Back in 1982, hobby state-of-the-art remote data access was 300 baud over POTS using a two cup acoustic coupler
Like that, yes. I spent hours on BBSs with the homebrew equivalent and a DEC VT-100 terminal. Not that many hours, mind, because our local calls were not free.
This is what the professionals used. 1200 baud over 4-wire leased-line (hellishly expensive, back then, the Leasing of the Line).
Nicely built, with the typical-of-the-day bulletproof line interface. I think the plastic resistors carriers are for machine pick-and-place, or maybe someone decided it was more reliable this way? The date code on the transformer looks like close to the end of 1982.
The big chips are
The interesting part is the D2920 Signal Processor. It was available in both EPROM and mask ROM versions, I don't know whether STC ever had masks made or whether they stuck to the EPROM devices. ROM size is 192 words of 24 bits each, and the Program Counter runs through the whole memory over and over again, the clock frequency and the amount of ROM used (there's an end-of-code instruction that loops back to the beginning) determines the sample rate. The crystal in this modem is 6.66666MHz, which is the maximum the 2920 can run at (the mask ROM 2921 can go up to 10MHz).
Each cycle has both a digital instruction (one of 13) and an analogue instruction (one of 8) but not all digital instructions can be used with all analogue instructions and vice versa. I suspect it takes a special kind of brain to actually understand this stuff. Here's an MSc thesis with some sample code.
I wrote a little program for my Expro 80 hoping to read the data from the 2920 but all I get is 0xFFFFFF. I suspect the EPROM did not survive the ravages of time (I also read the 8748 using the Expro software, that gives random data i.e. definite EPROM corruption).
I have no idea why the UA3302 Quad Comparator is on its own ceramic daughterboard.
The manual is stored in a plastic envelope glued to the bottom of the enclosure.
At the end of the era, 4-wire leased lines ran 64kbps and POTS modems were 56kbps to the exchange or 33 kbps modem-to-modem.
And at the time of writing even that is ancient history.
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