[1927] in linux-scsi channel archive
Re: Internal to External SCSI Connections
daemon@ATHENA.MIT.EDU (Larry M. Augustin)
Sat May 31 03:43:55 1997
Date: Sat, 31 May 1997 00:17:42 -0700
From: "Larry M. Augustin" <lma@varesearch.com>
To: edwards@ioa.com, linux-scsi@vger.rutgers.edu
Even though the Buslogic, and other Ultra Wide SCSI adapters like the
Symbios 8751SP, have 3 connectors there is only a *single* SCSI bus.
You can use only 2 of the 3 connectors simultaneously. Using three
connectors would be like making a T on the bus. So it makes no
difference whether you connect to the wide or narrow connector.
For example, if you use a narrow SCSI cable to connect an internal CD
ROM drive to the SCSI controller's 50-pin connector, and a wide cable
to connect to the SCSI controller's wide connector, you can't use the
external connector. You must be able to form a single chain with no
T's.
Narrow SCSI adapter back plates like the one you describe are
available from CS Electronics (http://www.scsi-cables.com).
Another option is a 68-pin to 50-pin adapter on the external wide
connector. CS Electronics makes a 68-pin to 50-pin adapter cable.
One thing to be extremely careful of when mixing wide and narrow
devices is termination. Remember, the SCSI bus must be terminated at
both ends, and all 68-pins must be terminated. That means if you use
a 68-pin to 50-pin adapter at the end of the chain you must terminate
the unused lines. CS Electronics has a nice picture of this at
http://www.scsi-cables.com/line/68to50.htm. Using active termination
is also a must for Ultra SCSI speeds. You'll also need Ultra SCSI
rated cables and have to stay within Ultra SCSI bus length limits and
minimum distances between connectors. All of these shouldn't be a
problem if you are using a good quality cable vendor.
In all of our systems we use a single wide SCSI cable internally that
terminates in active termination at the controller and at the wide
SCSI drive. Narrow SCSI devices like CD ROM drives are attached with
non-terminating wide to narrow adapters in the middle of the chain.
Narrow devices like CD ROM drives should never terminate the SCSI bus
since they typically use passive termination. The single internal
cable allows using the external wide connector. To make attaching
external narrow devices easier, we include a wide to narrow adapter
with active termination with all systems.
Larry
Larry Augustin
VA Research, Inc.
1235 Pear Ave., Suite 109
Mountain View, CA 94043
Tel: +1.415.934.3666
Fax: +1.415.964.7668
Email: lma@varesearch.com
Web: http://www.varesearch.com
>There exists out there somewhere (at least there used to - someone on
>this list bought one previously at a computer show) a connector adapter
>that fits into a PC expansion slot case opening. On the internal side
>of it connects a ribbon with a standard 50 pin connector. On the
>external side is a 50 pin dense socket to connect, in my case, a Jaz
>drive cable.
>
>If anyone knows where to purchase one of these items, please forward me
>a name, phone number (or address or whatever), so that I can buy one.
>
>I recently purchased a Buslogic BT-958 and need this item to hook up my
>Jaz drive. At least this seems like a clean way to set things up to me.
>But then I'm new with SCSI devices. Is there another way that to
>connect up the Jaz drive? If an adapter to convert the 68 pin external
>socket to a 50 pin dense connection were used, would this result in
>speed loss for SCSI-3 devices on the same chain or other side affects
>such as termination problems?
>
>Any help or pointers anyone can give is appreciated.
>
>Thank you,
> David
