> > I am developing (it is under construction to add additional features that I > need) an > Arduino-based central office simulator that will allow two devices to connect > to each > other as though connected by POTS lines. I began developing it to simplify > work in the > shop on a variety of broadcast equipment that uses modems, as the only phone > line > at the house is a cable-modem phone line that does not work with dialup > modems.
Some time back I was thinking of making something similar, although to me it's just a simple state machine and doesn't need a microcontroller. Heck, originally it was done by the telephone company using relays. I don't like using thousands of components when a couple of dozen will do :-) But the control circuit is the easy part, it's things like the power supplies, ringing generator, etc that are the major part of the work. The ringing voltage in particular is a pain, it's AC, and at a fairly low frequency (16.67Hz to 25Hz depending on country). Some devices, particular classic telephones with a mechanical ringer, do not work correctly on a 50Hz or 60Hz ringing voltage taken from a mains transformer. After sketching out a design, I realised that the cost of the main bits (line switching relays, the power supply for the ringing voltage, the 48V PSU for the line 'battery' voltage, etc was comparable to a cheap ready-build device to do much the same job. Of course the ready-built one would be difficult to keep going so I probably wouldn't go that route But anyway, telephone line simulators do turn up on Ebay and I was lucky enough to find a non-working one for (I think) $25. I figured that even if I couldn't fix it it would contain useful bits for power supplies, etc. After fixing the mains switch and reseating the socketed ICs it sprang to life and almost worked. After replacing a couple of LM311 comparators and a 3 terminal regulator it was perfect. This thing is _complicated_. IIRC there are 7 microprocessors/microcontrollers in it. Around 400-500 ICs spread over 3 main PCBs and 7 smaller PCBs (some of which are just switches or connectors). It does do a lot though, it will generate the correct call progress tones and ringing voltage for just about any countries system. It will deliberately introduce noise (from a shift register with XOR feedback) or degrade the line (handled by a classic DSP -- a TMS320 IIRC). Do I need all that for fooling around with old telephones and modems? No. But it was actually the cheapest solution and it is a nice, classic, piece of test gear. -tony
