Mark Garlanger wrote:
>
> D. Lance Robinson wrote:
> >
> > When mixing narrow and wide devices you must only use the first eight
> > scsi ids. If you don't, scsi arbitration will not work correctly. The
> > scsi bus uses the data bits to arbitrate for the bus. A narrow device
> > does not see the upper eight arbitration bits and can fowl things up.
> > Your wide devices can still work in wide mode, but their addresses
> > should be set between 0 and 7 where the narrow devices can see them for
> > arbitration.
> >
>
> Is this true?? I have never heard this mentioned before, I just looked
> through my Adaptec 2940UW Manual, in the SCSI ID Numbers section it
> says NOTHING like this... This would be a big hole in the SCSI standard
> if you can't have more than 7 devices if you use ANY narrow devices. My
> current system in which I have had no problems:
> IDs
> ============
> 0,8 - Wide
> 1,2,3,4,5 - Narrow
>
Since wide devices can still work with 16 bits of data, this isn't that
big of a problem. You may not be experience problems; but, as written in
the book "The SCSI Bus & IDE Interface", second edition, page 274: "When
you connect buses of different width, you can only use IDs that are
allowed by the narrowest bus segment. Otherwise, the devices on the
narrow bus cannot recognize devices with higher IDs and arbitration will
not work." This applies to mixing 8, 16 or 32 bit busses.
If a narrower device behaves well, it will sence a wider device
asserting BSY (from bus free) when arbitrating for the bus. Then the
'well behaved' narrow device will not try to arbitrate untill the next
bus free phase. Also, wider devices using a higher ID will see and yeild
to lower IDs when arbitrating. So, you may not run into trouble using
higher IDs when mixing bus widths; but, if you can avoid it...use the
lower IDs.
<>< Lance.
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