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Path: utzoo!watmath!wateng!padpowell
From: padpowell@wateng.UUCP (PAD Powell)
Newsgroups: net.periphs
Subject: Re: How long can an RS232 line be?
Message-ID: <1537@wateng.UUCP>
Date: Fri, 12-Oct-84 07:31:49 EDT
Article-I.D.: wateng.1537
Posted: Fri Oct 12 07:31:49 1984
Date-Received: Sat, 13-Oct-84 05:03:30 EDT
References: <487@oddjob.UChicago.UUCP>
Organization: U of Waterloo, Ontario
Lines: 47

Actually, you can run an RS232 line as far as you want.  What is critical is
the signal strengths remaining at the far end.

The standard (ANSI RS-232C), which I don't have beside me just now,
states what the maximum input voltage levels and max/min impedances are.
In addition, it specifies the minimum "received" signal values for a 1 and
0 value, together with the receiver max/min impedance.  The problem is to
figure out what YOU can get away with.

The usual DRIVER for an RS232 signal is an integrated circuit (ARRG!
mem fault- core dump! I can't remeber the number), which generates a
+11 to -9 volt signal.  The matching receiver will detect a 1 volt signal,
and has a hysterisis input, centered on about .5 volts, with about
.2 volts hysterisis.  Note that these values are typical, and can vary
all over the landscape.

The limiting factor on distance is your cable and speed.  The longer the
cable, the more total capacitance, the slower the driver can drive the
signal.  The following combinations have been tried here at Waterloo.
1. Unshielded Bell 4 conductor cable, using (TX,ground) (DX,ground)
	in the pairs.   RTS, DTR, etc. were unused.
	19.2K bps-  150 meters
	9600- 400 meters
	1200- ran out of cable, at least 2 KM.

2. Shielded 50 pair cable:
	TX, RD, with ground, rest jumbled together.
	About the same distances.

By the way, as an interesting experiment, I tried using the new RS422
drivers from Texax Instruments, in a 4 KM loopback mode.  Ran it at
9600 for a week with only a couple of bit errors.  This was in differential
mode.

I made up a small 6 component PC board that has a 422driver/receiver,
	and a small regulator, to convert from 422 to TTL levels
	(which are compatible with the RS232 receivers HINT),
	and use these instead of long distance modems.  The power supply
	is a $3.00 calculator battery replacement.  UGLY? Yerp.
	But it sure is cheaper than $200 for a line driver.  I have
	gotten paranoid lately and added 2 optical isolators for comfort.

	If you want isolation, you have to get a power source from the
	unit;  pin 25 of the RS232 connector is unused, and is usually
	a good place to stick a tap to the +12 or +5 supply.

Patrick Powell