ASR33 Teletype interfacing

From: James M. Walker <chejmw_at_acsu.buffalo.edu>
Date: Sat Oct 11 12:52:36 2003

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
 

Ian Primus wrote:
>
> 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 - 12:52:36 BST