Using a teletype as keyboard

I'm driving a Siemens T100 teletype machine (TELEX) by Arduino Mega. The PWM signals off pin9 are fed into an AN/URA17 converter/comparator, which is in series with a 60v/40mA current loop.

I've seen lots of RTTY programs driving a telex, but I can't seem to find how to use the telex as a keyboard feeding the arduino.

Being a noob, I guess I have to get rid of the current loop, or decrease this to 5V analog input on the arduino pins to get the 5bit Baudot code.

Any guidance to this project is welcome

TIA

The keyboard normally has outputs on a changeover set of contacts. The idea is that you feed negative and positive to the contacts and then the wiper gives an RS232 type signal that is easy to turn into TTL. Have a search for the publication "RTTY the easy way" from BARTG. You will find it very helpful.

It has been a long time for me but they were always operated in the current mode. We made adapters to use them with RS232. I am not familiar with your particular unit but we used similar looking units as terminals before the TV Typewriter. The ones I used were ASCII based although Baudot units were also very common. If you go back to antique computer age late 70's to mid 80's there were a lot of articles on this along with schematics for the adapters. The baud rate was 110, I assume this unit is as well. We used the paper punch and reader to save and load code into our microprocessors. This was very common with the Imsai 8080, Altair 8800 and comparable systems of the time. This link has a lot of good information and a source of the interface you require.

I used a real Teletype machine(ASR32, I think) with my SYM-1 single board computer. I think an opto-isolator with your current loop will let your Arduino see the signal on a digital pin. You will need to current limit for the LED in the opto-isolator.

Paul

IS the OP aware his mechanical Baudot machine also has a start bit and a stop bit? So, 5+2 = 7 bits. AS I recall from 50 years ago, the stop bit is 1.5 bit length to ensure the mechanical stop has time to engage.

Paul