SCSI Bus Problem?

From: Julius Sridhar <vance_at_ikickass.org>
Date: Mon Feb 11 18:30:06 2002

On Mon, 11 Feb 2002, Sellam Ismail wrote:

> > >SCSI Wide also supports plain old SCSI devices. I'm running such a
> > >configuration in my web server (3 SCSI Wide drives and 2 SCSI-2)
> > >
> > >Sellam Ismail Vintage Computer Festival
> >
> > Yup, sellam's right, be warned however that your SCSI bus will be hobbled
> > to the speed of the slowest device usually.
>
> Now that I didn't know. Is this only when a slower devices is being
> accessed simultaneously with a faster device, or does it revert to the
> lowest common denominator on the chain?

Ok, this is the way it works: there is a bus clock rate, and a bus width.
The bus will slow its clock rate to the clock of the slowest device, but
narrow (8-bit) and wide (16-bit) devices can coexist on a wide (16-bit)
bus. Suppose that you have two devices that run at 20MHz (ultra speed).
Even if one is ultra wide (20MHz, 16-bit) and one is plain-old ultra
(20MHz, 8-bit), the bus will be 16-bits wide. You just have to make sure
the narrow device's ID is in the lower three bits of the address space
(0-7). The wide devices can be addressed anywhere in the 4-bits of the
address space (0-f).

Peace... Sridhar
Received on Mon Feb 11 2002 - 18:30:06 GMT

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