[5174] in linux-scsi channel archive
Re: Termination questions
daemon@ATHENA.MIT.EDU (Robert Minichino)
Thu Nov 19 04:53:55 1998
Date: Wed, 18 Nov 1998 13:48:10 -0500 (EST)
From: Robert Minichino <rmini@joni.pasture.net>
Reply-To: Robert Minichino <rmini@joni.pasture.net>
To: ssaner@southwind.net
Cc: linux-scsi@vger.rutgers.edu
> I have what might be an elementary scsi question. I have a lot of
> hardware and electronics background, but I am new to scsi.
>
> I have an HP 5000i SureStore DAT drive and a Mylex BT-958 card. I also
> have a 50 pin ribbon cable to connect the two. My question is, do I
> need a terminator at the opposite end of the cable from the
> controller.
Absolutely; it may work without it, but it will not be reliable!
> The controller is auto-terminating, so it will take care of its end of
> the bus. According to its documentation, generally the device you are
> connecting it to will have built in termination that can be enabled or
> disabled. The tape drive documentation, however, does not indicate
> that it has internal termination. It suggests that you will need to
> put a terminator on the cable if the tape drive is the only, or last
device.
Your documentation is wrong. I have noticed a disturbing trend with
low-end hardware manufacturers (and even some more prominent manufacturers)
cutting costs by not including terminators, and then insisting that they
are not required. They absolutely are, regardless of how many devices
you have on the bus or its location on the bus, the SCSI signals will
still ring due to the impedance difference between the wire and the air.
At 10MHz and up for Fast and Ultra SCSI the effect becomes even more
pronounced, and may lead to data corruption. Granted, that tape drive may
only run at 5MHz, but if you have 10 or 20MHz devices on that bus as well
you may experience a multitude of problems. Also consider the fact that
the original Fast SCSI-2 (10MHz) specification called for differential
signaling, not single-ended as its currently implemented in most devices.
An unterminated 10MHz bus is pushing it to the limits, and I have rarely
seen it work in practice. I have, however, seen slow scanner busses
(2-3m, but less than 5MHz) work with passive termination on the card and
no termination on the scanner.
> A friend of mine told me that some of the newer cards would not
> require such a terminator and that I should try it without. I have not
> yet done that, but I wondered if someone has experience with this
> and could shed some light on it.
If and only if the SCSI card and the drive are the only two things on the
bus, the bus speed is set below 10MHz (or the device doesn't transfer at
10MHz or above), and data integrity and stability are not critical and
are not worth the minimal cost incurred in purchasing even a small passive
terminator, then use the bus unterminated. Me? My living depends upon
my workstations and servers, I terminate my busses properly. I HAVE seen
data lost like this, but granted those unterminated busses housed HDs.
--
Robert Minichino
Chief Engineer
Denarius Enterprises, Inc.
http://www.denarius.com/
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