Modems and external dialers.

[Eric Smith]

On Tue, Jun 4, 2019 at 8:30 PM allison via cctalk <cctalk at classiccmp.org>
wrote:

> Keep in mins the hardware for auto dial required some for of micro and
> that was a post 1974 thing for the most part.  A few before that had a
> lot of TTL state machine to do that.  They obviously weren't cheap.
>

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.