document  X3T9.2/90-003 R2
  TO: X3T9.2 Committee (SCSI)

FROM: Gerry Houlder (Seagate)

SUBJECT: SCSI-1 and CCS Implementation on SCSI-2 Target

While trying to decide what SCSI-2 features had to be deleted 
from a SCSI-2 target when its "operating definition" is changed 
to SCSI-1, I realized that the SCSI-1 wording "A reserved bit, 
byte, or field shall be set to zero or in accordance with a 
future extension to this standard" allows any SCSI-2 feature to 
legally be present in a SCSI-1 implementation.  We need to choose 
"de facto" definitions of CCS and SCSI-1.

The following table attempts to list the features that may cause 
compatibility problems and proposes which operating definitions 
(SCSI-1, CCS, SCSI-2) the feature is allowed (Y) or not allowed 
(N) to be implemented in.  If such a list can be agreed upon by 
the committee, perhaps it can be included in the SCSI-2 standard 
in an appendix or an implementors note in the CHANGE DEFINITION 
command.

Rev. 1: The table has been reduced to only those features which 
have to change to avoid compatibility problems, per discussions 
at Jan. 9-10 Working Group Meeting.  

             FEATURE                         SCSI1  CCS  SCSI2

   Single Bit Selection                        Y     Y     Y#
   Selection without ATN                       Y     Y     Y#

   Target initiated SDTR, WDTR messages        N     N     Y

   Asynchronous Event Notification             N     N     Y
   EXTENDED CONTINGENT ALLEGIANCE Condition    N     N     Y

   INQUIRY Command
      - EVPD bit (VITAL PRODUCT DATA)          N     N     Y
      - VPD PAGE CODE                          N     N     Y
      - Feature supported bits:                N     N     Y
        AENC, TRMIOP, RELADR, WBUS32, WBUS16,
        SYNC, LINKED, CMDQUE, SFTRES
      - ANSI VERSION Field                     1     1     2
      - RESPONSE DATA FORMAT Field             0     1     2

   REQUEST SENSE Command
    - Return 4 bytes if LENGTH = 0             Y     Y     N
    - Return 0 bytes if LENGTH = 0             N     N     Y

Y# means that SCSI-2 doesn't say what the target should do but 
says initiators shouldn't use these selection modes.  Since 
target response isn't defined in SCSI-2, I believe the target 
must act as defined in SCSI-1.  This is better than being 
"creative" and defining some new response.