X3T9.2/91-107





July 16, 1991

To:          X3T9.2 Committee
From:        John Lohmeyer, NCR Principal Member of X3T9.2
Subject:     Error in Implementors Note 1) in section 6.9 of SCSI-2 Rev 10e

There is a conflict between the last sentence in Implementors Note 1) in 6.9
and the fourth paragraph after the Implementors Note.  The latter paragraph
permits two possible implementations of the REQUEST SENSE command with a
pending unit attention condition:

"If a REQUEST SENSE command is received from an initiator with a pending unit
attention condition (before the target generates the contingent allegiance
condition), then the target may either:
       1)    report any pending sense data and preserve the unit attention
             condition on the logical unit, or,
       2)    report the unit attention condition, may discard any pending sense
             data, and clear the unit attention condition on the logical unit
             for that initiator."

Whereas the last sentence in the Implementors Note appears to be valid only
if the second option above is implemented:

       "IMPLEMENTORS NOTES:
        1)   Targets may queue unit attention conditions on logical units. 
             After the first unit attention condition is cleared, another unit
             attention condition may exist (e.g., a power on condition followed
             by a microcode change condition).  The initiator can clear all
             pending unit attention conditions by repeatedly sending the
             REQUEST SENSE command until a sense key other than UNIT ATTENTION
             is returned by the target."

A related Implementors Note is in the REQUEST SENSE command (7.2.14) points
out the need for initiators to accommodate either implementation:

       "IMPLEMENTORS NOTE: Some target implementations do not update sense data
       except on commands that return CHECK CONDITION or COMMAND TERMINATED
       status.  Thus when polling for a logical unit to become ready, the
       initiator should issue TEST UNIT READY commands until GOOD status is
       returned.  If desired, the initiator may issue REQUEST SENSE commands
       after the TEST UNIT READY commands that return CHECK CONDITION or
       COMMAND TERMINATED status to obtain the sense data."

I propose that the conflict be resolved by either:

       1)    Delete the entire third sentence in Implementors Note 1) in 6.9. 
                  OR
       2)    Replace the third sentence with: "The initiator can clear pending
             unit attention conditions by repeatedly issuing TEST UNIT READY
             commands followed by REQUEST SENSE commands until a sense key
             other than UNIT ATTENTION is returned by the target."

I prefer the first solution, but either solution resolves the conflict.