Modems and external dialers.

On Tue, Jun 4, 2019 at 8:30 PM allison via cctalk <cctalk at classiccmp.org> wrote: AFAIK the first commercially dialer for data use was the Bell 801A Automatic Calling Unit from 1964. It didn't contain a state machine any more complicated than a four-bit counter to send the appropriate number of dial pulses, though in 1964 even that was a fair bit of circuitry. As with other Western Electric equipment, it wasn't cheaply made, and also was not inexpensive to rent. The only thing preventing other companies from making less expensive alternatives was that customers were still not allowed to connect their own equipment directly to the PSTN at that time. The host computer presented one digit at a time to it over a four-bit parallel interface (using either EIA RS-232 levels or relay contact closures, depending on model). The 801C from 1965 used DTMF rather than pulse, and somewhat ironically its innards are actually _simpler_ than the 801A, because it doesn't need the four-bit counter. I suspect that it generated the DTMF using almost the same circuit as the original DTMF encoders in telephones, with a single transistor and multiple inductors. However, unlike the telephone, it needed circuitry to decode the four bit BCD input from the customer equipment to the two-of-four selections for the DTMF encoder.