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

[David Lawyer]

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 <http://www.jagshouse.com/modem.html>.  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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