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

From: Bill Davidsen
Date: Sat Dec 18 2004 - 10:19:59 EST

Manu Abraham wrote:
On Sat December 18 2004 11:01 am, Brad Campbell wrote:

David Lawyer wrote:

On Thu, Dec 16, 2004 at 02:01:38AM +0100, Pavel Machek wrote:


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.

I have used analogue modems back to back for years and have *never* come
across a modem that sourced anything other than it's ringing signal (via an
opto) from the phone line. Every single modem I have here will talk to the
others over a straight telephone cable.

What about power ? The opto-coupler will not work without power.

Analogue modems use a line transformer to couple to the phone network
usually with a decoupling capacitor on the phone end of the network to
prevent large current flows through the transformer. They use a standard AC

The capacitor is used to prevent DC saturation of the transformer core rather than doing current limiting, A capacitor cannot do current limiting. When the lag changes by changing the capacitance value, general concept is that a capacitor can limit current which is very much wrong.

I think you're quibbling over terminology here, blocking the DC component with a decoupling capacitor does result in less current even though it's not "current limiting" in some strict sense of the term. Less voltage results in less current, even if the load on the other side were purely resistive. None of this is germane to the original discussion of using back to back modems, of course.

bill davidsen <davidsen@xxxxxxx>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
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