CLARIFICATIONS
GFRAZIER at ausvm6.vnet.ibm.com
GFRAZIER at ausvm6.vnet.ibm.com
Tue Aug 16 13:58:02 PDT 1994
Just a couple more clarifications to recent e-mail:
1) Johnathan Vail wrote
If an INITIATOR SELECTs with ATN the next phase should be MESSAGE OUT.
Anything else and the transaction is broken by a phase mismatch.
Wouldn't this detect a broken ATN line?
-No, I don't think so:
If an initiator selects with ATN, the target MAY CHOOSE to ignore it and
proceed immediately to Command phase. The transaction is not broken. This
has been true since SCSI-1. It was not a problem until SCSI-3 required
initiators to set the LUN field to zero as my previous notes tried to
explain.
Again, please remember that I am *not* proposing that targets go to message
out phase without the ATN line.
2) Tom Wicklund wrote:
In the same way, I don't see why a SCSI-3 device might not accept
selection without ATN as a signal to revert to SCSI-1 operation. It
might fail a strict SCSI-3 compliance tester, but won't affect normal
operation on a "real" system.
Yes, there will be no problem under "normal" circumstances, however,
as a previous note says:
"... if the targets accept commands without an IDENTIFY
message, they expose themselves to data integrity problems because they
will route all commands from SCSI-3 initiators to LUN 0 whenever the -ATN
wire is broken or it's driver is defective."
I hope this clears things up.
Giles Frazier
IBM Austin
gfrazier at ausvm6.vnet.ibm.com
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