QAS question

Gerry.Houlder at Gerry.Houlder at
Tue Feb 27 12:49:57 PST 2001

* From the T10 Reflector (t10 at, posted by:
* Gerry.Houlder at

I will expand a bit on Bruce's response.

The initiator should raise ATN and send MESSAGE PARITY ERROR message in the
following Message out phase. This is the normal message parity error
response. Now for the hard part -- what will the target do with this?

The target will normally resend the previous message again. There will be a
vendor unique number of retries before the target gives up retrying the
message and eventually goes unexpected bus free.

Let us suppose the first resend of the 0x55 message is received by the
initiator without any parity error. ATN stays inactive. The target will
probably go to QAS. Will anyone else recognize this as a valid QAS phase?
It no longer satisifies the requirement of a single 0x55 message
immediately preceeded by a packet, so it probably should not be recognized
as a valid QAS. Thus no one but the target and maybe the initiator involved
with the last command will participate in the QAS cycle. If neither of
these has a command, the bus should change to BUS FREE.

Perhaps we would be better off to create a new message parity error rule
for 0x55 message. If a target sends 0x55 and a MESSASGE PARITY ERROR
message response is received, the target should change to COMMAND COMPLETE
message in the resend. If this is successful a normal bus free will occur
instead of QAS.

Another legal response the target can make is go to unexpected bus free
when a MESSAGE PARITY ERROR message is returned. (A vendor unique number of
retries can mean 0 retries.) Mandating this response could also be a simple
way to break the QAS cycle in a way that will not be misinterpreted by any
other devices on the bus.

Should we be thinking about choosing a single method for handling parity
error in the case of 0x55 message? This would surely simplify some of the
problems that can result in different actions for the several "allowed but
different" actions that exist today.

George_Penokie at on 02/27/2001 01:17:41 PM

Sent by:  owner-t10 at

To:   t10 at

Subject:  QAS question

* From the T10 Reflector (t10 at, posted by:
* George_Penokie at
OK all you QAS experts, here is a question I have received that I don't
have a good answer for.

What happens if the initiator on the I_T nexus detects a parity error on a
QAS message? And, to make things even more interesting, the device that
wants the bus does not see a parity error on the same QAS message?

Bye for now,
George Penokie

Dept 2C6  114-2 N212
E-Mail:    gpenokie at
Internal:  553-5208
External: 507-253-5208   FAX: 507-253-2880

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