Modems: An Odd Application

Although this may be counterindicated, can two modems be connected directly without going through a phone switch (PBX)? I mean, can there be a dedicated line between two modems, like a local connection? This may sound odd, but can two modems talk when DIRECTLY connected?

I’ve tried it a couple of times and never had any luck with it. Even if you force both modems off hook they don’t seem to talk without the voltage and signal the phone line provides.

I’ve always wound up using something like this Phone Line Simulator:

I think this used to be called a “null modem”, and was one early option for multiplayer gaming.

Actually it was a cable that plugs into the two serial ports in place of a modem, and has the relevant lines in the RS-232 cable crossed over so the transmit line from one went to the receive line of the other. It’s called a null modem cable because in technobabble null means “there ain’t no modem”, I have one, works fine. You have to buy or make one to do it. The modems themselves need phone company signals to communicate. On the other hand, you could whip up your own phone company “2 line central office”. The cable’s cheaper:p

ETA: I didn’t notice that “phone line simulator” comment. That’s what I suggested you could whip up. As I said, the cable’s cheaper…

Ah, that would explain why I could never get it to work, then.

Actually, they can, but not your general purpose consumer grade modems.

The consumer grade modems are designed 99.9% of the time to be used only only the POTS lines, and so are pretty much tied to them. Without the signaling, and line quality determination, they will generally just sit dumb and stupid.

If you need more then just a null modem cable for anything (as in, if you need a longer range serial connection), google for “Short Haul Modem”, which will get you to the non POTS dedicated industrial type modems.

Alternatively, you can use your general consumer modems, along with a POTS Simulator… Simulateur de ligne RTC et interconnexion R&D - FREELINK

Your terminology is a bit off here, most modems aren’t connected to a PBX, they are usually connected straight to a phone line. A PBX is a private exchange, not part of the public telephone system.

Very few modems would be able to talk to each other directly, because the telephone system is not symmetrical. A modem would typically only implement an FXO interface, and not FXS. I don’t think there would be any physical reason a modem couldn’t implement FXS, but it would require custom firmware.

You’re talking about a line driver, AKA a short-haul modem.

You’ll need a line simulator and you’ll need to put in the correct AT commands:

Typically, in the age of modems you wouldn’t do this, but instead would use the faster serial or parallel interfaces to connect machines. A serial port, even an old one, can do 115200 bps, which is going to be 3-4x what you sync up with those models over a purely analog transport. The only hardware you need is a null serial cable.

Which is also known as a null modem cable, which I described previously.

As other posters after my last have said, that won’t work if you have to go farther than an RS-232 serial port is good for. Longer cable = weaker signal at the ends. If you have to go farther than about 50 - 100 feet, use their suggestions. I was assuming something like a 2-player 2-machine game type application, in which the machines would be in the same room. Even 100 feet is really pushing it, but would probably work at somewhere in the neighborhood of 1/4 the max speed the computers can communicate thru their serial ports. The slowdown is due to the need for resending data that gets garbled. If you are using modern machines USB can connect them, although you will have to look to someone else for the “how to”, since I’ve never done anything at a machine level with USB.

If distance is an issue (which I dont see mentioned in the OP) then even when modem was king you’d go for 10base5 or 10base2 or some other networking scheme instead of an elaborate super-slow modem setup. Modems are designed for POTS system and make poor choices for local data transfers.

With modems over an analog line simulator device youre maxing at 28kpbs or so. With 1980s networking equipment youre at 10mbits.

Also, id be concerned about noise/distance over a line simulator device, especially if youre running untwisted plain-jane phone line for long runs.