up Options   
34
Table 43.................Auxiliary Connector Pin Assignments   
37
Table 44.............Remote SCSI ID Programming Combinations   
38
Table 45..................................J6 Pin Assignments   
38
Table 61........................Signal States and Bus Phases   
43
Table 62.....................................SCSI Bus Timing   
47
Table 63.........................................Status Byte   
50
Table 64........................................Status Codes   
51
Table 65.......................................Message Codes   
52
Table 66..............................IDENTIFY Message Codes   
57
Table 67.......SYNCHRONOUS DATA TRANSFER REQUEST Byte Values   
58
Table 68....Drive Responses to the SYNCHRONOUS DATA TRANSFER
REQUEST Message............................................58




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                            TABLES

Table 71.....................Typical CDB for 6-Byte Commands   
63
Table 72....................Typical CDB for 10-Byte Commands   
64
Table 73...........................CDB Operation Code Format   
65
Table 74.................................CDB Operation Codes   
65
Table 75........................................Control Byte   
66
Table 76.....................................FORMAT UNIT CDB   
71
Table 77......................FORMAT UNIT Defect List Header   
73
Table 78...................Initialization Pattern Descriptor   
74
Table 79.....................Initialization Pattern Modifier   
75
Table 710........................Initialization Pattern Type   
75
Table 711.....................FORMAT UNIT Drive Format Modes   
77
Table 712.........................FORMAT UNIT D List Formats   
78
Table 713.....FORMAT UNIT Defect Descriptor(s), Block Format   
79
Table 714.FORMAT UNIT Defect Descriptor(s), Bytes from Index
Format  80
Table 715..FORMAT UNIT Defect Descriptor(s), Physical Sector
Format  81
Table 716........................................INQUIRY CDB   
84
Table 717..............................INQUIRY Response Data   
86
Table 718................................MODE SELECT Command   
90
Table 719..................MODE SELECT Parameter List Header   
92
Table 720.MODE SELECT Parameter List Block Descriptor Format   
93
Table 721.............................MODE SELECT Page Codes   
94
Table 722..................Error Recovery Page (Page Code 1)   
95
Table 723...................MODE SELECT Error Recovery Modes   
100
Table 724..........................Disconnect-Reconnect Page   
101
Table 725.................................Format Device Page   
103





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                            TABLES

Table 726...MODE SELECT Rigid Disk Drive Geometry Parameters
(Page Code 4).............................................106
Table 727.......................................Caching Page   
108
Table 728..........................Maxtor Drive Control Page   
109
Table 729............Read-Ahead Control Page (Page Code 38h)   
111
Table 730.................................MODE SENSE Command   
115
Table 731.....................MODE SENSE Page Control Fields   
116
Table 732..............................MODE SENSE Page Codes   
117
Table 733...................MODE SENSE Parameter List Header   
118
Table 734..MODE SENSE Parameter List Block Descriptor Format   
119
Table 735..................MODE SENSE Page Header Page Codes   
120
Table 736.MODE SENSE Error Recovery Parameters (Page Code 1)   
121
Table 737.MODE SENSE Disconnect/Reconnect Control Parameters
(Page Code 2).............................................124
Table 738..MODE SENSE Direct-Access Device Format Parameters
(Page Code 3).............................................126
Table 739....MODE SENSE Rigid Disk Drive Geometry Parameters
(Page Code 4).............................................129
Table 740.................MODE SENSE Default Number of Heads   
130
Table 741.......................................Caching Page   
131
Table 742..........................Maxtor Drive Control Page   
132
Table 743......Read-Ahead Control Parameters (Page Code 38h)   
134
Table 744...........................................READ CDB   
136
Table 745................................READ (EXTENDED) CDB   
138
Table 746....................................READ BUFFER CDB   
140
Table 747.................................READ BUFFER Header   
142
Table 748..................................READ CAPACITY CDB   
144
Table 749..........................READ CAPACITY Data Format   
145
Table 750...............................READ DEFECT LIST CDB   
146

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                            TABLES

Table 751...............READ DEFECT LIST Defect List Formats   
148
Table 752............................READ DEFECT LIST Header   
149
Table 753READ DEFECT LIST Defect Descriptor(s), Block Format   
150
Table 754..READ DEFECT LIST Defect Descriptor(s), Bytes from
Index Format..............................................151
Table 755....READ DEFECT LIST Defect Descriptor(s), Physical
Sector Format.............................................152
Table 756......................................READ LONG CDB   
154
Table 757..................................READ LONG Command   
155
Table 758.................READ MANUFACTURERS DEFECT LIST CDB   
158
Table 759..READ MANUFACTURERS DEFECT LIST Defect Descriptors
Returned Format...........................................159
Table 760.................................REASSIGN BLOCK CDB   
161
Table 761..................REASSIGN BLOCK Defect List Header   
161
Table 762................REASSIGN BLOCK Defect Descriptor(s)   
162
Table 763.....................RECEIVE DIAGNOSTIC RESULTS CDB   
163
Table 764...................................RELEASE UNIT CDB   
165
Table 765..................................REQUEST SENSE CDB   
168
Table 766........REQUEST SENSE Nonextended Sense Data Format   
169
Table 767.........................Extended Sense Data Format   
170
Table 768................................Field Pointer Bytes   
172
Table 769...........................Actual Retry Count Bytes   
172
Table 770...................Format Progress Indication Bytes   
173
Table 771.............REQUEST SENSE Extended Sense Key Codes   
174
Table 772.............................Additional Sense Codes   
176
Table 773...................................RESERVE UNIT CDB   
181
Table 774....................................REZERO UNIT CDB   
182
Table 775...........................................SEEK CDB   
183





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                            TABLES

Table 776................................SEEK (EXTENDED) CDB   
186
Table 777................................SEND DIAGNOSTIC CDB   
188
Table 778............SEND DIAGNOSTIC Drive Self-Test Options   
189
Table 779.........GET DRIVE STATUS Subcommand Parameter List   
190
Table 780.......................GET DRIVE STATUS Data Format   
191
Table 781.............GET DRIVE STATUS Command Status Values   
192
Table 782.......PASS DRIVE COMMAND Subcommand Parameter List   
193
Table 783..............PASS DRIVE COMMAND Status Data Format   
194
Table 784...........PASS DRIVE COMMAND Command Status Values   
194
Table 785...PASS DRIVE COMMAND Standard Status Response Bits   
195
Table 786..............READ HEADER Subcommand Parameter List   
196
Table 787............................READ HEADER Data Format   
196
Table 788................TRANSLATE Subcommand Parameter List   
197
Table 789..............................TRANSLATE Data Format   
197
Table 790................................START/STOP UNIT CDB   
198
Table 791................................TEST UNIT READY CDB   
200
Table 792.........................................VERIFY CDB   
201
Table 793..........................................WRITE CDB   
203
Table 794...............................WRITE (EXTENDED) CDB   
205
Table 795...............................WRITE AND VERIFY CDB   
207
Table 796...................................WRITE BUFFER CDB   
210
Table 797......................Combined Header and Data Mode   
211
Table 798.....................................WRITE LONG CDB   
213
Table 799.................................WRITE LONG Command   
214
Table A1......................................READ Algorithm   
220

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                            TABLES

Table C1........................Defect Management Selections   
226
Table C2.....................Spares Versus Capacity, XT-8380   
230
Table C3.....................Spares Versus Capacity, XT-8760   
231
Table F1.................................Buffer Ratio Values   
240















































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1.0  INTRODUCTION


1.1  GENERAL DESCRIPTION

       The XT-8000S  Family disk  drives are  low cost,  high
       capacity,  high   performance,  random  access  storage
       devices which  use  nonremovable  5.25  inch  disks  as
       storage media.   Each disk surface employs one moveable
       head to  access the data tracks.  The total unformatted
       capacity of  these disk  drives ranges  from 410 to 769
       megabytes.

       The disk  drive  includes  the  Small  Computer  System
       Interface (SCSI)  controller embedded in the disk drive
       electronics.   Some of the resulting benefits of having
       an integrated  controller include  the elimination of a
       separate  controller   printed  circuit   board  (PCB),
       reduction in  the  number  of  associated  cables,  and
       elimination of the controller-specific power supply.

       Low cost  and high performance are achieved through the
       use of  a rotary  voice coil actuator and a closed loop
       servo system  using a  dedicated servo  surface.    The
       innovative MAXTORQ rotary voice coil actuator provides
       performance usually  achieved only  with larger, higher
       powered linear actuators.  The closed loop servo system
       and dedicated  servo surface  combine to allow state of
       the art recording densities (1,376 tracks per inch, and
       31,596 bits per inch) in a 5.25 inch package.

       High capacity  is achieved by a balanced combination of
       high areal  recording density, run-length limited (RLL)
       data encoding,  and high  density packaging techniques.
       Maxtor's   advanced    MAXPAK   electronic   packaging
       technique uses  miniature surface  mounted  devices  to
       allow all  electronic circuitry  to  fit  on  one  PCB.
       Advanced flexures and heads allow closer spacing of the
       disks; therefore,  a greater  number of  disks fit in a
       5.25 inch  package.   Maxtor's unique  integrated  disk
       drive motor/spindle  design allows  a deeper  head/disk
       assembly (HDA)  casting than conventional designs, thus
       permitting eight disks to be used.

       The disk  drive's electrical  interface  is  compatible
       with the  American National  Standards Institute (ANSI)
       SCSI standard  X3.131-1986, plus the Common Command Set
       (CCS) requirements.   Size  and mounting conform to the
       industry standard  5.25 inch form factor for floppy and
       Winchester disk  drives, and  the same  direct  current
       (DC) voltages and connectors are used.

    XT-8000S Product Specification & OEM Technical Manual


1.2  KEY DISK DRIVE FEATURES

       FEATURE                      BENEFITS

         Storage capacity  of 410  Ideal    for     multiuser
          to    769     megabytes,  systems    and    graphics
          unformatted               workstations

         Single PCB                High reliability,  ease of
                                    maintenance

         Rotary  voice  coil  and  Fast,    accurate     head
          closed loop servo system  positioning

         Separate microprocessor-  Fast  access  times,  high
          controlled servo          reliability,   and    high
                                    density packaging

         Thin film metallic media  Higher  bit   density  and
                                    resolution  and   improved
                                    durability

         Dedicated  head  landing  High   reliability,   data
          and shipping  zones with  protected from  mechanical
          automatic actuator lock   shock

         Brushless   DC   spindle  Maximum storage capacity
          motor inside hub

         Separate microprocessor-  Precise   speed    control
          controlled spindle motor  under all load conditions

         Dynamic  braking  during  Minimum head/disk  contact
          power-down cycle          time during spindown cycle

         Industry   standard   DC  No AC power required, ease
          power             supply  of integration
          requirements

         Industry        standard  Ease of integration
          physical    size     and
          mounting


1.3  KEY CONTROLLER FEATURES

       FEATURE                      BENEFITS

         Sophisticated, hardware-  Minimum time  on SCSI bus,
          based  control  of  SCSI  maximum bus use
          protocol






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       FEATURE                      BENEFITS
         Up to  4.8 megabytes per  Maximum SCSI bus use
          second   data   transfer
          rate      in      either
          asynchronous          or
          synchronous mode

         Synchronous  offsets  of  Provides high  performance
          up to 15 bytes            on long cables

         High performance  buffer  Simultaneous          data
          manager                   transfers  from   disk  to
                                    buffer, and from buffer to
                                    initiator

         In-line sector sparing    Maintains high performance
                                    over the  life of the disk
                                    drive, even after numerous
                                    reassignments

         Cylinder-oriented defect  Maximizes        formatted
          management                capacity available  to the
                                    system

         Programmable    cylinder  No   slipped   revolutions
          skewing                   when   crossing   cylinder
                                    boundaries

         Read-Ahead caching        Maximizes  throughput   on
                                    sequential READ commands

         45,056 byte  dual ported  Maintains maximum SCSI bus
          first-in,      first-out  transfer rate
          (FIFO)  buffer,   nearly
          two tracks capacity

         Hardware head switching   No head  skewing  required
                                    to     prevent     slipped
                                    revolutions     due     to
                                    software overhead

         Programmable              Flexibility    and    more
          alternates/zones          available capacity

         Dedicated microprocessor  SCSI  bus   unaffected  by
          for SCSI                  disk    drive    interface
                                    functions such as SEEK

         Error map on disk         Automatic           defect
                                    deallocation        during
                                    format, no need for manual
                                    entry

    XT-8000S Product Specification & OEM Technical Manual

       FEATURE                      BENEFITS
         ANSI    SCSI    Standard  Compatibility and  ease of
          X3.131-1986   Level    2  integration
          conformance

         Full  implementation  of  Availability  of   options
          CCS revision 4B           and industry compatibility

         Programmable 48-bit ECC   High data integrity

         Programmable   automatic  Flexibility and  high data
          retry  on   READ  errors  integrity
          with           automatic
          reallocation option

         Interleave    of     1:1  Maximum data throughput
          supported

         Fully       programmable  Optimum      SCSI      bus
          disconnect/recon-nect     performance
          capability

         Sector             sizes  User    flexibility    and
          programmable from 256 to  system compatibility
          4,096 bytes

         SCSI      implementation  Ease  of  integration  and
          compatible   with    all  upgrade
          Maxtor  Winchester   and
          optical disk drives

         Differential   interface  Superior  SCSI  bus  noise
          option                    immunity    and     longer
                                    transmission capability


1.4  SUMMARY OF SUPPORTED SCSI COMMANDS

       The commands supported by the disk drive are listed and
       discussed in  Chapter 7.0,  SCSI Command  Descriptions.
       For more  information on  Group 0  and Group 1 commands
       for direct-access  devices, see the reference documents
       below:

         ANSI X3.131-1986,  Small Computer  System  Interface
         (SCSI), American  National Standards Institute, Inc.,
         June 23, 1986.

         X3T9.2/85-52 Rev  4.B Common  Command Set  (CCS),  a
         document of the American Standards Committee.








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2.0  SPECIFICATION SUMMARY


2.1  PERFORMANCE SPECIFICATIONS


                                

                            Table 21
                   Performance Specifications



2.2  FUNCTIONAL SPECIFICATIONS


                                

                            Table 22
                    Functional Specifications



2.3  ENVIRONMENTAL SPECIFICATIONS


                                

                            Table 23
                      Environmental Limits



2.4  PHYSICAL SPECIFICATIONS


                                

                            Table 24
                       Physical Dimensions



2.5  RELIABILITY SPECIFICATIONS


                                

                            Table 25
                   Reliability Specifications







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2.6  ERROR RATES AT THE SCSI INTERFACE


                                

                            Table 26
                           Error Rates



2.7  DC POWER REQUIREMENTS


                                

                            Table 27
                      DC Power Requirements



2.8  STANDARDS AND REGULATIONS

       The  Maxtor   XT-8000S  Family  disk  drives  satisfy  the
       following standards and regulations:

       UNDERWRITERS LABORATORIES  (UL) = United States safety; UL
       478, Standard  for Safety, Electronic Processing Units and
       Systems.

       CANADIAN STANDARDS  ASSOCIATION (CSA)  = Canadian  safety;
       CSA  C22.2  No.  220,  1986,  Information  Processing  and
       Business Equipment (Consumer and Commercial Products).

       VERBAND DEUTSCHER  ELECTROTECHNIKER (VDE) = German safety;
       VDE 0806/8.81,  Safety of  Office Appliances  and Business
       Equipment.

       INTERNATIONAL   ELECTROTECHNICAL    COMMISSION   (IEC)   =
       International safety  commission; IEC  950 (formerly 380),
       Safety of Information Technology Equipment.

       FEDERAL COMMUNICATIONS  COMMISSION (FCC)  = United  States
       radiation emissions;  Part 15, Subpart J, Class B Consumer
       Computing Devices.

       CAUTION:     Connections between  equipment must  be  made
       with shielded  cables, and  a shielded  power cord must be
       used to connect AC power to the unit.

       CAUTION:     This  equipment   generates  and  uses  radio
       frequency energy,  and may cause interference to radio and
       television reception  if not  installed and used in strict
       accordance with the instructions in this manual.

      XT-8000S Product Specification & OEM Technical Manual


       The disk  drive has  been tested  and found to comply with
       the limits  for a  Class B computing device, in accordance
       with the  specifications in  Subpart J  of Part  15 of FCC
       Rules.   These rules  are designed  to provide  reasonable
       protection  against   radio   and   television   reception
       interference in  a  residential  installation.    However,
       there is  no guarantee that interference will not occur in
       a particular  installation.   If this equipment does cause
       interference to  radio or  television reception, which can
       be determined  by turning  the equipment  off and  on, the
       user is  encouraged to  try to  correct  the  interference
       using one or more of the following measures:

         reorient the receiving antenna
         reorient the computer with respect to the receiver
         move the computer away from the receiver
         plug the  computer into a different outlet, so that the
         computer and receiver are on different branch circuits

       If  necessary,  consult  the  dealer,  or  an  experienced
       radio/television technician,  for additional  suggestions.
       You may  find the  FCC booklet How to Identify and Resolve
       Radio TV  Interference Problems  helpful.  This booklet is
       available  from  the  United  States  Government  Printing
       Office, Washington,  D.C., 20402,  stock  number  004-000-
       00345-4.

       Maxtor is  not responsible  for any  radio  or  television
       interference caused  by unauthorized  modifications to the
       disk drive.   It  is the  responsibility of  the  user  to
       correct such interference.


























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3.0  FUNCTIONAL CHARACTERISTICS


3.1  GENERAL THEORY OF OPERATION

       The  disk  drive  consists  of  read/write,  control,  and
       interface electronics,  read/write heads,  a servo head, a
       head positioning actuator, a disk drive motor/spindle, the
       disk media,  and an air filtration system.  The components
       perform the following functions:

         interpret and generate control signals
         position the heads over the desired track
         read and write data
         provide automatic error correction to the data
         provide a contamination-free environment
         provide a controller to interact with the initiator
         maintain precise spindle rotation speed


3.2  READ/WRITE CONTROL AND SCSI CONTROLLER ELECTRONICS

       All the disk drive and controller electronics are packaged
       on  a   single  PCB.    This  PCB,  which  includes  three
       microprocessors,  performs   the  following   disk   drive
       functions:

         data separation
         reading and writing of data
         index detection
         head positioning
         head selection
         disk drive selection
         fault detection
         track zero detection
         recalibration to track zero on power up
         track position counter
         power and  speed control  for the  spindle  disk  drive
       motor
         braking for the spindle disk drive motor
         disk drive up-to-speed indication
         reduced write current on the inner tracks
         monitoring for WRITE FAULT conditions
         control of all internal timing
         generation of SEEK COMPLETE signals
         RLL encoding/decoding

       The PCB also performs the following controller functions:

         error detection and correction
         SCSI bus disconnect/reconnect functions
         SCSI bus arbitration
         defect handling
         data transfer



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         automatic retries
         data buffering
         command linking
         sector formatting


3.3  DISK DRIVE MECHANISM

       The HDA  is a sealed subassembly containing the mechanical
       portion of  the disk  drive.   A brushless  DC drive motor
       contained within  the spindle  hub rotates the spindle and
       is controlled  by a  dedicated microprocessor.   The motor
       and spindle  are dynamically  balanced  to  insure  a  low
       vibration level,  and dynamic  braking is  used to quickly
       stop the  spindle motor when power is removed.  The HDA is
       shock  mounted   to  minimize  transmission  of  vibration
       through the  frame.   The frame is the mechanical assembly
       holding the HDA and PCB.


3.4  AIR FILTRATION SYSTEM

       The disks  and read/write  heads are  assembled in a Class
       100 environment  and then  sealed within the HDA.  The HDA
       contains an  absolute filter mounted inside the casting to
       provide constant  internal air filtration (see Figure 31,
       Air Filtration System).  A second absolute filter, located
       on  the  HDA  top  cover,  permits  pressure  equalization
       between the internal and ambient air.


                                

                           Figure 31
                      Air Filtration System



3.5  HEAD POSITIONING MECHANISM

       The read/write  heads are  mounted on  a head/arm assembly
       which is  then mounted  on a  ball bearing supported shaft
       (see Figure  32, Head  Positioning Mechanism).  The voice
       coil, an  integral part  of the  head/arm  assembly,  lies
       inside the  magnet housing  when  installed  in  the  disk
       drive.    Current  from  the  power  amplifier,  which  is
       controlled by  the servo  system, induces a magnetic field
       in the voice coil.  This electrically induced field either
       aids or  opposes the  existing  fields  of  the  permanent
       magnets.   The interaction  of the  permanent and  induced
       magnetic fields  causes the voice coil to move.  Since the
       head/arm assemblies  are mounted  on the  voice coil,  the
       voice coil  movement is translated through the shaft pivot
       point directly to the heads.  The movement of the head/arm



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       assembly in  conjunction with  the  information  from  the
       servo  system   positions  the   heads  over  the  correct
       cylinder.


                                

                           Figure 32
                   Head Positioning Mechanism


       Movement of  the voice  coil actuator is controlled by the
       servo feedback  signal from  the servo  head.   The  servo
       information is  written onto  the disk  at the factory and
       provides the  following control  signals for the actuator:
       track-crossing signals  during a  seek  operation,  track-
       following signals during on cylinder operation, and timing
       information  such   as  index  and  servo  clock.    Servo
       information also provides the timing necessary to divide a
       track into  the sectors  used for data storage.  The servo
       control system  has a  dedicated microprocessor  for fast,
       optimized performance.


3.6  READ/WRITE HEADS AND DISKS

       The disk  drive employs  state  of  the  art  sliders  and
       flexures.   The configuration  of the sliders and flexures
       provides   improved    aerodynamic   stability,   superior
       head/disk compliance and an extremely high signal-to-noise
       ratio.

       The disk  media is  a thin,  nickel-cobalt, metallic  film
       deposited on  130 millimeter diameter aluminum substrates.
       The  coating  formulation  together  with  the  low  load-
       force/low  mass  heads  permits  highly  reliable  contact
       start/stop operation.   The  nickel-cobalt  metallic  film
       yields high  amplitude signals,  and very  high resolution
       performance compared  to conventional  oxide coated media.
       The metallic  media also  provides a  highly abrasion  and
       impact resistant  surface, decreasing  the  potential  for
       damage caused by shipping shock and vibration.

       Data  on  each  of  the  data  surfaces  is  read  by  one
       read/write head,  and each  head can access 1,632 physical
       tracks.     There  is   one  surface  dedicated  to  servo
       information in each disk drive.


3.7  TRACK AND SECTOR FORMAT

       The standard  track format  is organized into sectors (see
       Figure 33,  Track Format).  The sectors are addressed via




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       the logical block address (LBA) in the SCSI commands.  The
       method of encoding used is 2, 7 RLL.


                                

                           Figure 33
                          Track Format


       The data  field indicated in Figure 33 is 512 bytes.  The
       number of  sectors per  track for different data fields is
       summarized in  Table 31,  Data Field Size vs. Sectors per
       Track.


                                

                            Table 31
              Data Field Size vs. Sectors per Track



3.8  SCSI INTERFACE


3.8.1     Initiator-Target Configurations

       The SCSI  initiator interface  offers a  number of  unique
       advantages which  facilitate the  interconnection  of  the
       disk drive  with one  (or more)  computer systems.  Unlike
       traditional microcomputer  disk interfaces, such as ST506,
       SCSI   supports   multiple   peripherals   and   different
       peripheral types  all operating on the same bus structure.
       Figure 34, Typical SCSI Configurations, shows examples of
       typical configurations.


                                

                           Figure 34
                   Typical SCSI Configurations


       The disk  drive also supports multiple host configurations
       consistent with the established arbitration cycle outlined
       in the  SCSI standards.  Configuration changes are made by
       changing the  SCSI address  jumpers.  These jumpers should
       be set  and verified  when the  disk drive is installed in
       the system (jumper settings are covered in Chapter 4, User
       Selectable Options).






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       The SCSI implementation used on the disk drive is intended
       to  facilitate   high-speed  data   transfer  between  the
       initiator and the disk drive.  Interconnection between the
       initiator(s) and  the disk  drive is via a fifty-conductor
       ribbon cable  and uses  the single-ended  alternative (the
       single-ended alternative  allows up  to 6  meters of cable
       length).


3.8.2     Logical/Electrical Signal Definitions

       The SCSI  bus uses  +5 volts  (termination power), ground,
       and eighteen signal lines.  Nine signals are for the 8-bit
       data bus  with one data parity bit; the other nine signals
       are the  SCSI control lines which coordinate access to the
       bus for transfers of commands, data, status, and messages.
       The interface  signals are  listed below;  please refer to
       the published  SCSI standard  for further  details.    Pin
       assignments of  the connector  are  provided  in  sections
       3.8.3 Pin  Assignments and  Connector (Single  Ended)  and
       3.8.4 Pin  Assignments and  Connector (Differential) later
       in this chapter.

       Across the SCSI bus all initiator signals are low-true for
       the single  ended version.   The  signals are asserted, or
       active, at 0 to 0.5 volts DC, and deasserted, or inactive,
       at 2.5 to 5.25 volts DC.  This low-true logic is indicated
       by the negative sign which precedes the signal name.

       -RST
       The -RST  (reset) signal  is an OR-tied signal asserted by
       the initiator,  causing the  drive to  do a  "hard" RESET,
       self configure  and return  to the  IDLE condition.   This
       signal is normally used during a power-up sequence.  The -
       RST pulse should be at least 25 microseconds wide.

       -SEL
       The -SEL  (select) signal, accompanied by the disk drive's
       SCSI ID  bit (zero  through seven),  is  asserted  by  the
       initiator and causes the disk drive to be selected.  The -
       SEL line  must be  negated by the initiator after the disk
       drive asserts  the -BSY  line  in  response  to  a  proper
       selection.   The signal  can be  asserted by  the  arbiter
       (initiator or drive) in the ARBITRATION phase.  The signal
       is also  asserted by the disk drive during the RESELECTION
       phase.

       -BSY
       The -BSY  (busy) signal  is an  OR-tied signal asserted by
       the drive,  indicating that  the bus is being used.  It is
       also asserted  by the arbiter during the ARBITRATION phase
       and by  the  initiator  and  the  disk  drive  during  the
       RESELECTION phase.




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       -C/D
       Assertion of  the -C/D  (control/data) signal  by the disk
       drive  indicates   that  command,   status,   or   message
       information  is   to  be  transferred  on  the  data  bus.
       Negation of  this line  indicates that  data    is  to  be
       transferred on the data bus.

       -I/O
       When the  -I/O (input/output)  signal is  asserted by  the
       disk drive it indicates that information is transferred to
       the initiator from the disk drive.  Negation of the signal
       indicates that  information is  transferred  to  the  disk
       drive from  the initiator.   Note that IN means toward the
       initiator.

       -REQ
       When asserted by the disk drive, the -REQ (request) signal
       indicates that  a byte  is to  be transferred  on the data
       bus.  -REQ is negated following assertion of the -ACK line
       by the initiator.

       -ACK
       The  -ACK   (acknowledge)  signal   is  asserted   by  the
       initiator, following  the assertion  of the  -REQ line, to
       indicate that  data has been accepted by the initiator, or
       that data is ready to be transferred from the initiator to
       the disk drive.  -ACK is negated following negation of the
       -REQ line.

       -ATN
       The -ATN  (attention) signal  is asserted by the initiator
       to indicate the ATTENTION condition, which is a request by
       the initiator  for the disk drive to enter the MESSAGE OUT
       phase.

       -MSG
       The -MSG  (message) signal  is asserted  by the disk drive
       during one  of the message phases.  Messages may be either
       IN or OUT, depending on the state of the -I/O signal.

       -DB (7-0, P)
       The eight  bidirectional data  bus lines  (DB 7-0) and the
       parity line (DBP) are used to transfer 8-bit parallel data
       to  or  from  the  initiator.    Bit  seven  is  the  most
       significant bit.  Bits zero through seven are also used as
       SCSI  ID   bits  during  the  ARBITRATION,  SELECTION  and
       RESELECTION phases.  Data bus parity (DBP) is odd.


3.8.3     Pin Assignments and Connector (Single Ended)

       The disk drive communicates with an initiator system via a
       fifty-pin   connector,   J1.      The   logical/electrical
       configuration of the SCSI connector is given in Table 32,



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       Connector Pin  Assignments (Single  Ended).  Note that the
       minus sign (-) indicates low-true logic, and that all odd-
       numbered pins  are return (ground) pins for the associated
       even-numbered pins,  except for  pin twenty-five, which is
       not connected.   Pin  one is  located on  the  end  of  J1
       closest to  the DC  power connector, J3.  (See Figure 35,
       Connector Locations, Rear View of Drive.)


                                

                            Table 32
            Connector Pin Assignments (Single Ended)



3.8.4     Pin Assignments and Connector (Differential)

       The disk drive communicates with an initiator system via a
       fifty-pin   connector,   J1.      The   logical/electrical
       configuration of the SCSI connector is given in Table 33,
       Connector Pin  Assignments (Differential).    All  signals
       consist of  two lines  denoted +SIGNAL  and  -SIGNAL.    A
       signal is true when +SIGNAL is more positive than -SIGNAL.
       A signal  is false  when -SIGNAL  is  more  positive  than
       +SIGNAL.  All assigned signals shall be terminated at each
       end of  the cable  with a  terminator network.    Resistor
       tolerances in the terminator network shall be 5% or less.

       The DIFFSENS  signal of the connector is used as an active
       high enable  for the  differential drivers.   If a single-
       ended device  or terminator  is  inadvertently  connected,
       this  signal   is  grounded,  disabling  the  differential
       drivers.     See  Table  33,  Connector  Pin  Assignments
       (Differential).


                                

                            Table 33
            Connector Pin Assignments (Differential)



                                

                           Figure 35
             Connector Locations, Rear View of Drive


       Connection to  J1 is  via  a  nonshielded  fifty-conductor
       connector, consisting  of two  rows of  twenty-five female
       contacts on  0.1 inch  centers.   Figure 36,  SCSI  Cable




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       Connector, shows  the configuration  and dimensions  of  a
       suitable  mating  connector.    Recommended  strain-relief
       connectors are  AMP part number 1-499506-2, or Dupont part
       number 669002  (66900-250).   Use of  a keyed connector is
       recommended.


                                

                           Figure 36
                      SCSI Cable Connector



3.9  ELECTRICAL POWER INTERFACE

       This subsection  describes the  power-up sequence  for the
       disk drive,  and the  two connectors  associated with  the
       electrical power  interface.   These  connectors  are  the
       power connector, J3, and the frame ground connector, J4.





































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3.9.1     Power-Up Sequence

       DC power  (+5 volts  and +12 volts) may be supplied in any
       order.   Both power  supplies must  be present, and within
       the tolerances  of the  power sensing  circuit, before the
       motor will  spin up.  Typical current draw during power up
       is shown  in Figure 37, Motor Start Current Requirements.
       When the  spindle reaches  full speed,  the actuator  lock
       automatically  disengages.     The   disk  drive  performs
       automatic seek  calibration during  start up  for  optimum
       seek performance.   The  disk drive  spins up  and becomes
       ready in  20 to  30 seconds.   The disk drive executes its
       recalibration sequence  whenever power  is applied  or the
       START STOP  UNIT command  is  received.    You  may  delay
       starting the  spindle motor,  to minimize  power surges in
       multidrive configurations,  by using  the  jumper  options
       (see 4.2  Drive Power-Up  Options later  in this  manual).
       The  two  jumpers  controlling  the  disk  drive  power-up
       options are also covered in this section.


                                

                           Figure 37
                Motor Start Current Requirements



3.9.2     Power Connector

       The DC  power connector,  J3  (shown  in  Figure  38,  J3
       Connector), is  a four-pin AMP MATE-N-LOCK connector, part
       number 3505430-1.  The recommended mating connector is AMP
       part number 1-480424-0, using AMP pins part number 350078-
       4 (strip)  or part  number 61173-4 (loose piece).  J3 pins
       are numbered  and assigned as shown in Figure 38.  Figure
       35, Connector  Locations, Rear  View of  Drive, shows the
       location of J3 on the disk drive.


                                

                           Figure 38
                          J3 Connector



3.9.3     Frame (HDA) Ground Connector

       The ground connector, J4, is a Faston-type connection, AMP
       part number  61761-2.  The recommended mating connector is
       AMP part  number 62187-1.   (See  Figure 35.)  If wire is
       used, the  hole in J4 accommodates a wire size of 18 gauge
       wire, maximum.  This connection is on the rear of the HDA,



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       accessible through  an opening  in the  frame of  the disk
       drive.   Figure 35,  Connector Locations,  Rear  View  of
       Drive, shows the location of J4 on the disk drive.


3.10 MOUNTING AND INSTALLATION

       The  disk   drive  may  be  mounted  in  any  orientation.
       However, in  any final mounting configuration, ensure that
       the operation  of the three shock mounts is not restricted
       (these mounts  isolate the  HDA from  the frame).    Also,
       certain switching  power supplies  may emanate  electrical
       noise  which  degrades  the  specified  read  error  rate;
       therefore, it is suggested that the disk drive be oriented
       so that  the PCB  assembly is  not adjacent to these noise
       sources.

       Eight mounting  holes, four  on the bottom and two on each
       side, are  provided for  mounting the  disk drive  into an
       enclosure.  The size and location of these holes, shown in
       Figure 39,  Mechanical Outline,  Top and  Side Views, are
       identical to  industry standards.   Overall height, width,
       and depth,  along with  other key dimensions, are shown in
       Figure 39, and Figure 310, Mechanical Outline, Isometric
       View.


                                

                           Figure 39
             Mechanical Outline, Top and Side Views



                                

                           Figure 310
               Mechanical Outline, Isometric View


       The faceplate may be removed in installations that require
       it.   Remove the two C-clips and unplug the light-emitting
       diode (LED)  cable from  the PCB, as shown in Figure 311,
       Removable Faceplate.


                                

                           Figure 311
                       Removable Faceplate







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3.11 SHIPPING

       At power down, the heads are automatically positioned over
       the nondata,  dedicated landing zone on each disk surface.
       The automatic  shipping lock  solenoid is  also engaged at
       this time.   Maxtor  ships the  disk drive  in single- and
       multipack shipping  containers.   Users can  ship the disk
       drive installed  when the nonoperating shock and vibration
       limits are not exceeded.
















































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4.0  USER SELECTABLE OPTIONS

       Jumper locations are identified in Figures 41 and 42,
       PCB  Layout  (single  ended  version  and  differential
       version).


                               

                          Figure 41
              PCB Layout (Single Ended Version)



                               

                          Figure 42
                  PCB Layout (Differential)



4.1  SCSI ID SELECTION

       SCSI ID  jumpers (JP35  through JP37)  are provided  to
       configure each disk drive with a SCSI device ID for use
       in multiple SCSI device configurations.

       Table 41,  SCSI ID  Jumpers, is  a reference table for
       the SCSI  ID jumper  configuration,  the  ID,  and  the
       priority on  the SCSI  bus.   An ID  of  seven  is  the
       highest priority  in a  multiple device  configuration,
       and is usually used for the initiator.


                               

                          Table 41
                       SCSI ID Jumpers


       The disk  drive is shipped from the factory with a SCSI
       ID of  six.   This assures  that sufficient jumpers are
       available  for  any  address  except  seven,  which  is
       usually reserved for the host system.

       The SCSI  ID can  also  be  set  remotely  through  the
       auxiliary connector  (see 4.9 Auxiliary Connector later
       in this  chapter).  The SCSI ID jumpers must be removed
       to use the auxiliary connector option, and any shorting
       required for  a selected  SCSI ID  is done  outside the
       disk drive.






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4.2  DRIVE POWER-UP OPTIONS

       In order  to allow for system power supply constraints,
       which  may   require  minimizing   surge  current  when
       powering up multiple disk drives, three modes of start-
       up sequencing  are provided  (see Table 42, Summary of
       Power-Up Options).


                               

                          Table 42
                 Summary of Power-Up Options


       Start by ID Sequence:  Both JP14 and JP38 are out.  The
       disk drive  delays spinup  for approximately  11 to  13
       seconds multiplied  by the  SCSI ID.   This  allows for
       power-up sequencing.   A RESET condition results in the
       disk drive  delaying its  spinup according  to its SCSI
       ID.

       Wait for  START Command:   JP14  is out and JP38 is in.
       The disk  drive does  not spinup  until  the  initiator
       issues a  START STOP  UNIT command  with the  start bit
       equal to one.

       Start When  Power Is  Applied:   JP14 is in and JP38 is
       out.   The motor  starts as  soon as  power is applied.
       The disk drive is shipped in this configuration.


4.3  TERMINATOR POWER SELECTION

       Single Ended Version: Power to the terminators may come
       internally from  the disk drive, or externally from the
       SCSI bus.   If  JP41 is  in, the  terminator  power  is
       provided by  the disk  drive.    If  JP34  is  in,  the
       terminator power is external, from the SCSI bus.

       If both  JP41 and  JP34 are  in,  terminator  power  is
       provided from  the disk  drive and  to  the  SCSI  bus.
       Hence, the  disk drive  is at  one end  of the SCSI bus
       with  terminators   in  place  and  is  also  supplying
       terminator power for the device at the other end of the
       SCSI bus (see Figure 34, Typical SCSI Configurations).
       Additional discussion  of the  termination of  the SCSI
       bus and  location of  the disk  drive  on  the  bus  is
       presented in  4.8 Interface  Termination later  in this
       chapter.







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       Differential Version:   The   differential version does
       not provide  for termination  on the  PCB.   Therefore,
       JP34 is  shipped installed  and JP41  is not supported.
       This will  provide +5v  to the  SCSI bus  for  external
       termination.


4.4  WRITE PROTECT OPTION

       The write protect jumper, JP18,  is used to protect the
       data written  on the disk drive.  When the jumper is in
       data cannot  be written  onto the disk drive; only read
       operations can  be executed.  Installation of JP18 also
       grounds the  remote write  protect line, J2, pin seven,
       and prevents  remote control  of write  protect through
       J2, pin seven, on the ten-pin connector.

       As shipped  from the  factory  JP18  is  out,  allowing
       normal reading  and writing, or optional remote control
       through connector  J2.   See  4.9  Auxiliary  Connector
       later in this chapter, for details of the J2 connector.


4.5  PARITY OPTION

       The parity jumper, JP40, enables (in) or disables (out)
       odd parity  detection in the disk drive.  Odd parity is
       always generated  by the disk drive and provided to the
       SCSI bus.

       The disk drive is shipped with JP40 in, enabling parity
       detection.


4.6  OTHER JUMPERS

       The  other   jumper  positions   shown  are   not  user
       selectable.   These jumpers  are for  factory use only.
       The  user   should  avoid  changing  these  jumpers  or
       improper operation may occur.


4.7  SECTOR SIZE

       The disk  drive is  shipped from  the factory formatted
       with 512-byte  sectors.   Configuration parameters  are
       default  values   in  all   optional  cases.     It  is
       recommended that users reformat the disk drive with the
       user's sector  size, using  the P  list, and the user's
       configuration parameters.







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4.8  INTERFACE TERMINATION

       SCSI devices require proper interface termination.  The
       first device  and the  last device  on a SCSI bus daisy
       chain  must be terminated (see Figure 34, Typical SCSI
       Configurations).    Remove  the  terminators  from  any
       devices in  between.   For instance, if a disk drive is
       in the  middle of  a SCSI  bus daisy  chain, remove its
       terminators.   The disk  drive terminators are shown in
       Figures 41 and 42, PCB Layout.

       Terminator  pin  one  is  marked  with  a  dot  on  the
       terminator as shown in Figures 41 and 42, PCB Layout.
       The terminators  (RN17, RN18,  and RN19)  all look like
       the side  view inset in the figure.  The orientation of
       the terminator on the PCB is also shown.  Note that the
       PCB's hole/solder pad for pin one has a square outline,
       whereas all  the other holes/pads have a round outline.
       Also, note  that pin  one is  always the closest pin to
       the power connector, J3.

       NOTE: All terminator  packs (RN17 through RN19) must be
       oriented with  the dot towards J3 for the disk drive to
       work properly.

       As shipped,  the interface  signal lines are terminated
       with  three  removable  220/330  ohm  resistor  network
       packs.

       The devices  driving the  disk drive  inputs should  be
       open collector  devices capable  of sinking at least 48
       milliamps at  a voltage level of less than 0.5 volts DC
       (7438 or equivalent).

       Devices receiving  the disk  drive outputs should be of
       SCHMITT trigger type to improve noise immunity (74LS14,
       74LS240, or equivalent).  The initiator should not load
       the bus  with more  than one standard low power Schotky
       transistor-transistor  logic  (LSTTL)  input  load  per
       line, and should terminate all signals with 220/330 ohm
       terminators.

       NOTE: The   differential version   does not provide for
       termination on  the  PCB.    External  terminators  are
       available from  Maxtor.


4.9  AUXILIARY CONNECTOR

       Connector J2  in Figure  35, Connector Locations, Rear
       View of  Drive, is  an  auxiliary  connector  providing
       remote control  of the  write protect  feature and  the





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       SCSI ID, and the ability to carry the LED signal beyond
       the disk  drive.   The auxiliary  connector is  a  Berg
       68451-121, ten-pin  part.  The mating connector is a 3M
       3473-6010 part.

       Pin  assignments   are  as   in  Table  43,  Auxiliary
       Connector  Pin   Assignments  (also   see  Figure  35,
       Connector Locations, Rear View of Drive).


                               

                          Table 43
             Auxiliary Connector Pin Assignments


       When pin  seven is  connected to ground, the disk drive
       is protected  from writing, regardless of commands sent
       to the  disk drive  via the  SCSI interface.  The write
       protect terminal  may also  be connected to the cathode
       (negative lead)  of an  LED.   This LED should have its
       anode (positive  lead) connected  to a positive voltage
       source.   When the  disk drive enters the write protect
       mode, the  write protect  signal becomes true (low) and
       the LED lights.

       For remote write protect, JP18 must be removed (see 4.4
       Write Protect Option earlier in this chapter).

       When an LED is connected to pin one (+) and pin five (-
       ), that  LED functions  in the  same manner  as the LED
       which is  mounted on  the disk drive's faceplate.  This
       is typically  used in  cases where  the disk  drive  is
       mounted in a position where the disk drive's LED is not
       visible and the faceplate is removed.

       Pin three  is not  present so  that users  can key  the
       mating connector.   Pin  three of  the mating connector
       should be blocked for this purpose.

       The SCSI  ID  of  the  disk  drive  may  be  programmed
       remotely by selectively connecting pins six, eight, and
       ten to  ground, or  leaving them  open.    The  various
       combinations are  shown in  Table 44,  Remote SCSI  ID
       Programming Combinations.


                               

                          Table 44
           Remote SCSI ID Programming Combinations






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       SCSI ID jumpers (JP35 through JP37) must be removed for
       the remote SCSI ID option.


4.10 SPINDLE SYNCHRONIZATION CONTROL

       Spindle synchronization  is a feature that allows up to
       forty-eight disk  drives to  synchronize the rotational
       position  of  their  spindles.    Pin  assignments  for
       connector  J6   are  shown   in  Table   45,  J6   Pin
       Assignments.   For additional  information  on  spindle
       synchronization, call  Maxtor at (408) 432-1700 and ask
       for the Marketing department.


                               

                          Table 45
                      J6 Pin Assignments






































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5.0  LOGICAL CHARACTERISTICS

       The following  information  is  based  on  the  ANSI  SCSI
       standard (X3.131-1986), and the Common  Command Set (CCS).
       For more information, refer to these publications.


5.1  POWER UP AND BUS RESET

       The following  sections describe  the sequence  of  events
       during drive  self-test and initialization sequences.  The
       self-test  sequence   occurs  before   the  initialization
       sequence.


5.1.1     Self-Test Sequence

       The self-test  sequence is  executed upon  drive power up.
       The self-test  sequence  verifies  the  integrity  of  the
       hardware.    This  test  is  not  an  exhaustive  hardware
       diagnostic, but  checks  the  major  components  for  full
       function.  The drive does not respond to SELECTION for the
       first 250  milliseconds  of  the  self-test.    After  250
       milliseconds have elapsed, the drive responds to SELECTION
       with a  BUSY status for 2 to 3 seconds: this time is spent
       completing the remaining self-test sequences, initializing
       the SCSI  circuitry, and  enabling  the  SCSI  interrupts.
       After  the   self-test  is   complete  and  the  interface
       circuitry is  initialized (approximately  3 seconds),  the
       drive responds  to SELECTION  with CHECK  CONDITION status
       and the  appropriate sense  data  (i.e.,  UNIT  ATTENTION,
       POWER ON/RESET condition).

       The self-test sequence consists of the following events:

         Hardware  Reset   Test  -   This  routine   tests   the
         microprocessor, buffer  controller, disk  formatter, and
         SCSI reset  latch for the proper power up condition.  If
         any of  these tests fail, the drive can only be reset by
         a POWER UP condition.

         Microprocessor  Test   -   This   routine   tests   the
         microprocessor's internal  memory, timers,  and register
         bank switching for proper operation.

         Erasable Programmable Read Only Memory (EPROM) Checksum
         Test -  This routine  performs a checksum calculation on
         the controller  firmware EPROM  and compares  it against
         the checksum stored in the EPROM.

         Buffer Controller  Test - This routine tests the buffer
         controller for  proper operation.  All the registers are
         tested and  the chip  is engaged to access random-access
         memory (RAM).



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         Dynamic RAM  Test -  This routine tests the dynamic RAM
         by writing and reading different patterns to memory.  In
         addition, the  buffer controller  is tested  for  proper
         refresh operation  and parity detection.  This test also
         tests the memory parity interrupt.

         Disk Formatter  Test -  This  routine  tests  the  disk
         formatter chip  by  writing  and  reading  all  possible
         patterns to  each of  the disk formatter chip registers.
         After the  registers  are  tested,  the  interrupts  are
         tested to  ensure that  the formatter  chip generates an
         interrupt when a command completes.

         SCSI Controller  Test -  This routine  tests  the  SCSI
         controller  chip   by  executing   the  chip  diagnostic
         command.   After  the  diagnostic  test  completes,  the
         interrupts are  tested to  ensure  that  the  SCSI  chip
         generates  an   interrupt  when   a  command  completes.
         Finally, the registers are tested by writing and reading
         all possible  patterns to  each of  the SCSI  controller
         chip registers.

       If any portion of the self-test fails, except the hardware
       reset test,  the drive  can be  reset by  a SCSI bus RESET
       condition or  a power  up RESET condition.  The failure of
       the hardware  reset  test  is  considered  a  catastrophic
       failure and  the controller  can only be reset from such a
       failure by a power up RESET condition.


5.1.2     Initialization Sequence

       The initialization sequence is executed for any one of the
       following three reasons:

         a POWER UP condition occurs
         the SCSI bus -RST signal is asserted
         a BUS  DEVICE  RESET  message  (on  the  SCSI  bus)  is
       received

       After a  successful initialization, the first command from
       each initiator  is terminated with CHECK CONDITION status.
       The sense  data is  set to UNIT ATTENTION/POWER ON, RESET,
       or BUS DEVICE RESET OCCURRED.

       Until the  drive has  spun up,  any  command  sent  by  an
       initiator which requires a ready drive for GOOD completion
       status, is  terminated with  CHECK CONDITION  status.  The
       sense data  is set  to DRIVE NOT READY.  Commands that may
       complete with  GOOD status  prior to the drive being ready
       are REQUEST  SENSE, INQUIRY,  RESERVE UNIT,  RELEASE UNIT,
       START STOP UNIT, READ BUFFER, and WRITE BUFFER.

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       Note that the INQUIRY command results in the general model
       designation (XT-8000S)  being returned  prior to the drive
       being READY.   Afterwards,  the specific  model number  is
       returned.

       After a  RESET CONDITION occurs, the first command sent by
       the initiator  that causes a medium access loads the saved
       mode parameters into the current mode parameters.


5.1.3     Self Configuration

       When the  drive powers  up or a RESET CONDITION occurs, it
       configures itself  from  the  parameters  and  information
       saved on  the medium  from the  previous format operation.
       This includes  the model  number returned  in the  INQUIRY
       command and  the mode  parameters.    Refer  to  7.4  MODE
       SELECT, and  7.5 MODE  SENSE later  in this  manual, which
       describe the mode parameters.


5.1.4     UNIT ATTENTION Condition

       A UNIT ATTENTION condition is generated for each initiator
       whenever the  drive has  been reset (by a BUS DEVICE RESET
       message or a RESET condition), or when the mode parameters
       have been changed by other initiators.  The UNIT ATTENTION
       condition persists for each initiator until that initiator
       issues any  command other than INQUIRY.  The generation of
       a UNIT  ATTENTION condition  supercedes any  pending sense
       data.

       If an INQUIRY command is received from an initiator with a
       pending UNIT  ATTENTION condition, the last UNIT ATTENTION
       generated condition  is reported.  The drive  returns  the
       INQUIRY data  and does  not  clear    the  UNIT  ATTENTION
       condition.


5.2  BUFFERING SCHEME

       The drive  has 64  kilobytes of  RAM.   The drive  uses  4
       kilobytes of RAM for a scratchpad, 16 kilobytes of RAM for
       a write buffer and 45,056 bytes of RAM for a data buffer.

       The drive will use the data buffer for read-ahead cache if
       this feature  is enabled.   The  drive will  use the  data
       buffer for write operations if the transfer length exceeds
       16 kilobytes.

       Normally, the  drive uses  the split  buffering, that  is,
       separate buffers  for write  operations (write buffer) and
       read operations  (data  buffer)  to  optimize  throughput.




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       This is  particularly advantageous in systems that perform
       extensive read-modify-write processes.

       The   buffer   controller   is   dual   parted,   allowing
       simultaneous transfers  to and  from the initiator and the
       medium.


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6.0  SCSI PHASES AND MESSAGES

       The condition  on the  SCSI bus  can be divided into eight
       distinct phases:

       BUS FREE phase
       ARBITRATION phase
       SELECTION phase
       RESELECTION phase
       COMMAND phase \
       DATA phase     \  These phases are collectively termed the
       STATUS phase   /  Information Transfer phases.
       MESSAGE phase /

       The various  phases are  defined by  the state of the SCSI
       bus signals  -SEL, -BSY,  -MSG, -C/D, -I/O, -REQ, and -ACK
       (see Table  61, Signal  States and Bus Phases).  The SCSI
       bus can never be in more than one phase at a time.  Figure
       61, Signal  Sequence Chart  for SCSI  Phases,  shows  the
       signal sequence of the eight phases.  See the figure while
       reading the following sections.

       NOTE: The condition  of the  control lines for information
       transfer is  valid   only when the SCSI bus -REQ signal is
       asserted; therefore,  the disk  drive is  not "in a phase"
       unless the --REQ signal is asserted.


                                

                            Table 61
                  Signal States and Bus Phases



                                

                           Figure 61
              Signal Sequence Chart for SCSI Phases



6.1  BUS FREE PHASE

       The BUS FREE phase is used to indicate that no SCSI device
       is actively  using the  SCSI bus, and that it is available
       for subsequent  users.  The BUS FREE phase occurs when the
       drive releases -BSY following a RESET condition or certain
       message phases (i.e., COMMAND COMPLETE and DISCONNECT).








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6.2  ARBITRATION PHASE

       The ARBITRATION  phase allows  one  SCSI  device  to  gain
       control of  the SCSI bus so that it can assume the role of
       an initiator  or target  (drive).   The arbitrating device
       waits for  the BUS  FREE phase  to occur.  It then asserts
       its own SCSI ID bit and -BSY.  The arbitrating device then
       examines the  data bus.   If a higher priority SCSI ID bit
       exists on  the data  bus,  the  arbitrating  device  loses
       arbitration  and   releases  -BSY   and  the   data   bus.
       Otherwise, the  arbitrating device  wins  arbitration  and
       asserts -SEL.

       NOTE: Implementation of  the ARBITRATION phase is a system
       option.   Systems that  do not  implement this  option can
       have  only  one  initiator.    The  ARBITRATION  phase  is
       required for  systems that  use  the  disconnect/reconnect
       feature.


6.3  SELECTION PHASE

       If the initiator wins arbitration, it enters the SELECTION
       phase by  continuing to  assert its  own initiator SCSI ID
       bit and  asserting the drive's SCSI ID bit.  The initiator
       then  negates   -BSY  (-SEL   remains  asserted   by   the
       initiator).    If  the  initiator  expects  the  drive  to
       disconnect/reconnect, the  initiator must  assert the -ATN
       line prior to the negation of -BSY.

       If the  initiator does  not support  arbitration, then the
       SELECTION phase  is entered  from the BUS FREE phase.  The
       initiator asserts  only the  disk drive's  SCSI ID bit and
       asserts -SEL.

       During the  SELECTION phase, the drive maintains a negated
       -I/O line so that the SELECTION phase may be distinguished
       from the RESELECTION phase.

       The  drive   determines  that  it  has  been  selected  by
       detecting  its  SCSI  ID  bit  asserted  on  the  bus  (as
       determined by  the ID  jumpers, see  4.1 SCSI ID Selection
       earlier in this manual).

       If more  than two  IDs are  asserted on  the data  bus, or
       parity is  enabled and  bad parity  is detected, the drive
       does not respond to the SELECTION phase.

       The drive  asserts -BSY  after detecting  that it has been
       selected.   At this  point, the initiator must negate -SEL
       and may remove the IDs from the data bus.






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6.4  RESELECTION PHASE

       After disconnecting  to free  the bus  for other activity,
       the drive  reconnects when it is ready to transfer data or
       status across  the bus.   The drive arbitrates for the bus
       and, if it wins, reselects the initiator.  The RESELECTION
       phase is similar to the SELECTION phase, except that the -
       I/O signal  line is  asserted.   The drive asserts its own
       SCSI ID  bit and the SCSI ID bit of the initiator which is
       being reselected.   The  drive  releases  -BSY  (-BSY  was
       already asserted  during  arbitration)  and  continues  to
       assert -SEL.   The  initiator detects  that  it  has  been
       selected and  responds  by  asserting  -BSY.    The  drive
       detects that  the -BSY  signal is now true and responds by
       also asserting -BSY (at this point, both the initiator and
       the drive  are holding  the -BSY  signal low).  The  drive
       then releases -SEL and the initiator responds by releasing
       -BSY (-BSY  is still  being asserted  by the  drive).  See
       Figure 61, Signal Sequence Chart for SCSI Phases.

       After  reselecting  the  initiator,  the  drive  sends  an
       IDENTIFY message to identify itself to the initiator.

       If the  initiator does  not  respond  to  the  reselection
       within a selection time-out delay (see Table 62, SCSI Bus
       Timing), the  drive releases the bus and then rearbitrates
       for the  bus, trying  to reselect  the initiator.  It does
       this up  to 255  times, or until the initiator responds or
       the drive is reset.

       NOTE: The   drive  does  not  disconnect  if,  during  the
       SELECTION phase,  the initiator does not set its initiator
       SCSI device  ID on  the bus  and if the initiator does not
       send an  IDENTIFY message  out (with  bit six asserted) to
       the drive.


                                

                            Table 62
                         SCSI Bus Timing



6.5  INFORMATION TRANSFER PHASES

       The -C/D,  -I/O, and  -MSG signals are used to distinguish
       between  the   different   information   transfer   phases
       (COMMAND, DATA,  STATUS, and MESSAGE).  The drive controls
       these three  signals, and, therefore, controls all changes
       from one  phase to  another.   The initiator can request a
       MESSAGE OUT  phase by  asserting -ATN,  and the  drive can
       cause a BUS FREE phase by negating/releasing -SEL and -BSY
       (and all other SCSI bus signals).



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       The information  transfer phases use one or more -REQ/-ACK
       handshakes to  control the  information transfer.   Each -
       REQ/-ACK handshake  allows the  transfer of  one  byte  of
       information.  During the information transfer phases, -BSY
       remains true and -SEL remains false.  Additionally, during
       the information  transfer phases,  the drive  continuously
       envelopes the  -REQ/-ACK handshake(s) with -C/D, -I/O, and
       -MSG in such a manner that these control signals are valid
       for a  bus settle  delay (see  Table 62, SCSI Bus Timing)
       before the  assertion of  -REQ of the first handshake, and
       remain valid  until the negation of -ACK at the end of the
       last handshake.


6.5.1     Asynchronous Information Transfer

       The drive  controls the  direction of information transfer
       by  means  of  the  -I/O  signal.    When  -I/O  is  true,
       information  is   transferred  from   the  drive   to  the
       initiator.  When -I/O is false, information is transferred
       from the initiator to the drive.

       If -I/O  is true  (transfer to  the initiator),  the drive
       first asserts -DB (7-0, P) to their desired values, delays
       at least  one deskew  delay, plus  a cable skew delay (see
       Table 62,  SCSI Bus  Timing), and then asserts -REQ.  -DB
       (7-0, P)  remain valid  until -ACK  is true  at the drive.
       The initiator  reads -DB (7-0, P) after -REQ is true, then
       signals its  acceptance of  the data  by  asserting  -ACK.
       When -ACK  becomes true at the drive, the drive may change
       or release  -DB (7-0, P), and negates -REQ.  After -REQ is
       false, the  initiator then  negates -ACK.   After  -ACK is
       false, the  drive may continue the transfer by asserting -
       DB (7-0, P) and -REQ as described above.

       If -I/O  is false  (transfer  to  the  drive),  the  drive
       requests information  by asserting  -REQ.   The  initiator
       drives -DB  (7-0, P)  to their  desired values,  delays at
       least one deskew delay, plus a cable skew delay (see Table
       62, SCSI  Bus Timing),  and asserts  -ACK.  The initiator
       continues to drive -DB (7-0, P) until -REQ is false.  When
       -ACK becomes true at the drive, the  drive reads -DB (7-0,
       P) and  then negates -REQ.  When -REQ becomes false at the
       initiator, the  initiator may  change or release -DB (7-0,
       P), and negates -ACK.  The drive may continue the transfer
       by asserting -REQ as described above.


6.5.2     Synchronous Data Transfer

       Synchronous data  transfer is  optional, and  may be  used
       only if  previously agreed  to by  the initiator and drive
       through the  message system  (see 6.6.16  SYNCHRONOUS DATA
       TRANSFER REQUEST  Message later  in this  chapter).    The



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       messages determine  the use  of synchronous  mode  by  the
       initiator and  the drive, and establish a -REQ/-ACK offset
       and  a  transfer  period.    The  synchronous  mode,  once
       established, remains in effect for all DATA phases until a
       RESET condition or power cycle occurs, or until one of the
       devices elects to modify the agreement.

       The -REQ/-ACK  offset specifies the maximum number of -REQ
       pulses that  can be  sent by  the target in advance of the
       number of -ACK pulses received from the initiator, thereby
       establishing a  pacing mechanism.   If  the number of -REQ
       pulses exceeds  the number of -ACK pulses by the -REQ/-ACK
       offset, the   drive  does not  assert -REQ  until the next
       -ACK pulse  is received.   A  requirement  for  successful
       completion of  the DATA  phase is  that the number of -ACK
       and -REQ pulses be equal.

       The transfer  period specifies  the minimum  time  allowed
       between leading edges of successive -REQ and -ACK pulses.

       The drive  asserts the  -REQ signal  for a  minimum of one
       assertion period.  The drive waits at least the greater of
       a transfer  period from  the last  transition of  -REQ  to
       true, or  the minimum  of a  negation period from the last
       transition of  -REQ to false before the disk drive asserts
       the -REQ signal.

       The initiator  sends one pulse of the -ACK signal for each
       -REQ pulse  received.   The  initiator  asserts  the  -ACK
       signal for  a  minimum  of  one  assertion  period.    The
       initiator waits  at least the greater of a transfer period
       from the last transition of -ACK to true, or for a minimum
       of a  negation period  from the last transition of -ACK to
       false, before the initiator asserts the -ACK signal.

       If -I/O  is true  (transfer to  the initiator),  the drive
       first asserts  -DB (7-0, P) to their desired values, waits
       at least  one deskew delay, plus one cable skew delay (see
       Table 62,  SCSI Bus  Timing), and then asserts -REQ.  -DB
       (7+0, P) are held valid for a minimum of one deskew delay,
       plus one  cable skew  delay, plus one hold time (see Table
       62) after  the assertion of -REQ.  The drive asserts -REQ
       for a minimum of one assertion period.  The drive may then
       negate -REQ  and change  or release  -DB (7-0,  P).    The
       initiator reads  the value on -DB (7-0, P) within one hold
       time of  the transition  of -REQ  to true.   The initiator
       then responds with a -ACK pulse.

       If -I/O  is false  (transfer to  the drive), the initiator
       transfers 1  byte for  each -REQ  pulse received.    After
       receiving a  -REQ pulse,  the initiator  first asserts -DB
       (7-0, P)  to the desired value, delays at least one deskew
       delay, plus  one cable skew delay (see Table 62, SCSI Bus




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       Timing), and  then asserts  -ACK.  The initiator holds -DB
       (7-0, P)  valid for  at least  one deskew  delay, plus one
       cable skew delay, plus one hold time (see Table 62) after
       the assertion  of -ACK.   The initiator asserts -ACK for a
       minimum of  one assertion  period.  The initiator may then
       negate -ACK  and may  change or release -DB (7-0, P).  The
       drive reads the value of -DB (7-0, P) within one hold time
       of the transition of -ACK to true.


6.5.3     COMMAND Phase

       After being  selected and processing the IDENTIFY message,
       if any,  the drive normally switches to the COMMAND phase.
       The  6   or  10  bytes  of  command  information  (command
       descriptor  block,   or  CDB)  are  transferred  from  the
       initiator to the drive.

       If enabled,  parity is  checked on  each command byte.  If
       bad parity is detected, the command is aborted.  The drive
       switches to  the STATUS  phase,  returns  CHECK  CONDITION
       status, and  sets the sense data to ABORTED COMMAND/PARITY
       ERROR for  that initiator.  The drive then switches to the
       MESSAGE phase,  returns a  COMMAND COMPLETE  message,  and
       goes to the BUS FREE phase.

       After each command byte transfer, the -ATN bit is checked;
       if the  -ATN line  is active  the drive  switches  to  the
       MESSAGE OUT  phase and  receives  and  then  acts  on  the
       message.


6.5.4     DATA IN and DATA OUT Phases

       In commands  that require  a DATA phase (READ, WRITE, MODE
       SELECT, etc.),  the drive enters a DATA phase.  During the
       DATA IN  phase, data  is transferred from the drive to the
       initiator.  During the DATA OUT phase, data is transferred
       from the initiator to the drive.

       If bus  parity is  enabled and bad parity is detected, the
       command is aborted.  The controller switches to the STATUS
       phase, returns  CHECK CONDITION status, and sets the sense
       data to  ABORTED COMMAND/PARITY  ERROR for that initiator.
       The drive  then switches  to the  MESSAGE phase, returns a
       COMMAND COMPLETE message, and goes to the BUS FREE phase.

       After each  block, or group of blocks, is transferred, the
       -ATN bit  is checked;  if set,  the disk drive switches to
       the MESSAGE  phase  to  receive,  and  then  act  on,  the
       message.






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6.5.5     STATUS Phase

       After   completing    any   command    (successfully    or
       unsuccessfully, as  indicated by  the  status  byte),  the
       drive switches  to the STATUS phase and returns the status
       byte to  the initiator.   The  drive also  switches to the
       STATUS phase  for reporting  a BUSY, INTERMEDIATE-GOOD, or
       RESERVATION CONFLICT status.  The drive does not go to the
       STATUS phase  if it  is cleared  by a  BUS DEVICE RESET or
       ABORT message,  or by a "hard" RESET condition.  Following
       the STATUS phase, the drive enters the MESSAGE phase.

       The format  of the  status  byte  containing  the  command
       completion information  is defined  in Table  63,  Status
       Byte.


                                

                            Table 63
                           Status Byte


       The reserved bits are set aside for future standardization
       and is always set to zero.

       The status code field is used to specify the status of the
       completed command.  Table 64, Status Codes, gives the bit
       values for the status codes returned by the disk drive.


                                

                            Table 64
                          Status Codes


       Descriptions of the status codes are given below:

       GOOD -  This status  byte  indicates  that  the  operation
       completed as expected.

       CHECK  CONDITION  -  Any  error,  exception,  or  abnormal
       condition, that causes sense data to be set causes a CHECK
       CONDITION status.   The  REQUEST SENSE  command should  be
       issued following a CHECK CONDITION status to determine the
       condition.

       NOTE:   If any command other than REQUEST SENSE or INQUIRY
       is issued  following a  CHECK CONDITION, the sense data is
       lost.

       BUSY - The drive returns this status whenever it is unable
       to accept  a command.  For example, the drive returns this



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       status  when   it  is   busy  doing  self-tests  and  self
       configuration after  being powered  up or reset, or if the
       drive is busy executing a previously received command.

       INTERMEDIATE-GOOD -  This status  is  returned  for  every
       command in  a series  of linked  commands (except the last
       command),  unless   an  error,   exception,  or   abnormal
       condition,  causes   a  CHECK   CONDITION  status   or   a
       RESERVATION  CONFLICT   status  to   be  set.     If   the
       INTERMEDIATE-GOOD status  is not  returned, the  chain  of
       linked commands  is broken;  no further  commands  in  the
       series are executed.

       RESERVATION CONFLICT - This status is returned whenever an
       initiator attempts  to access  a drive that is reserved by
       another initiator.


6.5.6     MESSAGE Phase

       The MESSAGE  phase is  used to  transfer information about
       exception conditions  between the initiator and the drive.
       The MESSAGE IN and MESSAGE OUT phases are discussed below,
       followed by  descriptions of  the SCSI  messages.  Message
       codes supported  by the  drive are  shown  in  Table  65,
       Message Codes.


                                

                            Table 65
                          Message Codes



6.6  SCSI MESSAGES


6.6.1     MESSAGE IN Phase

       During the MESSAGE IN phase, a message is transferred from
       the drive  to the  initiator.   The drive  may enter  this
       phase at any time.


6.6.2     MESSAGE OUT Phase

       During the  MESSAGE OUT  phase, a  message is  transferred
       from the  initiator to  the drive.  The initiator requests
       that the  drive enter  the MESSAGE  OUT phase by asserting
       the -ATN  line.   The drive  frequently monitors  the -ATN
       line and  enters the  MESSAGE OUT  phase at  its  earliest





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       convenience in  response to the initiator's assertion of -
       ATN.

       After being  selected, the drive sets the control lines in
       preparation for  the MESSAGE  OUT phase.  If the initiator
       has -ATN  asserted, the  drive requests a message from the
       initiator  by  asserting  -REQ.    The  first  message  is
       expected to  be an  IDENTIFY message,  but the  drive also
       accepts a  BUS DEVICE  RESET, or an ABORT message.  If any
       other message  is received  the drive goes to the BUS FREE
       phase.   (See 6.6.16  SYNCHRONOUS  DATA  TRANSFER  REQUEST
       Message later in this chapter for additional information.)

       If, during  the selection,  the initiator  does not assert
       its ID  on the  bus, the  drive does  not examine the -ATN
       signal.   The drive  assumes the  initiator cannot support
       any message  except COMMAND COMPLETE, and does not support
       DISCONNECT-RECONNECT.     The  drive   also  assumes   the
       initiator ID  is zero,  and  saves  any  status  for  that
       initiator as initiator zero.

       NOTE: If the  initiator expects  the  drive to disconnect-
       reconnect, then  a MESSAGE OUT phase (the IDENTIFY message
       with bit  six true)  must occur  immediately  following  a
       SELECTION phase  which had  both the  initiator's, and the
       drive's, SCSI device ID asserted on the bus.


6.6.3     COMMAND COMPLETE (00h)

       The COMMAND COMPLETE message is sent from the drive to the
       initiator to  indicate that the execution of a command (or
       a series  of linked  commands) has  terminated,  and  that
       valid status  has been  sent  to  the  initiator.    After
       sending this  message successfully, the  drive goes to the
       BUS FREE  phase by  releasing -BSY,  unless the  initiator
       sets the -ATN line.

       NOTE: The command  may  or  may  not  have  been  executed
       successfully, as indicated in the status.

       If the  initiator rejects  this  message  with  a  MESSAGE
       REJECT message,  the drive  goes to the BUS FREE phase and
       does not consider this an error.


6.6.4     SAVE DATA POINTER (02h)

       When doing  disconnects, the  SAVE DATA POINTER message is
       sent before  every DISCONNECT  message .  If the initiator
       rejects this  message with  a MESSAGE  REJECT message, the
       drive does not disconnect.





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6.6.5     RESTORE POINTERS (03h)

       The RESTORE POINTERS message is sent from the drive to the
       initiator.   The message  acts to  restore to  the  active
       state the  most recently  saved pointers for the currently
       attached logical unit.  Pointers to the command, data, and
       status locations  for the logical unit are restored to the
       active pointers.  Command and status pointers are restored
       to the  beginning of the present command and status areas.
       The data pointer is restored to the value at the beginning
       of the  data area,  or to  the value at the point at which
       the last  SAVE DATA  POINTERS message  occurred  for  that
       logical unit.

       If the  initiator rejects  this  message  with  a  MESSAGE
       REJECT  message,  the  drive  immediately  terminates  the
       present command with a CHECK CONDITION status and sets the
       sense data  to HARDWARE  ERROR/MESSAGE REJECT  ERROR (04h,
       43h) for that initiator.

       When the  drive  reselects  the  initiator,  the  IDENTIFY
       message implies  that the  initiator  should  restore  its
       pointers.  Therefore, this message is not normally used in
       reselection.


6.6.6     DISCONNECT (04h)

       This message  is sent by the drive to inform the initiator
       that the  present physical path is about to be broken (the
       drive plans  to disconnect  by releasing -BSY), but that a
       later reconnect  is required  in  order  to  complete  the
       current operation.    This  message  does  not  cause  the
       initiator to  save the  data pointer.   If  the  initiator
       rejects this  message with  a MESSAGE  REJECT message, the
       drive does not disconnect.


6.6.7     INITIATOR DETECTED ERROR (05h)

       The INITIATOR  DETECTED ERROR  message  is  issued  by  an
       initiator to  inform the  drive that an error has occurred
       during an  operation.   This message should be sent by the
       initiator when a parity error is detected.


6.6.8     ABORT (06h)

       The ABORT  message is sent from the initiator to the drive
       to clear  the present  operation.   All pending  data  and
       status for  the issuing  initiator   is cleared  from  the
       drive, and  the drive goes to the BUS FREE phase.  Pending
       data and  status for  other initiators is not cleared.  No
       status or ending message is sent for the operation.  It is



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       not an  error to issue this message to a logical unit that
       is  not   currently  performing   an  operation   for  the
       initiator.


6.6.9     MESSAGE REJECT (07h)

       The  MESSAGE  REJECT  message  is  sent  from  either  the
       initiator or  the drive  to indicate that the last message
       received was inappropriate or has not been implemented.

       To create a MESSAGE REJECT message the initiator asserts -
       ATN prior  to releasing -ACK; the handshake of the message
       is then  rejected.   When the  drive sends this message it
       changes to  the MESSAGE  IN phase  and sends  this message
       prior to  requesting additional  message  bytes  from  the
       initiator.    This  provides  an  interlock  so  that  the
       initiator can determine which message is rejected.

       If the  initiator responds  to this message with a MESSAGE
       REJECT message,  the   drive  immediately  terminates  the
       present command with a CHECK CONDITION status and sets the
       sense data  to HARDWARE ERROR/MESSAGE ERROR (04h, 43h) for
       that initiator.


6.6.10    NO OPERATION (08h)

       The initiator  sends the  NO OPERATION message when it has
       no valid  message  for  the  drive  request.    The  drive
       receives and ignores this message.


6.6.11    MESSAGE PARITY ERROR (09h)

       The initiator  sends a  MESSAGE PARITY  ERROR  message  to
       indicate a  parity error  on one or more bytes of the last
       message sent  from the  drive.  The initiator asserts -ATN
       prior to  releasing -ACK  for the last byte of the message
       in error,  so that  the drive  knows which  message is  in
       error.   The drive  resends the  message.   If the Message
       Parity Error  is received again, the drive goes to the BUS
       FREE  phase  and  aborts  the  current  command  for  that
       initiator.   No further  reconnection  is  attempted,  and
       neither STATUS  nor COMMAND COMPLETE messages are returned
       for the  command.   The  sense  data  is  set  to  ABORTED
       COMMAND/SCSI PARITY ERROR (04h, 47h) for that initiator.










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6.6.12    LINKED COMMAND COMPLETE (0Ah)

       The  LINKED  COMMAND  COMPLETE  message  is  sent  to  the
       initiator to  indicate that  the  execution  of  a  linked
       command has completed and that the status has been sent.

       If the  initiator responds  with a MESSAGE REJECT message,
       the drive  goes to the BUS FREE phase and does not execute
       the next  command.   The sense  data is  set  to  HARDWARE
       ERROR/MESSAGE ERROR  (04h, 43h) for that initiator.


6.6.13    LINKED COMMAND COMPLETE (WITH FLAG) (0Bh)

       The LINKED COMMAND COMPLETE (WITH FLAG) message is sent to
       the initiator  to indicate  that the execution of a linked
       command (with  the flag  bit set to one) has completed and
       that the status has been sent.

       If the  initiator responds  with a MESSAGE REJECT message,
       the drive  goes to the BUS FREE phase and does not execute
       the next  command.   The sense  data is  set  to  HARDWARE
       ERROR/MESSAGE ERROR (04h, 43h).


6.6.14    BUS DEVICE RESET (0Ch)

       An initiator  may send the BUS DEVICE RESET message to the
       drive to  clear all  current commands on that SCSI device.
       The drive  clears all  commands, goes  through its initial
       power up  checks, its  self configuration, and goes to the
       BUS FREE condition ("hard" RESET).


6.6.15    IDENTIFY (C0h/80h)

       The IDENTIFY  message is  sent by  an initiator  after  it
       selects a  drive.   It is  sent by  the drive as the first
       message after  a reconnection.   In addition, this message
       specifies that  the sender  supports some  or all  of  the
       optional messages.    The  bits  in  Table  66,  IDENTIFY
       Message Codes,  show that the only truly changeable bit is
       bit six;  therefore, the command can have only two values:
       C0h if  the disconnect/reconnect  feature is supported, or
       80h if the disconnect/reconnect feature is not supported.


                                

                            Table 66
                     IDENTIFY Message Codes






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       If the  initiator responds  to this message with a MESSAGE
       REJECT  message,  the  drive  immediately  terminates  the
       present command with a CHECK CONDITION status and sets the
       sense key/error  code  to  HARDWARE  ERROR/Message  Reject
       Error (04h, 43h) for that initiator.

       NOTE: The  drive   does  not  disconnect  if,  during  the
       SELECTION phase,  the initiator does not set its initiator
       SCSI device  ID on  the bus, and if the initiator does not
       send an  IDENTIFY message  out (with  bit six  set) to the
       drive.


6.6.16    SYNCHRONOUS DATA TRANSFER REQUEST Message (01h)

       The  drive   can,  optionally,  perform  synchronous  data
       transfers, as discussed in 6.5.2 Synchronous Data Transfer
       earlier in  this chapter.   A  pair  of  SYNCHRONOUS  DATA
       TRANSFER REQUEST  messages (see Table 67 SYNCHRONOUS DATA
       TRANSFER REQUEST  Byte Values)  are exchanged  between  an
       initiator and the drive under the following conditions:

         a SCSI  device that  supports synchronous data transfer
         recognizes it  has not  communicated with the other SCSI
         device since receiving the last "hard" RESET

         A SCSI  device that  supports synchronous data transfer
         recognizes it  has not  communicated with the other SCSI
         device since receiving a BUS DEVICE RESET message

       SCSI devices  may  also  exchange  messages  to  establish
       synchronous data  transfer when  requested to  do so.  The
       messages exchanged  establish the  transfer period and the
       -REQ/-ACK offset.


                                

                            Table 67
          SYNCHRONOUS DATA TRANSFER REQUEST Byte Values


       The transfer period is defined as the minimum time between
       the leading  edge of a -REQ pulse and of its corresponding
       -ACK pulse.   The  -REQ/-ACK  offset  is  defined  as  the
       maximum number  of -REQ  pulses that  may  be  outstanding
       before the  corresponding -ACK  pulse is  received at  the
       drive.    A  -REQ/-ACK  offset  value  of  zero  indicates
       asynchronous mode;  a value  of  0Fh  yields  the  maximum
       number of outstanding -REQ pulses supported (fifteen).

       If the  initiator recognizes that negotiation is required,
       it asserts  -ATN and,  if  the  drive  implements  message
       transfers,  sends  a  SYNCHRONOUS  DATA  TRANSFER  REQUEST



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       message,  specifying  the  -REQ/-ACK  offset  and  minimum
       transfer period.   The  -REQ/-ACK offset is chosen to meet
       the data  handling requirements  of the  target, while the
       minimum  transfer  period  is  chosen  to  meet  the  data
       handling  requirements   of  the  initiator.    The  drive
       responds in  any of  the ways outlined in Table 68, Drive
       Responses  to   the  SYNCHRONOUS   DATA  TRANSFER  REQUEST
       Message.


                                

                            Table 68
Drive Responses to the SYNCHRONOUS DATA TRANSFER REQUEST Message


       The implied agreement remains in effect until a BUS DEVICE
       RESET  message  is  received,  a  "hard"  RESET  condition
       occurs, or  until one  of the  two SCSI  devices elects to
       modify the agreement.  Renegotiation at every selection is
       not recommended  since a significant performance impact is
       likely.     The  default   mode  of   data   transfer   is
       asynchronous.   The default  mode is  entered at power on,
       after a  BUS DEVICE RESET message, or after a "hard" RESET
       condition.   The SYNCHRONOUS DATA TRANSFER REQUEST message
       exchange can  only take  place following a SELECTION phase
       that includes  the SCSI IDs for both the initiator and the
       target.   Violation of  this rule  may make  data transfer
       impossible owing to disagreements among SCSI devices about
       the data transfer mode.


6.7  ERROR CONDITIONS

       Under several  error conditions,  the  drive  changes  the
       phase  to  BUS  FREE  without  correctly  terminating  the
       command (i.e.,  no DISCONNECT  or COMMAND COMPLETE message
       is sent).   The drive clears all information regarding the
       command, except  sense data (if any), and does not attempt
       to reconnect,  or in any other way terminate, the command.
       The initiator  must assume  that this  is  a  catastrophic
       failure and return the error to the system software.

       Sense data  may or  may not  be valid  when this condition
       occurs.   If the  initiator issues a REQUEST SENSE command
       and the  returned sense  key and additional sense code are
       not zero, the sense data is valid.










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6.7.1     MESSAGE OUT Phase Parity Error

       If the drive detects a parity error during the MESSAGE OUT
       phase it  retries the  MESSAGE OUT  phase once  using  the
       following sequence:

       1. Continue the  -REQ/-ACK handshake  until the  initiator
          negates -ATN (receives all of the message bytes).

       2. Instruct the  initiator to  resend all  of the  message
          bytes sent during the previous MESSAGE OUT phase by not
          changing the phase and asserting -REQ.

       3. The initiator  then resends all of the previous message
          bytes.

       If, after  one retry,  the message  is still  not received
       correctly, the  drive processes the error using one of the
       following sequences:

         If an  IDENTIFY message  is  not  received,  the  drive
          immediately goes  to the BUS FREE phase.  No sense data
          code information is set for this type of error.

         If  an   IDENTIFY  message   is  received,   the  drive
          terminates the  present command  with a CHECK CONDITION
          status and  sets the sense data to ABORTED COMMAND/SCSI
          PARITY ERROR  (0Bh/47h).   This error  does not prevent
          the initiator from retrying the command.


6.7.2     COMMAND Phase Parity Error

       When the  drive detects  a parity error during the COMMAND
       phase, it  retries the  COMMAND phase  one time  using the
       following sequence:

       1. Send the  initiator a RESTORE POINTERS message to reset
          the pointers to the start of the command.

       2. Attempt to receive the entire command again.

       If, after  one retry,  the command  is still  not received
       successfully, the  drive aborts  the command  using one of
       the following sequences:

         If the initiator does not send an IDENTIFY message, the
          drive immediately goes to the BUS FREE phase.  No sense
          data  information is sent for this type of error.

         If  the   drive  receives   an  IDENTIFY   message,  it
          terminates the  command with  a CHECK  CONDITION status
          and sets  the sense data to ABORTED COMMAND/SCSI PARITY




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          ERROR (0Bh/47h).   This  error  does  not  prevent  the
          initiator form retrying the command.


6.7.3     DATA OUT Phase Parity Error

       If the  drive detects  a parity  error during the DATA OUT
       phase, it  terminates the  command with  a CHECK CONDITION
       status, and  sets the  sense data  to ABORTED COMMAND/SCSI
       PARITY ERROR(0Bh/47h).   This  error does  not prevent the
       initiator from retrying the command.


6.7.4     Initiator Detected Error

       If the  drive receives an initiator detected error message
       at any  time during  the command, except during the STATUS
       phase or  COMMAND  COMPLETE  message,  it  terminates  the
       current command with a CHECK CONDITION status and sets the
       sense data  to ABORTED  COMMAND/INITIATOR  DETECTED  ERROR
       MESSAGE RECEIVED(0Bh/48h).   This  error does  not prevent
       the initiator from retrying the command.

       If the initiator sends an initiator detected error message
       immediately after  the STATUS  phase, the  drive  sends  a
       RESTORE POINTERS  message and  resends the status.  If the
       initiator sends  another initiator  detected error message
       in response  to the  resent status,  the drive immediately
       goes to  the BUS  FREE phase.   The  sense data  is set to
       ABORTED   COMMAND/INITIATOR    DETECTED   ERROR    MESSAGE
       RECEIVED(0Bh/48h).    This  error  does  not  prevent  the
       initiator from retrying the command.

       If the initiator sends an initiator detected error message
       immediately after  the COMMAND  COMPLETE message  is sent,
       the   drive immediately  goes to  the BUS FREE phase.  The
       sense data   is  set to ABORTED COMMAND/INITIATOR DETECTED
       ERROR MESSAGE  RECEIVED(0Bh/48h).   This  error  does  not
       prevent the initiator from retrying the command.


6.7.5     REJECTED Message

       When the  drive receives a MESSAGE REJECT message from the
       initiator, it  resends  the  original  message.    If  the
       message is  rejected again,  the drive  takes one  of  the
       following actions, based on which message was rejected:

         COMMAND COMPLETE - If the rejected message is a COMMAND
          COMPLETE message,  the disk  drive goes to the BUS FREE
          phase and does not consider this an error.






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         DISCONNECT -  If the  rejected message  is a DISCONNECT
          message,  the   drive  does  not  disconnect  from  the
          initiator and  continues the  current  command.    This
          condition does  not preclude  the drive from attempting
          to disconnect at a later time.

          NOTE:     The drive  does not send a DISCONNECT message
          to  an   initiator   which   does   not   support   the
          disconnect/reconnect option.

         IDENTIFY (Reconnect)  - If  the rejected  message is an
          IDENTIFY message, the drive immediately goes to the BUS
          FREE phase  and aborts  the current  SCSI command.   No
          further reconnection  is attempted,  and no  STATUS  or
          COMMAND COMPLETE  message is sent for the command.  The
          sense   data   is   set   to   HARDWARE   ERROR/MESSAGE
          ERROR(04h/43h).

         LINKED COMMAND  COMPLETE - If the rejected message is a
          LINKED COMMAND  COMPLETE message, the drive immediately
          goes to  the BUS  FREE phase and does not read the next
          command in  the linked  list.  The sense data is set to
          HARDWARE ERROR/MESSAGE ERROR(04h/43h).

         MESSAGE REJECT  - If  the rejected message is a MESSAGE
          REJECT message,  the drive  immediately terminates  the
          present command  with a CHECK CONDITION status and sets
          the  sense   data  to  HARDWARE  ERROR/  MESSAGE  ERROR
          (04h/43h).

         RESTORE POINTERS  - Since  the RESTORE POINTERS message
          is only  used in  an error recovery or retry situation,
          if the  rejected message is a RESTORE POINTERS message,
          the drive aborts the recovery or retry attempt, assumes
          the error  is unrecoverable,  and completes the command
          according to the error condition.

         SAVE DATA  POINTER -  The drive  assumes the  initiator
          does not  support this  message and does not attempt to
          disconnect from the bus during this command.


6.7.6     Initiator MESSAGE PARITY ERROR

       When the  drive receives  a MESSAGE  PARITY ERROR  message
       from the  initiator, it retries the operation by resending
       the original  message.   If the  message  cannot  be  sent
       successfully, the  drive immediately  goes to the BUS FREE
       phase and  aborts the  current SCSI  command.   No further
       reconnection  is   attempted  and  no  STATUS  or  COMMAND
       COMPLETE message  is returned  for the command.  The sense
       data  is   set  to   ABORTED  COMMAND/SCSI   PARITY  ERROR
       (0Bh/47h).




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6.7.7     RESELECTION Time-Out

       When the  drive attempts to reselect the initiator and the
       initiator does  not respond  within a  selection  time-out
       delay (as  defined in  the SCSI standard), the reselection
       is aborted.   The  drive attempts the reselection 254 more
       times and,  if all  attempts fail,  the drive  aborts  the
       current  SCSI   command.     No  further  reconnection  is
       attempted, and  no STATUS  or COMMAND  COMPLETE message is
       returned for  the command.   The  sense data   is  set  to
       HARDWARE ERROR/SELECT/RESELECT FAILURE (04h/45h).

       NOTE: The initiator must have an overall  command time-out
       delay to detect this error.


6.7.8     Internal Controller Errors

       If an  error occurs within the embedded controller that is
       related to  the  SCSI  hardware  or  firmware,  the  drive
       terminates the  present command  with  a  CHECK  CONDITION
       status  and   sets   the   sense   data      to   HARDWARE
       ERROR/INTERANALTARGET FAILURE  (04h/44h).  This error does
       not prevent the initiator from retrying the command.

































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APPENDIX A:  THE READ-AHEAD FEATURE


WHAT IS READ-AHEAD?

       Read-Ahead is a form of data caching where data is read
       from the  disk to  the buffer in anticipation of future
       operations.   This feature  can greatly  improve system
       access times  and throughput  in many  applications  by
       reducing the  time required  to make  data available to
       the host.   Read-Ahead  can be  switched on  or off  by
       means of the MODE SELECT command.


HOW DOES IT WORK?

       To understand  how Read-Ahead works, it is first useful
       to examine  how  data  is  stored  in  typical  systems
       applications.

       The tracks  on a  drive are divided into sectors as the
       smallest addressable  unit of  storage.  Data is stored
       as files  of various  lengths, with  one  sector  as  a
       minimum, but  most files  are spread across a number of
       sectors.  When a file spans multiple sectors, it can be
       stored and  retrieved most  quickly if  the sectors are
       written consecutively,  and so  most operating  systems
       allocate a  contiguous group  of sectors to a file, and
       write the  data in  a sequential  fashion  within  this
       allocated area.   Thus, although the drive is a random-
       access device,  files tend to be localized, and much of
       the data  transferred is  sequential.   As applications
       use larger  files (such  as the  multimegabyte graphics
       images created by CAD workstations), disk accesses tend
       to become more sequential.

       When retrieving  a file stored in this way, the highest
       performance is  achieved if  a single  READ command  is
       issued, with  the transfer length field set to the full
       length of  the file.   In  reality,  however,  this  is
       rarely the  case.   In operating  systems such as UNIX,
       READ commands  are usually issued with a fixed transfer
       length  (typically  1  to  8  kilobytes),  and  several
       successive commands  are required  to  retrieve  larger
       files.

       When an  access of  many  sectors  is  broken  up  into
       smaller commands,  it is  not possible  to transfer the
       data in  the most efficient way, because the time delay
       between the completion of one command and the beginning
       of the  next is  much longer  than the gap time between
       adjacent sectors.   By  the time  the next  command has





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       been received,  the disks  have already  rotated beyond
       the start  of the  next consecutive  sector, so  it  is
       usually necessary  to wait  for nearly  a full  latency
       period before  the disk  drive is  able to continue the
       data transfer.    Obviously,  this  additional  latency
       increases the  time required  to read  the entire file,
       and significantly degrades performance.

       As an  example, consider  the situation shown in Figure
       A1, Sequential READs without Read-Ahead.

       1. the host  issues a  command to read sectors zero and
       one
       2. some amount  of latency is required to arrive at the
       starting sector
       3. data is read from the disk to the data buffer
       4. when one  full sector is available in the buffer the
          drive starts to transfer data to the host
       5. after all  the data  has been  sent to  the host and
          status  information  has  been  provided,  the  host
          prepares to send the next command
       6. the second command (requesting a READ of sectors two
          and three) arrives, well after the heads have passed
          the start  of sector  two:  almost a full revolution
          is required before sector two rotates back around to
          the read/write head


                               

                          Figure A1
             Sequential READs without Read-Ahead


       Read-Ahead avoids  the loss  of performance represented
       by step  five by  taking advantage  of the disk drive's
       idle time  to read  additional data  into  the  buffer.
       There it  is available  for immediate  transfer in  the
       event that  the next READ command requests blocks which
       are already  in the buffer.  The probability of a "hit"
       (a match between data requested and data in the buffer)
       is  relatively   high  in   systems  which  store  data
       contiguously.

       When the  initiator sends  a READ command to the drive,
       it appears  to complete  normally.   In the background,
       however, while  the  system  is  obtaining  status  and
       preparing to  send the  next command,  the  disk  drive
       continues to read data from the disk to the buffer.  If
       the next  command is also a READ, the disk drive checks
       to see  if the  requested data is in the buffer, and if






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       so, the  data is  transferred to  the host immediately,
       without a delay for rotational latency.

       Of course,  not all  READ commands  request  contiguous
       sectors, so it is important that the Read-Ahead feature
       not impair  performance when  it is  not needed.    For
       example, if  the disk  drive is  reading ahead  and the
       next command  received is  anything other  than a  READ
       command (e.g. WRITE, SEEK, etc.), or the new command is
       a READ command for logical blocks which are not already
       in the  buffer, the  Read-Ahead  operation  immediately
       aborts and the new command is executed.

       Additionally, the  host system  can disable  Read-Ahead
       via the  MODE SELECT  command if  it is known that data
       accesses will be largely random rather than sequential.
       This technique  avoids the  overhead (approximately 100
       microseconds) associated  with checking  the buffer  to
       see if requested data is already present.


KEY READ-AHEAD PARAMETERS

       Size of  Read-Ahead Buffer:   The Read-Ahead buffer has
       45 kilobytes.   This is equivalent to approximately one
       and a half tracks.

       When Invoked:   If enabled, Read-Ahead is automatically
       performed after every READ or READ (EXTENDED) command.

       When Aborted:   Read-Ahead  is aborted  for any  of the
       following reasons:

         the buffer has been filled with 45 kilobytes of data
         a new  command is  received  which  is  not  a  READ
          command, or  which requires an access to a different
          area of the disk
         a data error occurs while reading ahead
         a SEEK  is required to continue reading ahead (e.g.,
         a cylinder boundary is encountered) with the RAMD bit
         set to zero in page code 38h


CONTROLLING THE READ-AHEAD FEATURE

       The Read-Ahead  feature is controlled by either setting
       the read cache disable bit (RCD) to zero in the caching
       page (page code 08h) or by setting the cache enable bit
       (CE) to  one in  the read-ahead control page (page code
       38h).  See Table 729 or Table 743.







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READ ALGORITHM

       The sequence  of a READ command varies depending on the
       answers to  the following questions:  (a) is Read-Ahead
       enabled?  (b) is the READ command requesting data which
       has been  prefetched?  and (c) does the transfer length
       exceed the  range of  data which  has been  prefetched,
       thus requiring  a media  access?   See Table  A1, READ
       Algorithm.


                               

                          Table A1
                        READ Algorithm



READ-AHEAD PERFORMANCE

       The performance  benefits of  Read-Ahead vary depending
       on the  operating system and application programs.  The
       greatest benefits  will be  seen in  UNIX-based systems
       which transfer  large blocks  of data  with  few  users
       (e.g., graphics  workstations), and  the least  benefit
       will be seen in systems with many users and short files
       (e.g.,  transaction   processing).    The  greater  the
       randomness of  disk accesses,  the less likely that the
       next  command   will  request   data  that   has   been
       prefetched.



























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APPENDIX B:  DISCONNECT/RECONNECT CONDITIONS

       This appendix  describes the conditions under which the
       disk drive disconnects from, and reconnects to the SCSI
       bus.   These are  the default  options provided  by the
       disk drive,  but may be modified by the initiator using
       the MODE SELECT command.

       The normal  DISCONNECT/RECONNECT options stress maximum
       throughput,  yet   do  not  monopolize  the  SCSI  bus,
       providing maximum bus use.  These options would be used
       in  a   multiple  initiator  environment.    Also,  the
       initiator should  be capable  of sustaining  a SCSI bus
       throughput which is equal to, or greater than, the disk
       drive's internal data transfer rate.

       To disable  disconnects, the  initiator should  not set
       bit six in the ID message that it sends at the start of
       a command.   If  the disk drive is processing a command
       which does  not support  disconnects (bit six in the ID
       message equals  zero), and any other command is active,
       the disk  drive immediately terminates the command that
       cannot disconnect with a BUSY status.  If normal system
       operation includes  multiple commands  sent to the disk
       drive simultaneously,  the initiator  should enable the
       DISCONNECT/RECONNECT option  (bit six in the ID message
       set to one).

       The disconnect/reconnect conditions are as follows:

         When the SCSI bus is anticipated to be idle (no -REQ
          is active)  for the  amount of time specified in the
          MODE SELECT  command bus inactivity limit field, the
          disk drive  automatically disconnects  from the SCSI
          bus.

         When a disk drive receives a command while a command
          from another  initiator is  active, the  disk  drive
          disconnects from  the bus  as  soon  as  the  entire
          command from the initiator has been received.

         READ commands  always disconnect  immediately  after
          receipt of  the command.   This allows access to the
          SCSI bus during the disk latency and/or seek time.

         When executing  a READ  command, if  the disk  drive
          anticipates that  the bus  will be  idle for  longer
          than the  time specified  in  the  MODE  SELECT  bus
          inactivity limit  field, the  disk drive disconnects
          from the  bus as  soon as the bus becomes idle.  The
          disk drive then reconnects when data is available to
          be transferred.   This  condition may  occur if:  1)





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          the read operation crosses a cylinder boundary and a
          seek operation  is required;  2) a seek operation to
          an alternate  track is required; or 3) the SCSI data
          transfer rate  is  greater  than  the  disk  drive's
          internal data  transfer rate and the buffer has been
          emptied.

         When the  drive receives  a WRITE  command, it reads
          data from the initiator until the internal buffer is
          filled.   Once the buffer is filled, or all the data
          has been  transferred, the  disk  drive  disconnects
          from the  bus.   The reconnect  occurs when the disk
          drive starts  to write  data to  the disk,  and  the
          internal buffer  becomes available  for more data to
          be transferred from the initiator.  A reconnect also
          occurs, after  all the  data has been written to the
          disk,  to   return  the  status  and  terminate  the
          command.







































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APPENDIX C:  DEFECT MANAGEMENT

       There are  four different  ways in which defects can be
       managed in the XT-8000S Family disk drives:

         Cylinder-Oriented.   Each cylinder  is  an  integral
          unit, with  the alternates per zone number of spares
          assigned to  the cylinder.  A defect anywhere within
          the cylinder  is reassigned  by invoking  one of the
          spare sectors,  which are  initially located  at the
          end of  the last  track of  the cylinder.    If  the
          number of  reassigned sectors  exceeds the number of
          spare  sectors,   one  track   of  the  cylinder  is
          relocated to  the area  of the disk drive defined by
          the alternate tracks per volume field.

         Track-Oriented.   Each track  is an  integral  unit,
          with one  or more  spares assigned  to the track.  A
          defect anywhere  within the  track is  reassigned by
          invoking  one   of  the  spare  sectors,  which  are
          initially located  at the  end of  the  track,  just
          prior to index.  If the number of reassigned sectors
          exceeds the  number of  spare sectors,  the track is
          relocated to  the area  of the disk drive defined by
          the alternate tracks per volume field.

         Disk  Drive-Oriented.     The  entire  drive  is  an
          integral unit,  with the  alternates per zone number
          of spares  assigned to  the disk  drive.   A  defect
          anywhere  on   the  disk   drive  is  reassigned  by
          relocating the  sector to  one of the spare sectors,
          which are  located in  the area  of the  disk  drive
          defined by the alternate tracks per volume field.

         Host Defect  Management.   The entire  drive  is  an
          integral unit, with no spares.  A defect anywhere on
          the disk  drive is deallocated by the host operating
          system,  and   future  access  to  this  sector  are
          inhibited.   An error occurs if you use the REASSIGN
          BLOCK command in this mode.

       Set  the   MODE  SELECT  values  of  tracks  per  zone,
       alternate sectors  per zone,  and alternate  tracks per
       volume,  as  shown  in  Table  C1,  Defect  Management
       Selections, to choose one of the four techniques.


                               

                          Table C1
                 Defect Management Selections

    XT-8000S Product Specification & OEM Technical Manual


       NOTE: The alternate  sectors per  zone,  and  alternate
       tracks per  volume, field  values  are  recommended  by
       Maxtor for most applications, but may vary per customer
       requirements  of  capacity  versus  performance.    For
       example, if  cylinder-oriented mode  is  selected,  the
       systems integrator  may elect  to increase  capacity by
       reserving fewer  alternate  sectors  per  zone.    This
       increases the  probability that  a track may have to be
       relocated because  of more  defects than  spares  in  a
       given cylinder,  so the  number of alternate tracks per
       volume may need to be increased.  Also, performance may
       be degraded as a result of the additional SEEKs imposed
       by track relocation.


REASSIGNMENT SEQUENCES

       When  a   defective  area   is  found,  a  reassignment
       algorithm is  invoked by  either the  REASSIGN BLOCK or
       FORMAT  UNIT   command,  and  multiple  blocks  may  be
       reassigned in  one  operation.    For  simplicity,  the
       following reassignment  sequences assume a single block
       reassignment.


Cylinder-Oriented Reassignment Sequence

       1. The designated  block is  located,  and  the  entire
          cylinder in  which it  resides is verified to ensure
          that there are no unflagged defects.

       2. The cylinder  is copied  to  the  reserved  cylinder
       (MAX-1).

       3. Flags in  the configuration area (cylinder zero) are
          updated, to  indicate completion  of the first phase
          in case  a power  failure or  RESET  interrupts  the
          process.

       4. The  affected  cylinder  is  reformatted,  with  the
          headers updated  to "slip"  the bad  block  and  all
          following blocks  (up to the end of the cylinder) by
          one.   This causes  some logical blocks to move from
          the last position on one track to the first position
          on the  next track,  but all  blocks are  contiguous
          within the cylinder.

       5. Flags in  the  configuration  area  are  updated  to
          indicate completion of the second phase.

       6. The content  of the reserved cylinder is copied back
       to the original cylinder.





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       7. Flags in  the  configuration  area  are  updated  to
       indicate completion of the operation.

       In the  event that there are insufficient spares in the
       cylinder,  one   of  the  tracks  in  the  cylinder  is
       relocated to  the area  defined by the alternate tracks
       per volume.   These are located in the area ending with
       cylinder MAX-2  (the starting  cylinder depends  on the
       number of  alternate tracks, which is always a multiple
       of the number of heads).  This restores to the cylinder
       those spares  which had been consumed by the defects in
       this track,   and reduces the probability that a future
       reassignment in  the same  cylinder results  in a track
       relocation.

       When a  SEEK is  performed to  an LBA  which  has  been
       reassigned, the following sequence applies:

       1. The drive  SEEKs to  the cylinder and head where the
          sector was  originally located,  and begins a search
          for the header.

       2. Upon reading  the first header encountered, the disk
          drive determines whether the target sector is within
          the  current  track.    Information  in  the  header
          indicates the  number of  the  last  sector  on  the
          current  track,  and  whether  the  track  has  been
          relocated.

       3. If the  target sector  was once  the last  one on  a
          track, and has been rolled over to the next track in
          the cylinder due to a reassignment, this is detected
          from the  "last sector"  information in  the  header
          that was  read,  and  a  head  switch  is  performed
          immediately.

       4. If the  track has  been relocated,  the  disk  drive
          SEEKs to  the alternate  area.  If the data transfer
          is long  enough to go beyond the end of the track, a
          SEEK back  to the  original cylinder  takes place to
          continue the transfer.


Track-Oriented Reassignment Sequence

       1. The designated  block is  located,  and  the  entire
         track in which it resides is verified, to ensure that
         there are no unflagged defects.

       2. The track  is copied  to a  track  in  the  reserved
       cylinder (MAX-1).

    XT-8000S Product Specification & OEM Technical Manual


       3. Flags in  the configuration area (cylinder zero) are
         updated, to indicate completion of the first phase in
         case a power failure or RESET interrupts the process.

       4. The affected  track is reformatted, with the headers
         updated to  "slip" the  bad block  and all  following
         blocks (up  to the  end of  the track)  by one.   All
         blocks are contiguous within the track.

       5. Flags in  the  configuration  area  are  updated  to
         indicate completion of the second phase.

       6. The content  of the  reserved area is copied back to
       the original track.

       7. Flags in  the  configuration  area  are  updated  to
       indicate completion of the operation.

       In the  event that there are insufficient spares in the
       track, the  entire  track  is  relocated  to  the  area
       defined by the alternate tracks per volume field.

       When a  SEEK is  performed to  an LBA  which  has  been
       reassigned, the following sequence applies:

       1. The disk  drive SEEKs to the cylinder and head where
         the sector  was  originally  located,  and  begins  a
         search for the header.

       2. Upon reading the first header encountered, the drive
         determines whether the track has been relocated.

       3. If the  track has  been relocated,  the  disk  drive
         SEEKs to  the alternate  area.   If the data transfer
         length extends beyond the end of the relocated track,
         a SEEK  back to  the original cylinder takes place to
         continue the transfer.


Disk Drive-Oriented Reassignment Sequence

       1. The designated  block is  copied to  a sector in the
       reserved cylinder (MAX-1).

       2. The header of the designated block is written with a
          pattern which indicates that it has been relocated.

       In the  event that there are insufficient spares in the
       reserved  area,   the  disk   drive  returns  an  error
       condition indicating that no spares are available.







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       When a  SEEK is  performed to  an LBA  which  has  been
       reassigned, the following sequence applies:

       1. The drive  SEEKs to  the cylinder and head where the
          sector was  originally located,  and begins a search
          for the header.

       2. Upon reading  the  target  header,  the  disk  drive
          determines that the sector has been relocated.

       3. The disk drive SEEKs to the alternate area to access
          the requested  LBA.   A SEEK  back to  the  original
          cylinder takes  place to  continue the  transfer, if
          necessary.


Host Defect Management Sequence

       1. The defective  block is  allocated to either a dummy
       file or a "bad block" list.

       2. The host  ensures that  no accesses are performed to
       the defective block.


DEFECT MANAGEMENT ASSUMPTIONS AND STATISTICS

       In order  to choose a defect management mode, it may be
       useful  to   examine  some   of  the   assumptions  and
       statistics which affect the media.

       The number of spares allocated must include the sum of:

         the initial  maximum number  of defects specified by
          the manufacturer (that is, primary defects)

         the additional  defects which  accumulate during the
          life of the drive (that is, grown defects)

       The  following   tables,  Table   C2  and  Table  C3,
       illustrate  the   effect  of   spares  and  defects  on
       capacity.


                               

                          Table C2
               Spares Versus Capacity, XT-8380

    XT-8000S Product Specification & OEM Technical Manual


                               

                          Table C3
               Spares Versus Capacity, XT-8760





















































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    XT-8000S Product Specification & OEM Technical Manual


APPENDIX D:  CDB BIT DEFINITIONS

                 Abbreviation Meaning
                 

                 ARRE       automatic    read     reallocation
                 enabled bit
                 AWRE       automatic    write    reallocation
                 enabled bit
                 BytChk     byte check bit
                 DCR        disable correction bit
                 DCRT       disable certification bit
                 DevOfl     device off-line bit
                 DPRY       disable primary bit
                 DT         defective track bit
                 DTE        disable transfer on error bit
                 EEC        enable early correction bit
                 FmtData    format data bit
                 FOV        format options valid bit
                 HSEC       hard sector format bit
                 ILI        incorrect length indicator bit
                 Immed      immediate bit
                 INS        inhibit save bit
                 PCF        page control field bit
                 PER        post error bit
                 PF         page format bit
                 PMI        partial medium indicator bit
                 PS         parameters saveable bit
                 RC         read continuous bit
                 RelAdr     relative address bit
                 RMB        removable media bit
                 SlfTst     self-test bit
                 SP         save parameters bit
                 SS         spare sector bit
                 SSEC       soft sector format bit
                 STPF       stop format bit
                 SURF       surface bit
                 TB         transfer block bit
                 3rdPty     third party reservation bit
                 UntOfl     unit off-line bit
                 VU         vendor unique bit
                 WP         write protect bit

    XT-8000S Product Specification & OEM Technical Manual
    XT-8000S Product Specification & OEM Technical Manual

       

    XT-8000S Product Specification & OEM Technical Manual


APPENDIX E:  UNITS OF MEASURE

          Abbreviation   Meaning
          

          A/m  amps per meter
          AWG  American wire gauge
          bpi  bits per inch
          dBa  decibel, A-weighted
          fci  flux changes per inch
          g    gram
          Gbyte     gigabyte
          Hz   hertz
          mA   milliamp
          A   microamp
          Mbit megabit
          Mbyte     megabyte
          m   micrometer
          msec millisecond
          sec microsecond
          nsec nanosecond
          Oe   oersted
          RH   relative humidity
          rpm  revolutions per minute
          tpi  tracks per inch
          xxb  binary values
          xxh  hexadecimal values

       
APPENDIX F:  BUFFER RATIO APPLICATIONS

WHAT ARE THE BUFFER RATIOS?

       The Buffer Ratios indicate to the target how full, on a
       READ command,  or how  empty, on  a WRITE  command, the
       buffer should be prior to attempting reselection.

       SCSI MODE  SELECT parameter  page 2  defines the Buffer
       Ratio  parameters   as  numerators   of  a   fractional
       multiplier that  has 256  (FFh) as  its denominator.  A
       value of  128 (80h) would equate to a ratio of 50%, and
       192 (C0h)  equates to a ratio of 75%, etc.  In standard
       SCSI, the  percentage is  applied to  the total size of
       the data  buffer.   In Maxtor's  implementation of  the
       Buffer Ratios,  the percentage is applied to either the
       data buffer  or the  data transfer,  whichever is less.
       In other  words, if  the data transfer is less than the
       disk buffer,  the Buffer  Ratio will  be applied to the
       transfer length.   If the data transfer is greater than
       the disk  buffer, the  Buffer Ratio  will be applied to
       the disk  buffer.  This implementation allows one value
       be used  for the  Buffer Ratios, regardless of the data
       transfer length.


WHAT ARE THE PERFORMANCE BENEFITS FROM BUFFER RATIOS?

       The performance  benefit from the Buffer Ratios is in a
       multi-target environment  where SCSI  "on-bus" time  is
       critical.   The Buffer  Ratios ensure  that the time on
       the SCSI  bus is  minimized and  the data transfer rate
       across the SCSI bus is maximized.


BUFFER FULL RATIO

       Without  the   Buffer  Full   Ratio  the  target  would
       reconnect to  the SCSI  bus as  soon as  data is in the
       buffer.   If the  SCSI bus transfer rate is higher then
       the disk  transfer rate,  the  transfer rate across the
       SCSI bus  will slow  to that  of the  disk.   This will
       increase the  total time  on the  bus for  a given data
       transfer.   Figure F1.  shows this condition where the
       "Hold-off Time"  is minimal and the "Total Time on Bus"
       is significant.   Note  that if  the SCSI  bus transfer
       rate is  lower than  the disk transfer rate, the Buffer
       Full Ratio  should  be  zero  (disabled).    This  will
       optimize performance  in  slower  systems  and  is  the
       default value for the Buffer Ratios.
                               
                          Figure F1
         Data Transfer Rate Without Buffer Full Ratio


       With Buffer  Full Ratio,  the target  will  "Hold  off"
       reconnecting to the bus until the desired percentage of
       data is  in the  buffer to ensure that the maximum SCSI
       bus rate  can be achieved.  This maximizes the SCSI bus
       rate and  minimizes total  "On-Bus" time.   Figure F2.
       shows that  by increasing  the "Hold-off Time" the SCSI
       bus transfer  rate is  maximized and the "Total Time on
       Bus" is  minimized,  with  the  "Total  Time  to  Data"
       remaining the  same.    Minimizing  the total  "On bus"
       time is  essential for  performance in  a  multi-target
       environment.


                               

                          Figure F2
          Data Transfer Rate With Buffer Full Ratio



BUFFER EMPTY RATIO

       The Buffer  Empty Ratio  applies to  how empty of Write
       Data the buffer should be before reconnecting to obtain
       more Write  Data.   Maxtor's SCSI  implementation "pre-
       reads" Write  Data into  the data buffer directly after
       the WRITE  command is decoded.  Because the size of the
       data buffer is 45,056 bytes the Buffer Empty Ratio will
       only apply  to WRITE  commands with data transfers over
       45,056 bytes  per command and a disconnect/reconnect is
       required for  the remaining  data.   If WRITE  commands
       have long  data transfers,  it is  recommended that the
       Buffer Full Ratio and the Buffer Empty Ratio be Equal.


WHAT VALUE SHOULD BE USED FOR THE BUFFER RATIOS?

       The value  chosen  for  the  Buffer  Ratios  should  be
       selected to  optimize the  difference between  the disk
       speed and  the SCSI bus speed.  In Figures F1 and F2,
       the difference  in speed between the disk data rate and
       the SCSI  bus are  reflected by a difference in slopes,
       with a steep slope representing a higher speed.

       Table F1  below shows  the calculated  values for  the
       Buffer Ratios with the given SCSI bus speed.
                               
                          Table F1
                     Buffer Ratio Values

       
GLOSSARY

       ack.  Acknowledge

       ADR.  Address

       ANSC.  American National Standards Committee

       ANSI.  American National Standards Institute

       arbitration winner.  The arbitrating SCSI device which has
          the highest SCSI address.

       assert.  A signal driven to the true state.

       async.  Asynchronous

       BCV.  Buffer control valid

       bit.  Binary digit

       byte.  Eight consecutive binary digits

       C/C.  Continuous/composite (format)

       CCS.  Common Command Set

       C/D.  CONTROL/DATA signal

       CDB.   Command descriptor  block, the  structure  used  to
          communicate requests from an initiator to a disk drive.

       cmd.  Command

       connect.   The function  that  occurs  when  an  initiator
          selects a target to start an operation.

       CRC.  Cyclic redundancy check

       CSA.  Canadian Standards Association

       DB (7-0,  P).   Eight data-bit  signals, plus a parity-bit
          signal, that form a data bus.

       DC.  Direct current

       DCR.  Disabled correction (signal)

       disconnect.   The  function  that  occurs  when  a  target
          releases control  of the  SCSI bus allowing it to go to
          the BUS FREE phase.

       DMA.  Direct memory access
       EBP.  Erase bypass

       ECC.  Error correction code

       ECL.  Emitter-coupled logic

       EDAC.  Error detection and correction

       EIA.  Electrical Industry Association

       ENDEC.  Encoder/decoder

       EPROM.  Erasable programmable read only memory

       ERA.  Erase all

       FCC.  Federal Communication Commission

       FIFO.  First-in, first-out storage and retrieval technique

       firmware.   Computer programs  encoded permanently  into a
          ROM

       FW.  Firmware

       G.  Constant of gravitation

       gnd.  Ground

       hard  error.     An   error  due   to  faulty   equipment,
          transmission techniques, recording media, etc.

       HDA.  Head disk assembly

       hex.  Hexadecimal

       HW.  Hardware

       initiator.   A SCSI  device, usually  a host  system, that
          requests that an operation be performed by another SCSI
          device.

       INTERMEDIATE status.   A status code sent from a target to
          an initiator  upon completion  of each command in a set
          of linked  commands, except for the last command in the
          set.

       I/O.  Input and/or output

       ISG.  Inter-sector gap

       ISO.  International Standardization Organization

       LBA.  Logical block address
       LED.  Light-emitting diode

       logical thread.   The logical path which exists between an
          initiator's memory  and a  bus device  LUN, even though
          the physical path may be disconnected.

       logical unit.   A  physical or  virtual device addressable
          through a target.

       LSB.  Least significant byte

       LSTTL.  Low power, Schotky transistor-transistor logic

       LUN.  Logical unit number, an encoded 3-bit identifier for
          the logical unit.

       C.  Microcomputer

       computer.  Microcomputer

       MFM.  Modified frequency modulation (encoding)

       MO.  Magneto optics

       MSB.  Most significant byte

       MSG.  Message

       MTBF.  Mean time between failures

       MTTR.  Mean time to repair

       MZCAV.  Maxtor Zoned Constant Angular Velocity (format)

       N.C.  No connection

       negate.  A signal driven to the false state

       nom.  Nominal

       OEM.  Original equipment manufacturer

       one.  True signal value

       parity.   A method  of checking  the  accuracy  of  binary
          numbers

       PC.  Polycarbonate

       PCB.  Printed circuit board

       peripheral device.  A peripheral that can be attached to a
          SCSI device
       PLL.  Phase-locked loop

       PLO.  Phase-locked oscillator

       PM.  Preventive maintenance

       P/N.  Part number

       POH.  Power On hours

       P-P.  Peak to peak

       PROM.  Programmable read only memory

       PTRN.  Pattern

       RAM.  Random-access memory

       reconnect.  The function that occurs when a target selects
          an initiator to continue an operation after disconnect.

       req.  Request

       reserved.   Bits, bytes,  fields and  code values that are
          set aside for future standardization.

       RLL.  Run-length limited

       ROM.  Read-only memory

       rsrv.  Reserved

       R/W.  Read and/or write

       SCSI.  Small Computer Systems Interface

       SCSI address.   The  octal representation  of  the  unique
          address (0 - 7) assigned to a SCSI device.

       SCSI ID.   The  bit-significant representation of the SCSI
          address, referring  to one  of the signal lines DB (7 -
          0).

       status.   One byte of information sent from a target to an
          initiator upon completion of each command.

       STD.  Standard

       SW.  Software

       sync.  Synchronization, synchronous
       target.     A  SCSI  device  that  performs  an  operation
          requested by  an initiator,  in this  manual, usually a
          disk drive.

       tbd.   To be  determined.  Values which are not defined as
          of the date this manual is published.

       TLA.  Top level assembly

       TTL.  Transistor-transistor logic

       typ.  Typical

       UL.  Underwriter's Laboratories, Inc.

       UNC.  Unified National Coarse

       UNF.  Unified National Fine

       VDE.  Verband Deutscher Electrotechniker

       vendor unique.   The bits, fields, or code values that are
          vendor specific.

       WORM.  Write once read multiple

       XFER.  Transfer

       zero.  False signal code


Rev B 									Anderson/Sandoval
10\16\93