20mA loop is (or, one hopes, was) an interface standard for telegraphs and teletypes. The basic idea is you drive 20mA pulses through the wire; this is quite a high current (although it doesn't sound like much). It needs to be high enough to trip relays on the other end, since real equipment doesn't use fancy technology like vacuum tubes for amplification.

Here's a trick I learnt in the army. Say you've got a 2 wire + ground circuit between your two machines (this is the common configuration). Then you've got 2 wires going a fair distance (say 2 kms) between them. One of them is tx (transmit), the other is rx (receive). Of course, one end's tx is the other's rx. So now you've run all the way across with the cable. The cable consists of 2 identical wires -- how do you tell which one is the remote's tx (and your rx), so you can wire it up correctly?

One way is to use some special equipment. Another is to try it first the wrong way, then swap the wires round.

Here's the third way: strip the ends of the wires (you need to do this anyway). Now get the remote to transmit. Tap each wire gently against a rock. It's a fair bet that the wire giving out the sparks needs to be connected to your rx.

This really works -- I told you 20 mA isn't that low!

Log in or registerto write something here or to contact authors.