31 Dec
2007
31 Dec
'07
12:47 a.m.
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.
-tony