Subject: Re: SCSI wiring
To: der Mouse <mouse@Rodents.Montreal.QC.CA>
From: Charles Hannum <mycroft@MIT.EDU>
List: tech-kern
Date: 08/09/2005 14:53:38
Quoting der Mouse <mouse@Rodents.Montreal.QC.CA>:

>>> I was thinking about connecting two SCSI buses with the data wires
>>> switched around, so that one bus's D7 (and therefore ID 7) isn't the
>>> same as the other's.  [...]
>> It would be easier to just reprogram the SCSI controller to use a
>> different ID.  Most of them support it.
>> More importantly, the arbitration isn't going to work right if both
>> systems think they're at ID 7 and the other is at ID 6.  They will
>> both try to use the bus at the same time.
> Won't you have the same problems if you teach one of the SCSI chips to
> use (say) ID 6?  Won't they both think they are "the" host on the bus?
> How would these two situations differ that makes one work and the other
> not?
> Or am I just exposing my ignorance of SCSI?  Is there a canonical
> reference somewhere I can go read to Learn All About It?

No, and yes.  If they know they're at different IDs, the higher one 
will always
get priority during arbitration.

Arbitration is covered in detail in the SCSI Parallel Interface (SPI) spec.

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