Teleco Question... More on my devious plan....

From: Ethan Dicks <>
Date: Wed Dec 1 14:37:51 1999

--- Arfon Gryffydd <> wrote:
> First, Thanks to all who have helped so far...

> I have some old modems (TRS-80, acoustic and etc.) which I would like to
> use (flashing LEDs are cool) so, I want to build a little telco emulator to
> interface with the modems in one of my Linux boxes.
> I figure an LM556 for the dial tone... A tone decoder for dialing... Not
> sure an easy way to decode pulse dialing.

For cheap? Either a stepping relay or some kind of PIC might do it. I
don't know how you'll do it for under $25, though. I have this one relay
that would do it - it looks like a clock face on the back with thirty or
so solder points. There are two relay inputs (110VAC) - step and reset.
With something like this, it would be possible to assign a number, 28, say,
and require that all the digits add up to that number. It would be, in effect,
like a 1920's central office, but with a one-dimensional stepper instead of
a three-dimensional stepper. Those things are fun to watch if you ever get
to see one in a science museum.

Perhaps you could simlate that relay in solid state? If you have a circuit
that will detect tones, perhaps you could run another connection from the
same line conditioners to a series of decade counters. The phone number
wouldn't be software settable, but it might accomplish the task. The trick
is that the phone company designates a minimum time between pulses on the
same number and a minimum interval between numbers. Old dial phones had a
mechanical governor to space the pulses jost so and to prevent the next number
from starting too soon. I'd tell you the timing, but I just don't know. If
I had to, I could hook up a scope/timer to this old dial phone I have.
> As for ring... I am thinking using two charged capacitors and switching
> them. That's the first method I came up with to limit the current cheaply.

The cheap-o boxes I've seen worked by running 110VAC through a half-wave
rectifier and current-limiting resistors. It's not perfect, but there's
lots of tolerance on the part of phone equipment.

My dad used to work for the phone company when he was right out of high
school. Much later, when we were growing up in the 1970's, he had a
box of old phones. We ran wires from my room to my next younger brother's
room and just hooked a couple of batteries in the circuit and had a non-
ringing intercom. I don't recall the voltage, but I think it was only 6v
or 12v. True telco voltage is, IIRC, -48VDC.

> Any suggestions? I'd like to do this for less that $25.00.

I've seen telco simulators made in electrical junctions boxes sell at
hamfests for about that. They didn't do any sort of dialling detection,
though. I used to make a Cadillac telco simulator... I still have a box
of parts and boards. We charged $800 for them. They would do half-
connections, failed connections, fake busy, etc. The only think I ever
used them for was to program an V.24 autodialer driver for our sync
datacomm products. The rig was two of our own MC68K serial boards in the
same BA-11, two Motorola 2400 baud sync modems and this box. Ny changing
the dialed phone number, I could change the behavior in the telco simulator
to force the modems to generate BUSY messages and NO CONNECT messages to
run the dialer software through its paces. It was a hoot.

The cheapest ones I've seen that did anything more than provide voltage and
fake dialtone were around $200.

Good Luck,


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Received on Wed Dec 01 1999 - 14:37:51 GMT

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