Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!ames!bionet!agate!shelby!neon!kaufman
From: kaufman@Neon.Stanford.EDU (Marc T. Kaufman)
Newsgroups: comp.sys.mac.hardware
Subject: Re: IIfx vs the scanner - scanner wins!
Keywords: SCSI Microtek terminator
Message-ID: <1990Dec12.184703.20250@Neon.Stanford.EDU>
Date: 12 Dec 90 18:47:03 GMT
References: <14862@arisia.Xerox.COM> <1990Dec12.161846.6240@eng.umd.edu>
Organization: Computer Science Department, Stanford University
Lines: 25

In article <1990Dec12.161846.6240@eng.umd.edu> russotto@eng.umd.edu (Matthew T. Russotto) writes:
>In article <14862@arisia.Xerox.COM> sanders.parc@xerox.com writes:
->Todays Macintosh support headache:

->Mac IIfx, System 6.05, 120 Mb Seagate HD, Microtek color scanner,
->  3 foot SCSI cable, black (IIfx) terminator between cable & scanner.
...
->  remove the SCSI terminator

>Sounds like the scanner is internally terminated.  That is bad.  Also,
>most Seagate drives come internally terminated-- three to four terminators is
>also bad.

I know that Apple says to remove internal terminators... but:
The black terminator is just a regular 220/330 ohm terminator with additional
capacitance to stabilize the +5 at the terminator, so that crosstalk between
SCSI signal lines is minimized.  Most internal terminators are powered locally,
so that the +5 is already stable (due to the low impedence and filtering in
the internal wiring).  Furthermore, the internal terminators can be placed
MUCH closer to the SCSI controller, thus minimizing the stub length.

Does anyone know the rationale for suggesting that internal terminators be
removed?

Marc Kaufman (kaufman@Neon.stanford.edu)