Re: Issue on connect 2 modems with a single phone line

From: David Lawyer
Date: Thu Dec 16 2004 - 11:59:18 EST

On Thu, Dec 16, 2004 at 02:01:38AM +0100, Pavel Machek wrote:
> Hi!
> > I want to try serial console in order to see the
> > complete Linux kernel oops.
> > I have 2 computers, one is a PC, and the other is a
> > Laptop. Unfortunately,my Laptop doesn't have a serial
> > port on it. But then, the each machine has a internal
> > serial modem respectively.
> > Then, can I use a telephone line to directly connect
> > the two machines via their internal modems (i.e. One
> > end of the telephone line is plugged into The PC's
> > modem, and the other end is plugged into The Laptop's
> > modem directly), and let them do the same function as
> > two serial ports and a null modem can do? If it is,
> > How to achieve that?
> You'd need phone exchange to do this. Most modems will not talk using
> simple cable. With 12V power supply and resistor phone exchange is
> quite easy to emulate, but...

Here's what I once wrote in Modem-HOWTO:

Most modems are designed to be connected only to telephone lines and
will not work over just a pair of wires. This is because the
telephone company supplies the telephone line with a 40-50 volt DC
voltage which powers part of the modem. Recall that ordinary
conventional telephones are entirely powered by the voltage from the
telephone company. Without such a DC voltage, the modem lacks power
and can't send out data. Furthermore, the telephone company has
special signals indicating a ring, line busy, etc. Conventional
modems expect and respond to these signals.

One way around this is to make a simple power supply to emulate a
telephone line. See Connecting two computers using their modems,
without a telephone line <>. In
most cases there are better way to connect computers together such as
using network cards or just cables (null-modem) between the serial
ports. Using modems has the advantage of increasing the distance as
compared to a null-modem cable, since it's using a twisted pair. But
it isn't nearly as fast as network cards.

David Lawyer
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