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)
> > >
> > > Yup, sellam's right, be warned however that your SCSI bus will be hobbled
> > > to the speed of the slowest device usually.
> >
> > Right. I'd certainly not mix wide and narrow devices on one channel.
> > Termination of the bus is also often a pain when you do this.
>
> It wasn't much of a problem for me. I'm using an Adaptec 2940UW (or is it
> a 2940W?) and it has both SCSI and SCSI Wide connectors on the card
> internally, and a connector on the back that may or may not be SCSI Wide.
> I think either you specify in the card's BIOS setup whether you want
> to use the internal connectors or a combination of one of the internal
> connectors and the external connector, or it auto-detects (I haven't
> played with it in a long time, but I know at one point I had a Syquest EZ
> Flyer attached to the external connector along with the internal SCSI Wide
> drives). If you attach drives to all three, you get problems (I know this
> from experience ;)
That's because that's one big bus. If you're using the internal 50- and
68-pin connectors, the high 8 bits are terminated on the card. Same if
you're using the external 68-pin and the internal 50-pin. If you're using
the internal *and* external 68-pin, termination is disabled on the card.
You can't have a SCSI-star, it has to be a bus, so therefore only two
connectors can be used at a time.
> So you just terminate each device at the end of whatever chains you use.
> If you use the internal SCSI and SCSI Wide chains, you terminate the last
> SCSI device and the last SCSI Wide device. If you use the external
> connector, apply termination to the last device on the external chain and
> the last device on the internal chain. It works perfectly.
In wide SCSI, there is high-byte and low-byte termination. Suppose you
have the following on your wide SCSI bus:
(Host ID 7) -> (Wide ID A) -> (Wide ID 9) -> (Narrow ID 0)
Then both bytes have to be terminated on the host, the high byte needs to
be terminated on ID 9, and the low byte on 0 (it's the only termination
device 0 knows how to do, so you just have to tell it to terminate.)
Now if you add another device at the end of the chain so that it's now:
(Host ID 7) -> (Wide ID A) -> (Wide ID 9) -> (Narrow ID 0) -> (Wide ID 1)
Then you wouldn't terminate device 0, but you would terminate the low byte
of 1, and device 1 will run in narrow mode, because 9 has already
terminated the high byte. Also, you can't have an ID > 7 on device 1.
Peace... Sridhar
> > What can't be done, is put 15 narrow devices on one wide channel. Well,
> > at least and have them all work anyway.
>
> Narrow devices are only able to address up to SCSI ID 7 as far as I know
> (otherwise they'd be SCSI Wide devices). Only Wide devices know that
> devices IDs from 8-15 exist.
>
> Sellam Ismail Vintage Computer Festival
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Received on Mon Feb 11 2002 - 18:43:52 GMT