Tom Wicklund wicklund at
Tue Aug 16 09:36:57 PDT 1994

Gerry Houlder writes:
 > This is the change proposed by Giles:
 > >"The first message sent by the initiator after the SELECTION phase
 > >shall be an IDENTIFY, ABORT, or BUS DEVICE RESET message. If a target
 > >receives any other message it shall issue a BUS FREE request."
 > >
 > >All that is needed is to reword the last sentence as follows:
 > >
 > > "If -ATN IS NOT ACTIVATED OR IF the target receives any other
 > > message..."    (CAPS indicate the change.)
 > This change means that SCSI-3 targets cannot function in SCSI-1 or SCSI-2
 > systems that use selection without ATN. This will absolutely require initiators
 > to put the drive in "SCSI-2 mode" or "SCSI-3 mode" before installing it in the
 > system. Or it could require target manufacturers to support even more different
 > and unique code versions than we have to now. I don't think this is a good
 > idea. The target manufacturer's problem of selling a SCSI drive to a
 > distributor and having that drive work in a wide variety of SCSI-1, SCSI-2, or
 > SCSI-3 systems and have it be able to plug and play (not necessarily a
 > reference to the Plug and Play SCSI proposal) will become impossible.

I don't see how this will be true.

SCSI-2 already requires that the IDENTIFY message be used to identify
the logical unit -- (see section 7.2.2), yet I'm sure most SCSI-2
devices will also accept SCSI-1 selection without the IDENTIFY message
and use the LUN field in the CDB (especially when popular systems like
Sun issue selection without IDENTIFY during system boot).

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.

More information about the T10 mailing list