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From: John Foust <jfoust_at_threedee.com>
Date: Thu Oct 7 13:21:09 1999

I just tried to answer a question from someone on the Greenkeys
mailing list, regarding interfacing an ASR-33 to RS-232. My answer
is below. I hope I got it right.

- John

At 11:30 AM 10/7/99 -0400, Barry Hall wrote:
>Hello John and Christian,
>
> I was referred to you by Don Robert regarding connecting an ASR 33 to a
>Microcomputer RS232 Serial COM port. Any help you can give me would be
greatly
>appreciated.

The ASR-33 usually has what's referred to as a "20 milliamp current loop"
interface. It's not RS-232. You can buy converter boxes from places
like <http://www.blackbox.com/>, like the one at
<http://catalog.blackbox.com/blackbox/templates/blackbox/class13itemgroup598
guest.asp?param=95&ig_id=598&title=High%2DSpeed+RS%2D232%26lt%3B%26gt%3BCurr
ent%2DLoop+Interface+Converter&related=>.

There's an explanation at <http://telebyteusa.com/catalog/appnotes/a65a.htm>,
 <http://www.mouse.demon.nl/ckp/serial/cl.htm>,
<http://telecom.tbi.net/data-if.html>
and <http://www.bb-elec.com/techlibr.asp>.

I'm sure someone has a web page with a homebrew conversion between
current loop and RS-232. The voltage levels and swings aren't the
same, so it's not just a matter of wiring.

The circuit at
<http://www.ibhsoftec.de/english/screenshots/current_loop_converter.html>
might be right.

- John
Received on Thu Oct 07 1999 - 13:21:09 BST