Modem to Modem without a line simulator? - test-drb@ccmp.vtda.org

31 Dec 2007

...
   The current is used for the carbon mike and to tell
the
 phone system that the phone is off the hook. Line connected
 modems don't have any use for the current and only have
 a load resistor there to keep the line connected.  
Something I forgot about earlier :

I think the GPO Modem 13A (A plinth mounted under a normal dial-type 
phone) could be strapped to draw its power from the line.  I don't know 
the full deatails, I do have one of these modems (300 baud CCITT 
oriignate only), but I have no diagrams other than the connection lable 
stuck inside. But there's certainly a transformer-isolated PSU circuit on 
the board, and one setting of the straps seems to connect the input of 
this to the line

-tony