Messages in this thread | | | Date | Thu, 16 Dec 2004 00:58:29 -0800 | From | David Lawyer <> | Subject | Re: Issue on connect 2 modems with a single phone line |
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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.
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