Modems and external dialers.

[John Labovitz]

This talk of auto-dialers reminded me of a couple of things from modem culture…

I ran a BBS when I was a teenager in Maryland in the early 80s. We only had one phone line (like most everyone else), but being a BBS aficionado I’d heard of a technique called ‘callback’ that some BBSes implemented, which allowed for using a POTS line for both dial-up modem and for voice. The idea was that if you wanted to dial into a BBS with callback, you’d dial up, let it ring once, hang up, then call back; the modem on the BBS system would only answer on the *second* call. The software (in my case, the BYE modem/IO handler that run on my CP/M system) would watch the ring-detect line on the serial port, and once it went high, it would set a timer for a little more than six seconds (2 for the ring, 4 for the silence). If the timer expired, it would set a flag to answer on the *next* ring — which would have been the second call. If a ring came in before the timer fired, it would ignore the call — assuming it was *not* a computer calling in, but rather a voice call. And if no call came in within a few seconds more, the state would reset for a new call. It worked pretty well, as long as the BBS callers knew how to use callback. After a few months of those shenanigans my parents allowed me to order a second dedicated line, so they wouldn’t be woken up in the middle of the night.

The other thing I remember was discovering variable baud rates. The UART in my Heathkit H89 could be set to *any* rate between 1 and 9600 (?) baud. (Setting it to 1 baud was very educational!) Similarly, my Hayes Smartmodem would auto-detect both local and remote speed. It turned out that even though the modem was rated for 300 baud, it could actually be pushed to 450 or sometimes 600 — and some other modems (like the PMMI 103, a popular S-100 modem) would do the same. So I advertised both 300 & 450 baud for my BBS. 450 felt so fast! :)

—John