How does TTY work?

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?

This paper from BB Electronics may help

Thanks Bernie.

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