Teletype services

tony duell ard at
Thu Nov 5 15:12:48 CST 2015

> 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.


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