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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