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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: comp.periphs.scsi
Subject: Re: Need help on electrical characteristics of SCSI bus
Keywords: scsi bus
Message-ID: <16050@milton.u.washington.edu>
Date: 8 Feb 91 02:39:02 GMT
References: <fausett.665954939@aivax>
Distribution: comp.periphs.scsi
Organization: University of Washington, Seattle
Lines: 19

In article <fausett.665954939@aivax> fausett@aivax.radc.af.mil (Mark L. Fausett) writes:

> ..... what is the proper cable impedance, and is the termination
>220 or 330 ohms (or something else??)

	The original recommended termination was 330 Ohms to ground 
_and_ 220 Ohms to +5V; the Thevenin equivalent 130 Ohms to +3V was
also recommended.  I understand that the SCSI-2 spec includes
a third recommendation, 110 Ohms to 2.85V.  None of these
values is critical (I have heard from one manufacturer that he
plugged in all his resistor packs backward; everything worked,
and the product actually shipped that way for a while.)

	I have found that a voltmeter on each pin of the cable
gives 3V or ground on almost every pin (two pins may have +5V
terminator power).  Suspect a broken wire or faulty termination
resistor, and track it down with the VOM.

	John Whitmore