Modem Collection

From: Tony Duell <>
Date: Fri Jul 11 17:54:54 1997

In message <> writes:
> Just curious (again!) if anyone is collecting the early modems used on
> computers and what might have been the earliest commercial phone modem.

I've got a few (I'm interested in telegraph and telephone history as well as
computers, but my main collection is computer-related). It's not a large
part of my collection, though.

In the UK, modems were _rented_ from the GPO (== post office, who also ran
the telephone system) at one time, like all other telephone equipment. Some
of these modems are interesting technically - the earlier ones were about
14" square and 6" high, and contained plug-in modules for the PSU, modulator,
demodulator, and controller. It was all discrete components (no chips), and
some of the control used PO 3000-style relays (similar to the ones used in
telephone exchanges). The demodulator was strange. It filtered the incoming
signal (a multi-stage LC filter housed in a tobacco-tin sized box!), and then
mixed it with a local oscillator to shift the frequency up. The output of that
was filtered again, and fed to a 2-diode discriminator. The output of that
was buffered and became the data output. It was basically a superhet FM
receiver modified to work at the right frequency.

Another strange GPO modem was the 13A. It's a 1" (approx) high plinth that was
fitted under a Type 746 'phone that had extra 'voice' and 'data' buttons
fitted. I know nothing about the circuitry in that one, but it's a rather neat

There were also things called 'isolation boxes' that were used to prevent
faults in the user's equipment damaging the modem (or worse still the 'phone
line). Typically they contained a number of zener diodes from each line of the
RS232 connector to ground (2 diodes in inverse series per line), and some 50mA
fuses in series with each line that would blow if there was overvoltage on that

Such things are probably useless now, but I like to have all the bits of a
classic setup, including little details such as this.

> I have several devices that are called modem eliminators and I believe

The term 'modem eliminator' normally meant 'null modem cable', and was a
passive device.

Received on Fri Jul 11 1997 - 17:54:54 BST

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