🚀 go-pugleaf

In article <1992Jul26.052449.18091@metapro.DIALix.oz.au> bernie@metapro.DIALix.oz.au (Bernd Felsche) writes:
>
>How do you prevent two devices responding to the same
>low 8-bits of address?
>
>i.e. devices 4 and 12.
>
>The high bit is only visible on the 16bit bus. Is there some
>magic which differentiates between 8 and 16bit addresses,
>which is visible in the 8bit space?

Assuming your host adapter was at SCSI ID 0 and it wanted to
address a device at SCSI ID 4, in the SELECTION phase it would
assert DB(0) and DB(4); to address a device at SCSI ID 12, it
would assert DB(0) and DB(12) (at least it would if this were
legal :-), my reading of revision 10c of the SCSI-2 spec (most
recent I have) is that the range is explicitly limited to 0-7).

Since in neither case is the ID bit for the "other" device
asserted, it's not clear to me why it would care (it shouldn't
even "see" a SELECT or RESELECT that didn't have its SCSI ID
asserted).  The spec says that a device shouldn't respond if
_more_ than 2 bits are asserted, but it shouldn't care if only
1 is, in fact that's how the single initiator option works.
Obviously, the host adapter itself would have to be in the 0-7
range if any devices only understood 8 address bits, and target
to target operations wouldn't work too good...

ted

-- 
Ted Goldblatt    ted@telematics.com     (305) 351-4367
    Telematics Intl., Inc.   Ft. Lauderdale, FL