I have a situation where I'm trying to connect two computers via their dial-up modems. I've built a line voltage simulator (9v @ 40ma) into the line that connects them, and I've tested with a telephone to listen and make sure the modems generate DTMF tones, and by pressing keys on the phone, that the modems can receive DTMF tones.
The problem is that when I connect both the modems together, the receiving modem doesn't detect anything from the transmitting modem. There must be some difference between the tones/voltage/frequencies generated by a normal telephone, and those generated by the source modem.
I'd like to measure the DTMF tones generated by the phone, and those generated by the source modem so I can compare them and perhaps isolate the problem. But I'm not sure how to measure a telephone line with an oscilloscope.
The line is made up of a tip-and-ring pair, how can I get a nice measurement of the audio on the line?
From what I understand it should work just hooking up the wires to the probes - at least by using the scope's math function and subtracting one channel from the other. The problem is, I never get any signal at all, except background AC noise. The main things I don't understand is what to do with the ground probe, as the cable doesn't have a ground, and if there can even be any signal without a device attached on the other side of the cable to complete the circuit between the two cables