Messages in this thread | | | From | Manu Abraham <> | Subject | Re: Issue on connect 2 modems with a single phone line | Date | Sat, 18 Dec 2004 11:45:59 +0400 |
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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). - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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