2 wire analogue leased line modems

[Thickie]

Does anyone know why when a two wire private circuit is looped the modems do not loop, but they do on a 4 wire one.
Also what is the BT pinouts to run two modems back to back.
I can do it on 4 wire but not two ? help.
I am trying 2 maze/dowty 1496 modems.

 

[svermill]

I can't answer your specific questions about the modems, but a 2-wire circuit is itself a loop. Most analog modems are 4-wire, which allows you to loop tx to rx. A 2-wire modem would necessarily tx in part of the spectrum and rx in another. So your local modem isn't listining on its own tx "frequency." It's listening on that of the remote modem.

 

[svermill]

I guess I should also have mentioned that some modems are half-duplex, where one side transmits and then the other. Of course, that wouldn't work in a loopback either. When transmitting, such modems don't receive.