Simulated telephone

Brent Hilpert hilpert at
Tue Aug 24 12:51:34 CDT 2010

On 2010 Aug 24, at 8:51 AM, Jim Scheef wrote:
> I would like to play^H^H^H^H use some of my vintage portables the way 
> they were intended. For example, the Radio Shack Model 100 and the HP 
> 110 Portable have 300bps internal modems. I have several other modems 
> thru which these machines could talk to a *NIX but I do not want to 
> pay for multiple phone lines in my home. How can I simulate a POTS 
> connection between these modems? I don't need a dial tone or ringing, 
> etc., as I can just tell one machine to connect and the other to 
> answer. Way back I heard that a few modems would work if the tip and 
> ring wires of the modems were crossed and connected, but most need 
> something more than that. Is there some simple circuit I could build? 
> Surely I'm not the first to want to avoid phone bills for such 
> play^H^H^H^H testing.

If both modems in a given connection are older ('80s), lower-speed 
(300-1200/2400) it will likely work by simply connecting them together 
and commanding one off-hook in answer mode (ATA ?), then the other 
off-hook in originate mode (ATD ? ATH1 ?). I have successfully done 
this with particular modems up to 14.4Kbps but might be forgetting the 

The older, lower-speed modems are likely to be transformer-coupled onto 
the line and will inject audio energy onto the line without need for DC 
line current. Some higher-speed modems do this as well and will work, 
other higher-speed modems use opto-couplers and need the DC line 
current to modulate.

You shouldn't need to worry about 'crossing' tip and ring. Technically 
it is more correct to connect tip-tip and ring-ring, although it is not 
likely to matter one way or the other. At the electrical level it is 
not a null-modem-type situation where one has to swap xmtr-rcvr 

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