                                                             BSR X3.131-198_
NOTE:  Revision 17B consists of changes made by the          X3T9/84-40 REV 1B
X3T9.2 task group at their December 10, 1985 meeting.        X3T9.2/82-2
These changes were made in order to make the X3T9.2          REV 17B
draft proposed standard consistent with the ISO/TC97/SC13 
draft proposal.  See page 1.1 for a changed page list.







                                draft proposed
                          American National Standard
                          for information systems -


                    SMALL COMPUTER SYSTEM INTERFACE (SCSI)


                              December 16, 1985







Secretariat

Computer and Business Equipment Manufacturers Association


Abstract:  This standard defines mechanical, electrical, and functional 
requirements for attaching small computers with each other and with low- to 
medium-performance intelligent peripherals such as rigid disks, flexible 
disks, magnetic tape devices, printers, and optical disks.  The resulting 
interface facilitates the interconnection of small computers and intelligent 
peripherals and thus provides a common interface specification for both 
systems integrators and suppliers of intelligent peripherals.






POINTS OF CONTACT:

William E. Burr (X3T9.2 Chairman)      John B. Lohmeyer (X3T9.2 Vice Chairman)
U.S. Department of Commerce            NCR Corporation
National Bureau of Standards           3718 N. Rock Road
Technology A-216                       Wichita, KS 67226
Gaithersburg, MD 20899                 (316) 688-8703 
(301) 921-3723
                              CHANGED PAGE LIST

At their December 10, 1985 meeting, the X3T9.2 task group made a number of 
changes to Revision 17 of this document.  (Revision 17A was distributed at the 
meeting proposing some of the changes; the remainder of the changes were made 
at the meeting.)  The primary reason for making these changes is to keep this 
document consistent with the ISO/TC97/SC13 SCSI draft proposal.  The pages 
that have changed since Revision 17 are listed below:

   Page                                  Change
  -------  -------------------------------------------------------------------

     1     Changed revision number and date.
   1.1     Replaced changed page list.
     2     Editorial revisions to the foreword.
  7-7.1    Revised the Table of Contents to reflect the document changes.
     9     Added description of the contents of appendixes D - G.
    12     Moved section 4.3.2, Shielded Connectors into Appendix D.  Merged 
           section 4.3.1 into section 4.3.
 12.1-12.2 \
   17-19    \  Deleted pages 
 19.1-19.4  /  (Information moved to Appendix D).
 21.1-21.2 /
   20-21   Deleted second line of Table titles.
    24     Redrew figures 4-5 through 4-7.
    26     Redrew figure 4-9.
    37     Clarified that multiple messages may be sent during a message phase.
    38     Clarification to the MESSAGE OUT phase error handling.
    41     Redrew figures 5-1 and 5-2.
    43     Editorial clarification.
    49     Deleted "immediately".
  50-50.1  Clarified that unit attention condition is on a per logical unit 
           basis.  Also clarified "other" command is other than REQUEST SENSE.
   65-66   Added ISO version field to the INQUIRY data.
   93.1    Revised Table 8-14.1 to reference the latest X3B5 document number 
           and to refer to Appendix F for additional standards.
   94-99   Editorial clarifications to the RESERVE and RELEASE commands.
   113     Clarified which status codes are to be returned on SEARCH DATA 
           commands.
   119     Clarified that READ BLOCK LIMITS returns the target's capability as 
           opposed to its current setting.
   132     Added code values 0BH through 0DH to Table 9-14.1.
 132.2-133 Editorial clarifications to the RESERVE UNIT and RELEASE UNIT 
           commands.
  147-149  Editorial clarifications to the RESERVE UNIT and RELEASE UNIT 
           commands.
 174-176.2 Revised Appendix A to make the figure legible.
  182-183  Redrew figures C1 and C2.
  184-192  Added Appendix D, Recommended Shielded Connectors.  Most of this 
           information was previously in section 4.3.2.
  193-194  Added Appendix E, Conformance.
  195-196  Added Appendix F, Additional Medium Type and Density Code Standards.
   197     Appendix G was previously Appendix D.  Editorial clarification.



                                   FOREWORD
    (This Foreword is not part of American National Standard X3.131-198_.)

The development of comparatively inexpensive VLSI device controllers have 
recently changed the economics of interfaces for small system storage devices. 
Where expensive controller logic was once shared among as many devices as 
possible, in many cases it now makes economic sense to build a controller in 
each device.  This is particularly true for high-performance storage devices, 
where the intimate interactions of the recording medium, the recording 
mechanism, and the recording code cause intersymbol interference and error 
recovery problems that are highly specific to the chosen technologies, and are 
best resolved within the device itself.  

Moreover, the number of types of storage devices for small computers, and the 
industry that builds them, have grown dramatically in the past few years.  In 
particular, the emergence of physically small, but comparatively high-capacity 
and high-performance fixed-medium magnetic disk devices (virtually non-
existent in the late 1970s, but a multibillion dollar business in the mid-
1980s) has driven the development of small computer systems and caused a need 
for other classes of devices, such as streaming cartridge tape drives, for 
backup and data interchange.

Because device interfaces are very specific to device types, many device level 
interface standards would be required to service all small computer device 
types.  Because backplane buses reside at the center of computers, and have 
dramatic performance effects, many different ones are needed for different 
system requirements.  To connect every backplane bus to every device interface 
through a controller would require an almost unbounded number of specific 
controller products.  

In addition, in many systems today, it is not the computer which is "central", 
it is the storage facility.  That is, one or two large capacity storage 
subsystems serve several computers.  An interface adapted to this reality was 
needed.  

By 1982, all the needs given above were widely recognized in the industry and 
by the members of X3T9 and its Task Groups.  A commercial small system 
parallel bus, the Shugart Associates System Interface (SASI), generally met 
the small system requirements for a device-independent peripheral or system 
bus and had enjoyed significant market success.  It was offered to X3T9.2 as 
the basis for a standard.  X3T9.2 chose the name Small Computer System 
Interface (SCSI) for that standard and began work at its April 1982 meeting.  
The present SCSI dpANS is a formalization and extension of the SASI.  Many 
existing SASI devices are SCSI compatible.

Since April 1982, X3T9.2 has held plenary sessions, at two month intervals, 
plus numerous informal working meetings.  The original SASI has been extended 
in a number of ways, including:

  (1)  A differential electrical option has been added to allow use of longer 
cables (up to 25 meters) in environments where common mode noise is a concern.

  (2)  A synchronous transfer option has been developed allowing maximum 
transfer rates in the 3 to 4 megabyte per second range.

  (3)  An optional "extended" command set has been added, allowing for very 
large capacity storage devices (a block address space of 232 blocks versus 221 
for SASI), and Inquiry commands that allow self-configuring driver software. 

  (4)  Command sets for magnetic tape (both start/stop and streaming), 
printers, processors, optical disks, and read-only optical disks have been 
added to the proposed SCSI standard, in addition to those for magnetic disks.  

Although it might have been premature in April 1982 to claim that SASI was 
then a de facto standard, this surely is the case for SASI/SCSI today.  SCSI 
compatible host adapters, controllers, and peripheral devices are now widely 
manufactured around the world.  Host adapters are available for most small 
computers with accessible backplane buses.  SCSI controllers are widely 
available for all the de jure and de facto standard magnetic disk and magnetic 
tape device interfaces.  Small high-capacity fixed-medium magnetic disks, 
rigid removable-medium magnetic disks, high-capacity Bernoulli-effect flexible 
disks, and other closed-loop high-capacity flexible disk products, as well as 
optical disk products, are all available with integral SCSI controllers.  SCSI 
subsystems that integrate both a rigid-disk and a streaming-tape drive into a 
single package are also available.  SCSI interface chips are available, and 
some disk controller chip sets also provide SCSI support.

Since a large number of companies have implemented and tested the SCSI during 
the development of the proposed standard, no separate test program has been 
deemed necessary.  At the February 1984 meeting of X3T9.2, representatives of 
the following companies stated, for the record, that their companies had 
implemented and tested SCSI:  Adaptec, Adaptive Data, Data Technology Corp., 
Fujitsu America, Inc., NCR Corp., and Shugart Corp.  This is not a complete 
list of companies implementing products using SCSI nor is it a promise by 
these companies to offer SCSI products.  

The SCSI fills an urgent need, provides for the future, and is consonant with 
actual commercial practice.  Most important, the timing is right.  SCSI 
catches the floodtide of new, high-performance storage devices for small 
systems, and promises to bring a measure of needed order to what would 
otherwise be a chaotic and fragmented market.  

This standard specifies the mechanical, electrical, and functional 
requirements for a small computer input/output bus interface, and command sets 
for peripheral device types, particularly storage devices, commonly used with 
small computers.

Suggestions for improvement of this standard will be welcome.  They should be 
sent to the Computer and Business Equipment Manufacturers Association, 311 
First Street, NW, Suite 500, Washington, DC 20001.

This standard was processed and approved for submittal to ANSI by American 
National Standards Committee on Computers and Information Processing, X3.  
Committee approval of the standard does not imply that all committee members 
voted for its approval.







At the time it approved this standard, the X3 Committee had the following 
members:

TO BE DETERMINED




















































Subcommittee X3T9 on I/O interfaces, which reviewed this standard, had the 
following members:

Delbert L. Shoemaker (Chairman)
Ron Tranquilli (Vice Chairman)

Bob Bender                         G. Atterbury (Alt)
John Blagaila                      Charles Brill (Alt) 
Fred Ciechowski                    William E. Burr (Alt) 
George Clark                       Roger Cormier (Alt) 
Stephen W. Cooper                  Hank Dorris (Alt) 
Louis C. Domshy                    Thomas A. Fiers (Alt) 
Robert Dugan                       Henry Ginter (Alt)
Ross H. Jaibaji                    William J. McClain (Alt)
Patrick Lannan                     William Mosenthal, Jr. (Alt)
Gene Milligan                      Kirk Moulton (Alt)
Tom Morrow
Mike Newton
Gary S. Robinson
Arnold John Roccati
Floyd E. Ross
Holly S. White


(Note: The name lists on this page and the following page are incomplete and 
they will be updated upon final approval of this standard.)






























Task Group X3T9.2 on Lower-Level Interfaces, which developed this standard, 
had the following members:

William E. Burr (Chairman)
John B. Lohmeyer (Vice Chairman)

Ezra R. Alcudia                    Keith Amundsen (Alt) 
J. L. Amstutz                      Karen Anneberg (Alt) 
Bob Bender                         Charles Brill (Alt)
John Blagaila                      Larry Boucher (Alt)
Tom Briggs                         Paul Clement (Alt) 
David T. Cornaby                   George E. Clark (Alt)
David F. Craft, Jr.                Steve Cooper (Alt) 
Jay Cunningham                     Gary Crowell (Alt) 
Willard S. Davidson                Jon Ericson (Alt) 
Terry Dawson                       Tom Fiers (Alt)
Phil Devin                         Stephen Fitzgerald (Alt) 
Louis C. Domshy                    Marty Francis (Alt) 
Norm Dornseif                      William Homans (Alt) 
Alan Ebright                       J. V. Howell (Alt) 
Anita  Freeman                     Skip Kilsdonk (Alt)
Abe Gindi                          Jim Korpi (Alt)
William A. Horton                  Lawrence J. Lamers (Alt) 
Frank Krulc                        Keith Mueller (Alt)
Patrick E. Lannan                  Don Nanneman (Alt) 
Daniel Loski                       Doug Nolff (Alt) 
William C. Mavity                  Richard Reiser (Alt) 
Gene Milligan                      William H. Roberts (Alt) 
Bob Mortensen                      Floyd E. Ross (Alt)
Gary S. Robinson                   D. Michael Robinson (Alt)
Don Rodgers                        Jay Seashore (Alt) 
Arnold J. Roccati                  Chuck Spatafore (Alt)
Jack Schiffhauer                   Jeff Stai (Alt)
Ralph H. Schultz                   Paul Stavish (Alt) 
Moshe Segal
Delbert L. Shoemaker 
Tim Slaton 
Robert N. Snively
Adrienne Turenne 
Norm Zimmerman 
















                      T A B L E   O F   C O N T E N T S 

  1. Scope...................................................................8

  2. Referenced Standard.....................................................9

  3. Glossary and Conventions................................................9
     3.1  Glossary...........................................................9
     3.2  Editorial Conventions.............................................11

  4. Physical Characteristics...............................................11
     4.1  Physical Description..............................................11
     4.2  Cable Requirements................................................11
     4.3  Connector Requirements............................................12
     4.4  Electrical Description............................................22
     4.5  SCSI Bus..........................................................25
     4.6  SCSI Bus Signals..................................................27
     4.7  SCSI Bus Timing...................................................30

  5. Logical Characteristics................................................31
     5.1  SCSI Bus Phases...................................................31
     5.2  SCSI Bus Conditions...............................................38
     5.3  SCSI Bus Phase Sequences..........................................40
     5.4  SCSI Pointers.....................................................42
     5.5  Message System Specification......................................42

  6. SCSI Commands .........................................................49
     6.1  Command Implementation Requirements...............................50
     6.2  Command Descriptor Block........................................50.1
     6.3  Command Examples..................................................55

  7. Command Descriptions for All Device Types..............................57
     7.1  Group 0 Commands for All Device Types.............................57
     7.2  Group 1 Commands for All Device Types.............................76
     7.3  Group 2 Commands for All Device Types.............................79
     7.4  Group 3 Commands for All Device Types.............................79
     7.5  Group 4 Commands for All Device Types.............................79
     7.6  Group 5 Commands for All Device Types.............................79
     7.7  Group 6 Commands for All Device Types.............................79
     7.8  Group 7 Commands for All Device Types.............................79

  8. Command Descriptions for Direct-Access Devices.........................80
     8.1  Group 0 Commands for Direct-Access Devices........................80
     8.2  Group 1 Commands for Direct-Access Devices.......................104

  9. Group 0 Command Descriptions for Sequential-Access Devices............117

 10. Group 0 Command Descriptions for Printer Devices......................140

 11. Group 0 Command Descriptions for Processor Devices....................152

 12. Command Descriptions for Write-Once Read-Multiple Devices.............155
     12.1  Group 0 Commands for Write-Once Read-Multiple Devices...........155
     12.2  Group 1 Commands for Write-Once Read-Multiple Devices...........164


 13. Command Descriptions for Read-Only Direct-Access Devices..............170
     13.1  Group 0 Commands for Read-Only Direct-Access Devices............170
     13.2  Group 1 Commands for Read-Only Direct-Access Devices............171

 14. Status ...............................................................172


                        L I S T   O F   F I G U R E S 

 4-1  Nonshielded SCSI Device Connector.....................................13
 4-2  Nonshielded Cable Connector...........................................15
 4-5  Termination for Single-Ended Devices..................................24
 4-6  Termination for Differential Devices..................................24
 4-7  Differential Driver Protection Circuit (Optional).....................24
 4-8  SCSI ID Bits..........................................................25
 4-9  Sample SCSI Configurations............................................26

 5-1  Phase Sequences without Arbitration...................................41
 5-2  Phase Sequences with Arbitration......................................41
 5-3  Simplified SCSI System................................................42


Appendixes

 Appendix A................................................................174

 SCSI Signal Sequence Example

   Appendix A Figures
     A1:  SCSI Timing Chart ...............................................175

 Appendix B................................................................177

 Typical Bus Phase Sequence

 Appendix C................................................................179

 SCSI System Operation

     C1.  Host Memory / Host Adapter / SCSI Controller Relationship........179
     C2.  SCSI READ Command Example........................................180
     C3.  I/O Channel Concept..............................................181

   Appendix C Figures
     C1:  Snapshot Prior to Initial Selection..............................182
     C2:  Snapshot Prior to Data Transfer..................................183

 Appendix D................................................................184

 Recommended Shielded Connectors

     D1.  Shielded Connector, Alternative 1................................184
     D2.  Shielded Connector, Alternative 2................................184
     D3.  EUROCARD Boxes...................................................184


   Appendix D Figures
     D1:  Female Shielded SCSI Cable Connector, Alternative 1..............185
     D2:  Male Shielded SCSI Device Connector, Alternative 1...............187
     D3:  Shielded SCSI Device Connector, Alternative 2....................189
     D4:  Shielded SCSI Cable Connector, Alternative 2.....................190

 Appendix E................................................................193

 Conformance

     E1.  Alternatives.....................................................193
     E2.  Levels of Conformance............................................193
     E3.  Options..........................................................194
     E4.  Statement of Conformance.........................................194

 Appendix F................................................................195

 Additional Medium Type and Density Code Standards

 Appendix G................................................................197

 Future Standardization    
































































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