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
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

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