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From: kaufman@Neon.Stanford.EDU (Marc T. Kaufman)
Newsgroups: comp.dcom.modems
Subject: Re: Big problems with modems that aren't grounded. What to do?
Message-ID: <1990Dec4.172322.17585@Neon.Stanford.EDU>
Date: 4 Dec 90 17:23:22 GMT
References: <1990Dec4.023427.25652@panix.uucp>
Organization: Computer Science Department, Stanford University
Lines: 29

In article <1990Dec4.023427.25652@panix.uucp> alexis@panix.uucp (Alexis Rosen) writes:
>I posted a query here a month or so ago, complaining about some very strange
>problems with a bunch of 2400bps modems connected to my unix box.

>While I'm not sure of this, it now seems likely that the problems are the
>result of poor grounding. I think this because the Telebit performs flawlessly,
>and the US Robotics nearly so. They both have three-prong (grounded) plugs on
>their transformers. The other three modems (a Smart One, a Prometheus, and a
>Maxxum) lack grounding, and they frequently die (although the Smart One is
>rather less susceptible).

>The problem is that the modems and the computer frequently fail to recognize
>that DCD and/or DTR control lines have been raised. Thus, the modems may not
>answer, and if they do, the getty may not complete the open.

RS-232 pin 7 is the logic ground reference point for signals (e.g. DCD, DTR).
In my experience, it is preferrable to ISOLATE the modem from system ground
to avoid ground-loop voltage offsets.  In any event, the green wire ground is
not intended to carry current -- it is a protective circuit in case of failures
in the modem power supply.  It should be connected to the case and/or chassis,
but NOT to logic ground in the modem.

A more probable scenario is incorrect wiring of some of your AC outlets --
reversal of the Hot and Neutral leads.  I have seen equipment that couples
logic ground to the presumed Neutral lead (usually through .1 uf and 10-12K
in parallel).  If your plug is on another AC phase and wired incorrectly,
you will see 60 Hz AC superimposed on your modem signals.

Marc Kaufman (kaufman@Neon.stanford.edu)