ACA Processing

Denis Brockus denis at
Tue Mar 21 16:31:19 PST 1995

Bill Galloway writes
> I have a question about ACA.  If a SCSI-3 target detects a parity
> error during the command phase and returns check condition status,
> how does the target know what ACA processing the initiator requested?
> The target cannot rely on the NACA bit in the control field if the parity
> error was on that byte or on the opcode byte.
> ACA only works if both initiator and target know that ACA is active.

If a parity error is detected during command phase you can
    1) Retry the phase by explicitly or implicitly performing a
       RESTORE POINTERS and going back to command phase.
    2) Create an unexpected bus free condition.

I personally would skip the retries and go for the unexpected bus free.
Optionally, sense data may be prepared. A contingent allegiance does not
go into affect. If the initiator issues a REQUEST SENSE as the next
request, following the unexpected bus free, the sense data will be
available. Otherwise it is lost.


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