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From: kaufman@Neon.Stanford.EDU (Marc T. Kaufman)
Newsgroups: comp.dcom.modems
Subject: Re: Extending RS-232: plug in drivers?
Message-ID: <11880@polya.Stanford.EDU>
Date: 21 Sep 89 16:28:05 GMT
References: <2395@loral.UUCP>
Sender: USENET News System <news@Polya.Stanford.EDU>
Reply-To: kaufman@Neon.Stanford.EDU (Marc T. Kaufman)
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Organization: Stanford University, Computer Science Dept.
Lines: 35

In article <2395@loral.UUCP> jcb@loral.UUCP (Jay Bowden) writes:
>I am looking for cheap ways to extend RS-232 signals to
>go about 2-3 km.  I've seen no-power-needed plug-in
>drivers from Black Box, but they are about $90 per side,
>and that seems excessive.  Anyone know of a cheaper
>soloution?

I 'invented' one of these things in 1980, and at the time my company was
building them the all-up cost to manufacture, INCLUDING the external plug-in
transformer AND packaging, was $5.00

So, to make it easier for the rest of you, here is a circuit that will
extend a full duplex RS-232 circuit on 2 wires.  Use a fast slew-rate
op-amp for best results at high speed and long range. (the op amps need to
be fed with +/- 12 volts, or whatever your RS-232 chips use.  you can derive
this from the other pins, such as RTS and DTR, or just supply it from outside):

           |\
 TxD-------|+\             2 K
           |  \-------+--/\/\/\/--+------------------line + (to other line+)
 Gnd---+---|- /       |           |
 (0V)  |   | /     2K >           |    |\
       |   |/         <           +----|+\
       |              |                |  \------- RxD
       |              +----------------|- /
       |              |                | /
       |           1K >                |/
       |              <
       |              |
       +--------------+------------------------------line - (to other line -)

As you can see, the thing is basically a DC bridge.  I have run this circuit
at 19.2 KB on 2 miles of wire.  Have fun with it.

Marc Kaufman (kaufman@Neon.stanford.edu)