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SCSI Description
Maxtor Atlas 10K III 5-7
Auto Contingent Allegiance condition (ACA) is the SCSI-3 term for the condition of
the drive immediately after it returns a CHECK CONDITION status code. While it
is in this condition, the drive:
• Keeps the status information (sense data) that characterizes the nature of,
and the reason for, the command's failure.
• Suspends execution of all commands that were queued behind the
command that failed (includes commands from all initiators, tagged or
untagged).
• Discards and returns BUSY status for both tagged and untagged drive
access commands from other initiators.
The Auto Contingent Allegiance condition remains in effect until cleared by one of
the following:
• TARGET RESET message received from any initiator
• ABORT TASK SET message received from the allegianced initiator
• Subsequent command received from the allegianced initiator
After receipt of an ABORT TASK SET message from the “allegianced” initiator, the
drive clears any queued command sourced by that initiator and processes the
remaining queued commands.
For Parallel SCSI, as its first action following receipt of the CHECK CONDITION
status code, the initiator sends a REQUEST SENSE command to retrieve the sense
data. Any other command from the initiator causes the drive to set the sense key to
NO SENSE and the additional sense code to NO ADDITIONAL SENSE
INFORMATION. These actions happen prior to resuming execution of queued
commands, or, if there are no commands queued, before processing a new command.
Also for Parallel SCSI, after receipt of the REQUEST SENSE command or any other
command from the “allegianced” initiator, the drive resumes processing of the queued
commands in the normal manner if the QErr bit in the MODE SELECT Control
Mode page is set to 0, its default value. If the value of QErr is 1, the drive aborts all
queued commands and generates a Unit Attention condition for initiators with
commands in the queue. A Unit Attention condition is not generated for the
“allegianced” initiator if commands belonging to it are flushed from the queue.