30 Dec
2005
30 Dec
'05
4:17 p.m.
Anyone remember the old Bell 103 modems? IIRC, the
design was something
like a big pile of passives and only a couple of transistors. But then,
300 baud was as good as they got. WE was pretty good at designing circuits
that used the minimum of active components; IIRC, the DTMF touch-tone
dialing circuit was a single transistor.
I thought the touch-tone keypad was 2 transistors, one for the column
frequencies, one for the row frequencies. But I've never seen one.
As regards old/odd modems. The GPO 300 baud Modem (Modem 2B IIRC) was a
metal case about 15" square and 6" high. Inside were 4 modles that slid
in from the font. One was a PSU, one was the modulator, one the
demodulator, and the last the control. The Control circuitry was relay
based with a few transistors.
The demodulator was a bit odd by today's standards. The incoming signal
was filtered, then mixed with a local oscillator to shift it up in
frequency. That was fed into an FM discriminator (I can't remember which
one, it was one of the standard ones that I've seen in FM radios), the
output of that went to a schmitt trigger and thesce to the RS232 output.
The filters were a string of LC tuned circuits contained in a flat metal
can about the size of a tobacco tin.
I have a much more modern modem which, IIRC, does 1200 baud full-duplex,
and uses a 8088 as the DSP (!). And a leased-line modem stuffed with
interesting logic (some AMD 2900-series bit-slice parts and an 8*8
multiplier chip from what I remember)
-tony