Current Loop Adapter wiring

[Dwight Elvey]

The input transistor would turn off if you short the leads. The DC voltage you read
is about right, both in voltage and polarity..
There is not enough voltage in the loop to cause the optical coupler 4100 to operate.
You need to have a separate loop supply. A DC wall wort, a 1000 uf cap and a
resistor to set the current limit, would most likely work.
The resistor would depend on the wall wort voltage. I'd look for a 12 volt one.
Put the capacitor across the leads ( watch polarity ).
Say, after the capacitor was on, the voltage was 15V. I'd try a 750 ohm resistor
first in series. Break the loop someplace and put this in there.
I suspect you'll find things will work as originally suspected.
Dwight

 

[NeXT]

Oh my god I'm so f$&%ing frustrated right now.
[forget the editing. I'm six different ways of pissed off right now]

I'm sorry. This is really getting on my nerves.

 

[NeXT]

Took a week break to sort some things out and wait for some voltage regulators and resistors to come in.
I now have the loop adapter hanging off a breadboard which powers the loops with the following circuit:

Finally, the teletype shuts up and can receive (hurrah!) but transmitted data as corrupt.

That should be "the quick brown fox jumped over the lazy dog" but most of the characters are wrong. I forgot that while the model 3 can input 8-bit ASCII it only outputs 7-bit. After changing the data bits I'm sending and receiving now. I'll just add the power circuit to the adapter but yeah, in summary:

-Model 33 teletypes DO NOT power the send or receive loops
-Optocoupled current loop adapters do NOT power the loops by default
-You CAN power the loops using +12v DC and two 470 ohm resistors