From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261749AbULPQzh (ORCPT ); Thu, 16 Dec 2004 11:55:37 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261744AbULPQyA (ORCPT ); Thu, 16 Dec 2004 11:54:00 -0500 Received: from zoot.lafn.ORG ([206.117.18.6]:41476 "EHLO zoot.lafn.org") by vger.kernel.org with ESMTP id S261749AbULPQx2 (ORCPT ); Thu, 16 Dec 2004 11:53:28 -0500 Date: Thu, 16 Dec 2004 00:58:29 -0800 From: David Lawyer To: Pavel Machek Cc: Park Lee , linux-kernel@vger.kernel.org Subject: Re: Issue on connect 2 modems with a single phone line Message-ID: <20041216085828.GG1189@lafn.org> References: <20041215184206.43601.qmail@web51505.mail.yahoo.com> <20041216010138.GC6285@elf.ucw.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20041216010138.GC6285@elf.ucw.cz> User-Agent: Mutt/1.5.5.1+cvs20040105i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org 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