ASR33 Teletype interfacing

From: Ian Primus <ian_primus_at_yahoo.com>
Date: Sat Oct 11 07:56:30 2003

On Thursday, October 9, 2003, at 10:07 AM, James M. Walker wrote:

> Hello,
> as for the "Line or Local" issue, you stated that the local side worked
> fine. As in when
> you type on the keys it prints what you type.
>
> A rather simple test is to take a "fresh" 9 volt battery, find the two
> "RX
> lines and connect
> them to the battery, with the TTY in line mode the basket should
> quiet, if
> it does, then
> connect the keyboard leads between one side of the battery and the
> "RX" line
> and try
> typing again. If that works then your interface is lacking in the
> ability to
> provide the
> 20/60 MA required by the TTY.
>
> Jim
> WB2FCN

I found a 9 volt battery in my workshop, and tested it on a multimeter,
and it came out to be 9.02 volts. Perfect! Then, I connected some
scraps of telephone wire to the barrier strip on the teletype, and
hooked the RX lines to the battery. It didn't help, so I switched the
wires on the battery, and that worked. The teletype stopped "running
open". Typing produced no effect. Then I unhooked one of the leads from
the battery, twisted it together with one of the TX lines, and
connected the other TX line to the radio battery. Once again, the
teletype was quiet, and this time, typing produced output on the page.
It looks like everything is working then, I just need to build an
interface that supplies the proper loop current. While I had the
battery connected, I typed three lines in Line mode to test it, then I
switched over to local mode and typed a couple lines, to make sure that
it did, in fact, still work in local mode. It did. But, when I turned
off the teletype, and went to unhook the battery, it was hot! I
unhooked the leads and tested it again on the multimeter - 8.07 volts.
I had no idea that the teletype used that much power just for the
current loop interface. 20mA isn't very much, but I guess for a little
radio battery it is. At least I know now that the teletype works
properly, now all I need is a better interface. Thanks for the help!

Ian Primus
ian_primus_at_yahoo.com
Received on Sat Oct 11 2003 - 07:56:30 BST