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Path: utzoo!linus!decvax!decwrl!amd!fortune!hpda!dmsd!bass
From: bass@dmsd.UUCP (John Bass)
Newsgroups: net.periphs
Subject: Re: How long can an RS232 line be?
Message-ID: <150@dmsd.UUCP>
Date: Tue, 16-Oct-84 01:15:05 EDT
Article-I.D.: dmsd.150
Posted: Tue Oct 16 01:15:05 1984
Date-Received: Thu, 18-Oct-84 19:24:23 EDT
References: <487@oddjob.UChicago.UUCP>
Lines: 24

At SRI in the late 70's we ran some 2000-3000 ft lines running at 2400/4800
baud with little trouble. Some would work at 9600. The lines were 2 twisted
pairs with foil shield. Some terminals had "marginal" rs232 drivers/receivers
and worked better after using different chips.

Looking at the signals at the computer end the receiver waveforms were nearly
sine waves. Some of the installations also showed 4-15v of 60hz noise ...
much of that was ground noise ... both earth ground variances and inductive
pickup from the lines running along side power lines.

I have heard that some folks have used an RC filter to waveshape the signal and
cut out some of the 60hz noise ... but don't know what was used.

The biggest problems were caused by lines talking to themselves when a remote
terminal was powered off. The floating end of a 2000ft twisted pair lines
passes back all sorts of garbage into the receiver on the computer end. The
logon banner would couple back into the receiver, echo/getty/login output
from the input would keep the line talking for hours on end. A resistive
termination to ground at the terminal end helped on some lines.

Some lines had repeated driver/receiver failure ... ground differential?
inductive kicks?

Have fun .. John Bass