Re: Extending the range of null modem to 100ft.

[Eric Smith <eric@brouhaha.com>]

Andy McFadden <fadden@fadden.com> writes:
> If you take two standard phones and plug them together, nothing happens.
> There's no power on the line, and you can't get enough out of a piezo-
> electric microphone to drive a phone signal.
> 
> For this to work, one or both modems must be supplying current on the
> phone line.

Depending on the modem design, they may not *need* any DC current on the
line.  This is especially common with modern high-speed modems; they
have to go to a lot of trouble to avoid passing DC through their
DAA transformer, because the DC causes distortion that severely affects
high-speed modulation and demodulation.  Almost all modems now run
the transformer "dry", or use capacitive coupling instead of a transformer,
which also blocks DC.

Of course, when doing that they have to have the off-hook relay (or
optoisolator) cause some current drain on the line to get the phone company
to recognize that the line is off-hook, but two back-to-back modems won't
care about that.

Naturally you have to tell the modem not to wait for dial tone, ringing,
etc., since you won't get those.

Eric