What is SCSI-3? (RE- CLARI

MGauthier MGauthier at iit.nrc.ca
Tue Aug 16 22:51:00 PDT 1994

Subject:     What is SCSI-3?  (RE: CLARIFICATIONS)

I have a question regarding how the SCSI-3 standards are written.
There seems to be a bit of confusion (mine perhaps) between "strictly
SCSI-3 behaviour" versus the behaviour of a SCSI-3 device that is also
SCSI-2 and SCSI-1 compatible.  Do the SCSI-3 specifications (SAM & friends)
specify only "strictly SCSI-3" behaviour or do they also fill out the
details and exceptions of what to do to be SCSI-2 and SCSI-1 compatible?
If the former, they wouldn't specify a realistic device, since most will
strive to be SCSI-1 and SCSI-2 compatible.  That is, they wouldn't specify
SCSI-3 as it is intended since it is meant to be compatible.  If the latter,
then much of the recent discussion should have been covered by the specs,
which isn't the case.

Did I miss anything?

Maybe this is a more fundamental question.  What is SCSI-3?  Is it (A)
a protocol/standard that, if adhered to, will interoperate with SCSI-1,
SCSI-2 and SCSI-3?  Or is it (B) a protocol/standard that is related to
but distinct from SCSI-1 and SCSI-2, that if adhered to is only guaranteed
to interoperate with other SCSI-3 devices but somehow allows (or not?) in
its design to be made compatible with SCSI-2 and SCSI-1 -- implying that
realistic SCSI devices have to implement something that will interoperate
with SCSI-3, SCSI-2 and SCSI-1 yet is neither of those three (strictly

For example, if SCSI-3 defines the LUN field of command bytes strictly
as reserved, then strictly speaking, SCSI-3 is not compatible with SCSI-1.
A device could be made that interoperates with both SCSI-3 and SCSI-1,
but *strictly speaking*, it wouldn't be SCSI-3 since under certain
conditions it wouldn't operate according to SCSI-3 specs.  So ironically,
we are encouraged to make SCSI-3 devices that don't follow specs
(for backward compatibility).

It would be nice to have (A), but if (B), shouldn't there be a standard
or guidelines on how to make a SCSI-3 device interoperable with SCSI-2
and SCSI-1?  (solutions don't all seem so obvious as to not be written

[In this question SCSI-3 is viewed with SIP and SPI at the protocol and
 interconnect layers.  SAM r.12 doesn't address this question.  I don't
 have a copy of SIP, and SPI is rather low-level with little behavioural
 specifications (none or little that depend on versions of SCSI that is).]


Marc E. Gauthier
Software Engineering Lab, Institute for Information Technology (SEL,IIT)
National Research Council Canada, Bldg M-50,  Ottawa ON Canada  K1A 0R6
+1 613 991 6975   fax:  +1 613 952 7151   email:  mgauthier at iit.nrc.ca

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