Modem Quick Question

ok, you know how if you have to phones in a house, on the same line and if you’re talking to a person through one phone the another person can pick up the phone and listen in, and converse also. Is it possible to do this with a modem? Say, have two computers in the same house connected to the same server through the same line? say, just have the modem “pick up”? I hope this makes sense. cheers. ps. no broadband responses.

WAG: One of your computers would be getting (and discarding) unrequested data or neither one would have a clear connection. I’m betting on the former.

      • No, with regular software it is not possible to operate more than two modems on a phone connection (that is, one on each end). The reason is because every time one sends something, it “asks” the other if the something was received without error, and each modem is only set to normally “assume” that there is one other modem on that line. Regular modems are not set up to allow managing more than one set of communications at once. If you -could- do this I do not know.

I have my modem to dial no matter whether the line is busy. Isn’t there some command line to just pick up the line, like how hyper terminal pick ups the line if someone’s on the line?

while you can’t use two seperate computers modems with the same phone line, you can SHARE one of the modems between the computers using windows “Internet connection sharing” software which ships with winme 2000, and xp.

You need to connect the computers through a lan card or USB but then the computers can each use the modem independently though they will be splitting the bandwidth. This works much better with dsl or a cable connection but is very usable even with dialup.

They also have modem routers that can share a 56k connection. Probably not worth it unless you have a small business and occasionally send and recieve small files via FTP or something related that doesn’t require Broadband or T1-3 connection, or ISDN.

Yes, it is possible. I did it back in the 70s and early 80s using 300 and 2400 baud modems. However, I can’t say if it is workable with today’s modems. It depends too strongly on the programming of the unit. In my experience, external modems (connected to the PC via a serial cable) are far more likely to implement the full extended DC Hayes command set (IIRC, that’s the original name; the current name for the AT ccodes eludes me - am I showing my age?)

Don’t expect a clear transcript with an unmodified modem (usually modified firmware, not hardware) and specialty software – which isn’t that hard to write, if you have a copy of the protocols, and which is probably available in the source code of a lot of old private and full-featured commercial software (of the era when I did it) This technique had numerous uses when modems were used as much for for equipment, sensor, or private (often hard-wired) ad-hoc networks than BBSs and remote dial up. (BBSs didn’t even exist until ca 1978) The source for programs that do exactly what you want was once available in many public archives, and should at least give you an idea of how to configure your hardware settings.

There are issues, however. First, your modem missed the handshake negotiation, so your software/firmware would have to silently deduce the agreed settings by listening. (Of course, if your modem changes the local line load enough by ‘picking up’, it may force a renegotiation which it can eavesdrop on) Your software will have to be written to utilize the settings the other modems negotiate, rather than participating in the negotiation, as all common software expect today.

Unfortunately, your chances of picking up a communication at 300 (or the harder to eavesdrop 2400) baud are pretty slim these days. I can only guess that the task is trickier at ca. 50K. At those speeds, you’re pushing the envelope of the phone line (in those analog protocols, anyway), so errors and retransmits are rather common. When a negotiating modem “mishears” something, it can ask for a retransmit, but when yours “mishears” it’s stuck. You’d be surprised how much info can be garbled by a relatively small “mangled packet” rate sometimes - even in text transmission, Murphy dictates hat you’ll miss many of the most important bits – it’s adequate for a project or last resort, but its unlikely to be a wonder device.

It’s really not mysterious, and it would probably make an excellent educational project. However, I wouldn’t expect to be able to do it without some knowledge of programming. The code is straightforward, as are the protocols you’ll be implementing (though they’ll seem mind-boggglingly perverse until you’re done), so it’s “doable” – but make no mistake, it’ll be a challenge.

The necessary “primitives” (subroutines and functions) are probably already in your firmware, but frankly with todays fast processors, I personally would do as much work as possible with my own code on the system CPU rather than inside the modem. A modem guru would (of course) think I was an idiot, and I wouldn’t be surprised if the hackers and phreakers have long since written flashable BIOSes for their preferred modems (e.g. US Robotics Courier) that handle the entire process transparently.