Analog modem emulator?

Tony Duell ard at
Tue Aug 2 13:52:11 CDT 2005

> It won't work to just plug the modems together by their phone jacks.  You
> still need line voltage or else they won't work.

That depends on the modems.....

I've not seen schematics for any recent modems, but all the old ones (up 
to 2400 bits/second) used a transformer to couple the audio, that 
transformer being capacitively coupled to the line. There was no DC path 
through that part of the circuit, therefore, so the audio transmission 
shouldn't depend on line voltage.

If the unit uses relays for off-hook switching, etc, then there shouldn't 
be any problem with that part of the cirucit needing line voltage either 
(semiconductor switches might need the line voltage for bias). 

Obviously the ring detector detects ringing voltage from the line.


More information about the cctalk mailing list