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

From: Manu Abraham
Date: Sat Dec 18 2004 - 02:49:28 EST

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:
> >>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.
> 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.


> analogue signal. Nothing more than an audio transformer linkage.
> Now, sometimes a modem needs coaxing to ignore the lack of dial/call
> progress tones, but they should always talk to each other regardless of
> line voltage.
> ATA on one end and ATD on the other will normally get them talking.
> As a test I just looped my internal AMR winmodem to my Xircom Realport V90
> modem and got a solid 28.8k link. No problem.
> If the fluid is salty enough you could probably get analogue modems to talk
> over wet string (I have certainly passed RS485 over wet string before).
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