How does TTY work?

[The_Harlequin]

I know it is 20mA current loop, but I don't understand it. I have a TTY-port like this:

TTY IN+
TTY IN-
TTY OUT+
TTY OUT-
20mA
20mA

If I for an example want to loop this, how should I do? The function I want is if I send ASCII characters on the send line I will receive the same characters on the receive line. Please can anyone explain how it works?

 

[bernie_carlton]

This paper from BB Electronics may help

 

[danw]

Thanks Bernie.

Two pairs of wires for full duplex - each end has a receive and transmit.