ISTR that the ADAPTEC BIOS setup allows maximal rates to be set in advance, so
you don't waste a lot of time trying to run a slow device fast. Nevertheless,
it's at least possible, if not always practicable, to operate the SCSI at
rates compatible with the fastest the devices can handle. This, I believe, is
intended to alleviate accomodating slow devices at the expense of fast ones.
I don't think it was ever the committee's intention to slow the entire bus
down to that of the slowest device, though they did make provision for
different rates based on the signalling protocol. The controller has to
"know" the limits for synchronous transfer, however.
Dick
----- Original Message -----
From: "Jeff Hellige" <jhellige_at_earthlink.net>
To: <classiccmp_at_classiccmp.org>
Sent: Monday, February 11, 2002 5:16 PM
Subject: Re: SCSI Bus Problem?
> >On Mon, 11 Feb 2002, Julius Sridhar wrote:
> >
> >> A narrow device will not slow down a wide bus to narrow width. But a
> >> 10 MHz device will slow down a 20 MHz SCSI bus to 10MHz.
> >
> >You're contradicting yourself here. Either that or the semantics of your
> >sentence are confusing :)
>
> Both the 20Mhz (Ultra) and 10 Mhz SCSI buses are narrow.
> Standard SCSI runs at 5 Mhz, Fast-SCSI at 10 Mhz, Ultra at 20Mhz and
> Ultra2/Wide is 40Mhz. The various versions of Wide go up to 160Mhz
> and 30 devices now. 20Mhz and below is still narrow width, just
> different speeds. The bus will slow itself to the speed of the
> slowest item on the bus. To put a narrow item on a wide bus requires
> some sort of adapter so the narrow item only changes the speed of the
> bus, not the data width of the whole bus.
>
> Jeff
> --
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>
Received on Mon Feb 11 2002 - 20:14:59 GMT