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From: phys169@canterbury.ac.nz
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Serial-port connections
Message-ID: <1990Aug21.112025.8970@canterbury.ac.nz>
Date: 21 Aug 90 01:29:59 GMT
References: <54123@iuvax.cs.indiana.edu> <8978@uhccux.uhcc.Hawaii.Edu>
Organization: University of Canterbury
Lines: 23

In article <54123@iuvax.cs.indiana.edu> bobmon@iuvax.cs.indiana.edu (RAMontante) writes:
> how far can the remote device be from the the P.C., 
I've managed 600metres at 4800 baud. It depends a lot on noise sources, baud
rate, and whether you use currenht loop drivers.

> and how should it be connected?
You may manage with a simple null-modem cable, but try it thoroughly first
(include parity error detection at both ends, sends lots of data, try it for
several days, e.g. I had an arc-welding machine nearby that was only used
occassionally, but when it was, there were problems. Also check the voltage
levels of the earth lines of both machines - check for mains hum ( a few volts
can cause errors, I had about 50v difference in one case, that zapped interface
chips!).
 
Better still, use 20mA or 40mA current loop circuits with opto-couplers. These
used to be popular on terminals and minicomputers; the interface to RS232 is
easy (consisting od a few cheapo resistors, protective diodes, batteries, and
opto-couplers - also called opto-isolators). You can reliably go over 1000
yards with 20mA loops, since it provides a better signal/noise ratio and avoids
mains hum in "earth loops" between equipment. But, for 200ft, definately try
plain RS232 first.

Mark Aitchison, Physics, University of Canterbury, New Zealand.