Valid SCAM Selection?

John Lohmeyer John.Lohmeyer at
Wed Jun 18 12:22:19 PDT 1997

* From the SCSI Reflector (scsi at, posted by:
* John Lohmeyer <John.Lohmeyer at>
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
SCAM selections.

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 Logic Inc.             Voice: 719-533-7560
4420 ArrowsWest Dr.              Fax: 719-533-7036
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