Valid SCAM Selection?
John.Lohmeyer at symbios.com
Wed Jun 18 12:22:19 PDT 1997
* From the SCSI Reflector (scsi at symbios.com), posted by:
* John Lohmeyer <John.Lohmeyer at Symbios.com>
At 11:11 AM 6/18/97 -0700, Tony wrote:
>Would the reference to the I/O signal in SPI r15a section 10.3.1 under "To
>perform a selection the initiator PIA shall:" "a) negate the I/O signal so
>this can be distinguished from reselection;" be used in the definition of
I think there are three kinds of 'selections':
1. Normal Selection
2. Normal Re-Selection
3. SCAM Selection
A Philadelphia lawyer might argue that SCAM Selection should follow the
Normal Selection rules (instead of the Normal Re-Selection rules), because
we didn't call it a SCAM Re-Selection. However, I don't see how the rules
for Normal Selections and Re-Selections can be extrapolated to apply to
I personally believe that one should not assert signals that are not
mentioned in the protocol. However, depending on I/O remaining false
during SCAM is not very safe either. There are a lot of early SCAM
implementations that use protocol chips that were designed prior to SCAM's
invention. This is probably why some designs are asserting I/O -- "the
silicon made me do it".
As for who should change their designs, I think both should.
As T10 Chair, I would be remiss if I did not mention that there is a
process called a "Request For Interpretation" whereby you can formally ask
for interpretations of ANSI standards. Unfortunately, the RFI process is a
lot of work and the answer is not binding in any way -- it does not change
the standard. In the SCAM selection case, I'd expect you'd get an answer
back to the effect that both implementations are wrong.
I do think that T10 should examine this issue and attempt to nail down a
direction (to allow I/O true or not) in SPI-2. I'll add an agenda item for
this topic to the SCSI WG meeting on Wednesday July 16th.
John Lohmeyer E-Mail: john.lohmeyer at symbios.com
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