On Thu, 9 Oct 97 20:29:19 -0000,
#hea <hea@ix.netcom.com> wrote:
> =

> On Thurs 10/9/97, Ken Nakata wrote:
> >Just a minor nit-picking; I always thought lower SCSI ID had higher
> >priority, i.e. you usually want the host adapter to have the lowest
> >priority (in wide SCSI, it's conceivable that the host is ID 7 instea=
d
> >of the lowest priority ID 15, because otherwise you won't be able to
> >talk to non-wide devices connected to wide bus).
> =

> Actually, the higher SCSI ID has higher priority.  Macs use initiator =
ID =

> 7, primarily =

> because ID 0 was typically used by the drive - i.e. no jumpers).  It w=
as =

> also convenient =

> to use ID 7 on the wide bus (as you point out, it works for both).  =


From=20the comp.periphs.scsi FAQ:
----------
=3D=3D=3D=3D
QUESTION: How does device ID numbering work with WIDE vs NARROW devices?=

ANSWER From: Gary Field (gfield@zk3.dec.com)
=3D=3D=3D=3D

Narrow SCSI devices can only use IDs 0 through 7. WIDE SCSI devices on
a SCSI-3 system with 68 pin P cables, can use IDs 0 through 15. It is
generally wise to reserve 0-7 for narrow devices though.
SCSI-2 only specified the use of IDs 0-7 even for WIDE devices, but SCSI=
-3
allows 0-15 for WIDE devices. All devices on one bus must have unique ID=
s
of course.
The arbitration priorities are as follows:
highest
ID 7
...
ID 0
ID 15
...
ID 8
ID 23
...
ID 16
ID 31
...
ID 24
lowest

A WIDE device that is set to ID 10 knows not to respond to selection
for ID 2 because the parity bit P1 (for  bits 8-15) will not be set
by the initiator. During a selection of ID 10, the P parity bit
(for bits 0-7) will not be set by the initiator, but the P1 bit will be.=


----------

So, we are both incorrect! ;-) ID-7 has the highest priority of all
(at least in SCSI-3 spec).

Cheers,

Ken