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Connecting a pair of Dialup Modems

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@rt:
Hi Guys :)

I’m trying to connect two terminals together, each with their own 56k dialup modem, and an ordinary phone cable connected between them.
One is an old XP laptop running Win XP using the Hyperterminal program, and it’s internal modem, and the other is a 56k portable travel modem.

Both modems can echo back to their respective terminal programs, and both also respond to AT commands with “OK” etc.

I’ve had no luck getting them to connect though. There are a few small articles about, and a YouTube video demo of the same,
and they all use ATA for the terminal doing the answering. The command used to dial varies from a simple “ATD” to something that might cause
the modem to ignore the fact that there is no dial tone without a real phone exchange. Some examples are “ATX3D” or “ATX1D”.
I hear that the function of the “X” command can vary between modems. I can get both modems to make noises by answering,
and one modem to make noises by dialling with some variations of the “ATD” command. It doesn’t matter if I have to dial with one of them all the time.

Ultimately, they both quit with a “NO CARRIER” message, and I have never heard anything that suggests to me they are responding to each other.
Is there a physical (electrical) reason for this (assuming everything is working), or a possible incompatibility that would prevent them even connecting to each other?
Cheers, Brek.

CJay:
Seem to remember 56K modems won't work back to back, at least not at 56K, as they rely on some trickery with the digital telephone exchanges so you might need to persuade them to connect at a slower speed.

But, first, are you expecting them to connect over an actual phone line or just a piece of phone wire?

If they've not got the relevant voltages on the line then they may just not work at all.

@rt:
With a straight phone wire. I have seen this done on YouTube, and know the fellow who did it.

56k should connect at 33.6 or so if that’s the uplink speed of each I assume.

bd139:
You can do this with older Hayes modems. I had a 28.8k building to building ppp link running for several years on some Hayes Optima modems over one spare pair. ATX1D (blind dial no busy detect) Needs to be configured or it’ll wait for a dial tone. That had Pegasus mail running over it (yuck) as part of a supposed loose air gap.

It should negotiate 33.6k max but no guarantees with 56k/v90 cack. Depending on the chipset you might be able to turn that off and negotiate 33.6k max.

Berni:
The phone line injects power into the wires. That's where phones get there power from.

You can get two phones to talk to each other by connecting a 9V battery in series with one of the phones terminals before connecting them with together with a phone cable. The battery pushes current around the loop to power both of them, munch like the telephone exchange would. Audio is transmitted by varying how much current is drawn from the line.

Perhaps the battery trick works with modems too in order to trick them into thinking they are on a real line. Hopefully they don,t also expect to hear a dial tone too, but perhaps that can also be bypassed by simulating the line ringing by injecting 100V AC on the line for a short moment. When you pick up a ringing line you don't get any dial tone, it's straight to audio(apart from the caller id signal nowadays)

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