ASR33 Teletype interfacing

From: Ian Primus <>
Date: Sun Oct 12 21:01:21 2003

On Saturday, October 11, 2003, at 01:52 PM, James M. Walker wrote:

> Hi Ian and list,
> Sorry there Ian, I should have mentioned that this technique is only
> for testing purposes and also that the battery gets "HOT". This is an
> old
> trick I used while doing field setups and testing of Shio-To-Shore
> sites
> that used ARQ and FEC with ASR-32/33 teletypes. The 20 MA loop can be
> connected to your interface as described previously. All that is needed
> is a 115v primary to 115 v secondary transformer, a full wave bridge to
> rectify the AC and around 35 - 60 uF cap for filtering. All this
> output
> is fed to the collector of an MJE-340 NPN transistor emitter, the
> collector
> connection goes to your ASR-(*) TTY. The base goes to the output from
> the
> tx terminals on the interface. I think I have an Orcad drawing of the
> circuit if you would like it. As I recall it is an RS-232 to 20 MA
> interface
> and also includes an opto-isolator in the circuit. However at least you
> did
> get to verify that your TTY is indeed working correctly! Good "Oh!
> Jim
> Real Radio in Buffalo

I would definitely be interested in seeing your circuit. I am quite
confused as to the proper way to go about doing this, from what I have
seen, it looks like there are several quite different ways of doing
what I want to do, and none of which I have been able to figure out
thus far. That little thing I built with the resistor and the diode I
somewhat understand, but then again, I also understand that it is _not_
a very good solution, and it looks like it might be dangerous to the
computer if I wasn't careful. Electronics aren't my strong suit,
although I am currently working on learning more. A friend and I are
building a little computer based around the Z80 CPU, and I am actually
beginning to understand memory decoding and addressing. But, analog
electronics is something that I have done very little with, save for
simple power supply repair and simple audio amplifiers. At the moment,
20mA current loops are over my head. Anything to help me understand
what is going on here would be greatly appreciated.

Also, yesterday I went to test the teletype again (I was cleaning the
type cylinder), and I happened to turn it into Line mode, and it didn't
run open like it had before. Then, I realized that the scraps of wire
were still connected to the terminals, with one of the RX lines tied to
one of the TX lines, and the other two wires were dangling, not
connected to anything. Disconnecting the RX and TX lines caused it to
run open again. Does this mean that the Teletype is supplying enough
loop current to drive the selector magnet, or what?

There seems to be forty different possible configurations for the
teletype, and a the Teletype itself involves circuits and mechanisms
that I don't understand. (IMHO, the ability to get that little cylinder
to rotate and bang out the proper letter is amazing in itself, even
more amazing is the fact that it's all done with levers and cams...)

Once again, any help to clarify things would be greatly appreciated,
one of these days I hope to be able to more fully comprehend these


Ian Primus
Received on Sun Oct 12 2003 - 21:01:21 BST

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