From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262849AbULRHrO (ORCPT ); Sat, 18 Dec 2004 02:47:14 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262847AbULRHrO (ORCPT ); Sat, 18 Dec 2004 02:47:14 -0500 Received: from penta.pentaserver.com ([216.74.97.66]:54410 "EHLO penta.pentaserver.com") by vger.kernel.org with ESMTP id S262852AbULRHqn (ORCPT ); Sat, 18 Dec 2004 02:46:43 -0500 From: Manu Abraham Reply-To: manu@kromtek.com Organization: Kromtek Systems To: Brad Campbell Subject: Re: Issue on connect 2 modems with a single phone line Date: Sat, 18 Dec 2004 11:45:59 +0400 User-Agent: KMail/1.6.2 Cc: David Lawyer , Pavel Machek , Park Lee , linux-kernel@vger.kernel.org References: <20041215184206.43601.qmail@web51505.mail.yahoo.com> <20041216085828.GG1189@lafn.org> <41C3D5AD.7090507@wasp.net.au> In-Reply-To: <41C3D5AD.7090507@wasp.net.au> MIME-Version: 1.0 Content-Disposition: inline Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <200412181145.59211.manu@kromtek.com> X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - penta.pentaserver.com X-AntiAbuse: Original Domain - vger.kernel.org X-AntiAbuse: Originator/Caller UID/GID - [0 0] / [47 12] X-AntiAbuse: Sender Address Domain - kromtek.com X-Source: X-Source-Args: X-Source-Dir: Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org 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. Manu > 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).