SEL Compatibility

Gene_Milligan at Gene_Milligan at
Tue Feb 29 12:41:08 PST 2000

* From the T10 Reflector (t10 at, posted by:
* Gene_Milligan at
I understand that during the SPI-4 working group there was a proposal to
initiate training with the assertion of SEL. I assume the proposal was to do
this only during data phases and I also assume only when certain modes have been

This led me to look at SEL to see if there were any backward compatibility
issues. It appears there are not if new devices have their protocol automation
designed fresh for SPI-4 and that prior devices are not snooping the bus (e.g.
QAS from SPI-3).

But what about our old unspecified nemesis - expanders. SPI-3 and prior states
<<The SCSI bus phases are defined such that the SCSI bus is never in more than
one phase at any given time. In the following descriptions, signals that are not
mentioned shall not be asserted.>> Except for Arbitration, Selection and Message
with QAS phases SEL is not mentioned. Therefore I assume designers could have
detected a protocol error if SEL popped up in some other phase. But for the
prior designs what phase are the other phases in, in particular the data phase,
if SEL pops up? Except with broken new hardware/firmware I assume the issue will
not occur except in the eyes of an expander. Does someone know what all
expanders will do or at least the bulk of expander designs?


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