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From: rpw3@redwood.UUCP (Rob Warnock)
Newsgroups: net.dcom,net.periphs
Subject: Re: Ringing RS-232 dilemma
Message-ID: <224@redwood.UUCP>
Date: Wed, 13-Nov-85 08:48:03 EST
Article-I.D.: redwood.224
Posted: Wed Nov 13 08:48:03 1985
Date-Received: Fri, 15-Nov-85 05:28:48 EST
References: <4134@fritz.UUCP>
Organization: [Consultant], Foster City, CA
Lines: 39
Xref: watmath net.dcom:1468 net.periphs:904

Concerning undesired "echoing" on long unterminated terminal cables:

+---------------
| Is there some device which can be placed in the line to prevent
| the ringing but will not interfere with normal operation?  We
| tried placing 10K resistors between pins 2 and 7 and between 3
| and 7 but this didn't help.
| 	-- Art Zemon ...! {decvax, ihnp4, ucbvax} !trwrb!felix!zemon
+---------------

The problem is that 1000' of cable is quite a high capacitance. In order
to prevent the coupling from transmit to receive from driving the receive
line above threshold, you must bias the receive line well below threshold,
so that the cross-coupled spikes don't get it up above zero.

Try a 10K resistor between pin 3 (receive data) and a negative voltage
well below the RS-232 threshold, say -6 to -12 volts. That should do it.

You say you don't have any source of negative volts? Try DTR (pin 20).
It should be negative when the line is disconnected. But maybe you want
the resistor to be more like 27K, so no more than a milliamp or so of
excess current is drawn, worst case, when things are operating normally.

I used to work for a stat-mux company, and ALL inputs on our boxes had
current sinks on them, just to solve this problem. If this implies that
at one time we HAD the problem, you're right.

Historical note: Some sites would not see any problems for days or weeks,
then suddenly it would start. Seems that it would only happen when the
operator would send a broadcast message to the users, which would cause
the data to start looping...


Rob Warnock
Systems Architecture Consultant

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