Path: utzoo!utgpu!watserv1!watmath!att!att!linac!uwm.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!rice!uw-beaver!ubc-cs!van-bc!
From: lphillips@lpami.wimsey.bc.ca (Larry Phillips)
Newsgroups: comp.sys.amiga
Subject: Re: 2091 Interface
Message-ID: <2180@lpami.wimsey.bc.ca>
Date: 31 Oct 90 21:02:04 GMT
Lines: 61
Return-Path: <lpami!lphillips>
To: van-bc!rnews

In <304@rusux1.rus.uni-stuttgart.de>, wschmidt@phobix (Wolfram Schmidt) writes:
>
>So what happens if you have multiple drives, one internal and some external?

Same thing applies... two terminators.. one at each end of the bus.

>The bus is terminated at both ends.

This is the way it should be.

> The host adapter is terminated, too.

The 'too' is a little misleading. In many cases, the host adapter _IS_ at te
end of the bus. If it is not, the proper way is to remove the terminators from
the host adapter. In practice, there are other solutions that do not follow the
spec, but that work just fine.

>Otherwise the manufacturer of the host adapter should have supplied a
>terminator for the external port.

Why? When you buy an external device, it will usually have a terminator, either
separately or installed in the device. It is up to you as to whether you
install it, leave it in, or remove it.

> At least my 2090 has terminators on board.

Right.. it is considered to be on one end of the bus, and was in fact, shipped
that way in the 2500, when 2090s were the controller supplied.

>So when I soon connect an external SCSI tape drive the bus will have three
>terminators. Any comments (especially form peolpe who design such things)?

It will if you leave all the terminators in place. Now in practice, you can
often 'get by' with terminating the bus in a unit that is not on the end of the
run. While the spec states that each physical end of the bus should be
terminated, and while it should work that way, it will often work if it is
configured differently.

You might try one of the following...

1. Leave the terminators off the internal drive(s), leaving the terminators on
   the host adapter and terminating the external device. (I ran this way for
   ages when I had a 2090)

2. Remove the terminators from the host adapter, leaving the terminators on
   _one_ of the internal drives. They may, however, be soldered in.

3. Leave the host adapter teminators in place, and run a ribbon cable from the
   host adapter to the internal drives and from there, out the back to the
   external device, terminating in the external.

-larry

--
It is not possible to both understand and appreciate Intel CPUs.
    -D.Wolfskill
+-----------------------------------------------------------------------+ 
|   //   Larry Phillips                                                 |
| \X/    lphillips@lpami.wimsey.bc.ca -or- uunet!van-bc!lpami!lphillips |
|        COMPUSERVE: 76703,4322  -or-  76703.4322@compuserve.com        |
+-----------------------------------------------------------------------+