Path: utzoo!news-server.csri.toronto.edu!rutgers!cmcl2!adm!smoke!gwyn
From: gwyn@smoke.brl.mil (Doug Gwyn)
Newsgroups: comp.sys.apple2
Subject: Re: Multi SCSI card system..
Message-ID: <15442@smoke.brl.mil>
Date: 11 Mar 91 18:48:14 GMT
References: <15420@smoke.brl.mil> <1991Mar11.045531.3866@nntp-server.caltech.edu>
Organization: U.S. Army Ballistic Research Laboratory, APG, MD.
Lines: 33

In article <1991Mar11.045531.3866@nntp-server.caltech.edu> toddpw@nntp-server.caltech.edu (Todd P. Whitesel) writes:
>>Why would one WANT to remove the bus terminating resistors?
>So you can get more than 2 of them on a bus together. A properly terminated
>SCSI bus has terminators on each end, and nowhere in between. With 2 DMA SCSI's
>that means you have 1 computer on each end of the bus and all the devices in
>between them with internal termination removed and no external terminators.
>Try to add more DMA SCSI's and you will run into problems because of the extra
>termination.
>>Lack of proper termination was one of the main problems with
>>the previous (non-DMA) Apple SCSI card.
>External terminators fixed that. Apple should have included one free with the
>Rev. C SCSI, but it doesn't really matter now.

No, in fact proper termination does require terminators precisely at the
two ends of the SCSI bus and nowhere in between.  Thus, the terminating
resistors DO belong (permanently) on the Apple SCSI Card.  Putting them
one "system cable" length from the card for the old (non-DMA) SCSI card
was electrically inferior to having them on the card.

The scenario of more than two SCSI computer interfaces (bus controllers)
strikes me as unrealistic.  I have heard of only a few installations to
have even two controllers, and Apple's operating systems do not properly
interlock their shared access to device contents (for example, local
caching is not coordinated among the systems).

It is terminators at each peripheral that may need to be installed or
removed; Apple's use of external terminators makes that convenient but
expensive.  Some people leave the internal termination installed in the
last drive on the SCSI bus (farthest from the interface card), to avoid
having to purchase an external terminator.  If you seldom reconfigure
the SCSI chain, this is adequate.  For frequent reconfiguration, external
termination is the only way to go.  In all realistic cases, the SCSI
interface card should retain its built-in termination.