Current Loop Converter - test-drb@ccmp.vtda.org

7 Oct 1999

ftp://ftp.bb-elec.com/bb-elec/literature/232pclin.pdf

This is the full schematic of an industrial one. I built one with two
transistors years ago.. I will dig up my design and post it (used it on a
DG-116)

-----Original Message-----
From: John Foust &lt;jfoust(a)threedee.com&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
Date: Thursday, October 07, 1999 2:23 PM
Subject: SGI Hardware Developer Handbooks available

...

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/class13itemgroup59 8
...

guest.asp?param=95&ig_id=598&title=High%2DSpeed+RS%2D232%26lt%3B%26gt%3BCur
r
...
 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