Modems and external dialers.

[Grant Taylor]

On 6/5/19 3:08 PM, John Labovitz via cctalk wrote:
> 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.

That's a decidedly different meaning for callback than I'm used to hearing.

I'm used to it being applied in a security context.  As in you would 
call into a system, identify yourself, hang up, and then it would call 
back the number stored on file for the account.  So calls going in 
opposing directions.

I guess this could have also been used to alter which end paid for 
calls.  The first call would be short and relatively inexpensive.  The 
second call (back) would be longer and could be expensive and take 
advantage of better long distance rates as applicable.

> 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.

Intriguing technique.



-- 
Grant. . . .
unix || die