Modem to modem without a line simulator

ard at ard at
Mon Dec 31 02:47:11 CST 2007

> This question is bugging me from another forum.
> Is it possible to connect two modems (eg: Hayes 2400 to Hayes 2400) 
> using a 'dead' or isolated pair of copper wire and have them be able to 
> communicate?
> I always thought that this can not work because the line that the modems 
> are connected to has to have some current.  The phone system works 
> because an action on one end of the phone (talking into the carbon 
> microphone) causes a reaction on the other end.  Without some sort of 
> current on the line, how can this work?  Hence the need for 'line 
> simulator' circuits

Most direct-connect modems do not require any power from the telephone 
line -- in fact the modem circuitry is coupled to the line through an 
isolating transformer, which is iteself capacitor-coupled to the line. So 
the DC conditions on the line have no effect on the modem circuit

Of course such modems do 'loop the line' (provide a DC path between the 2 
line wires) when off-hook. But that's to tell the telephone exchange that 
the modem is indeed off-hook, not for any particular requirement of the modem

So if you can get onee modem to ignore the lack of dial tone/rigning tone,
andthe other one to answer without seeing a rining voltage on teh line,
then just connecting them together should work. 

I think there have been a few -- a very few -- modems that do requeire a 
DC voltage on the line for correct operation. Foe those, you can often 
fake it by connecting a suitable DC supply in series with a suitable 
limiting resistor between the line wires.

> Some people are claiming that it works without the line simulator.  I'd 
> like to understand why.
> (I could get out the multimeter if things get really desperate, but 
> maybe somebody can tell me that the line current thing only applies to 
> acoustically connected modems, not direct connect.  Or the direct 
> connect modems put enough juice on the line to make it work.  Or 
> something else that might make sense.)

Telephones, of course, do draw power from the line, which means 
accoustically-coupled modems need line power for the asscoicated telephone.


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