> On Jun 18, 2020, at 7:21 PM, Pete Turnbull via cctalk <cctalk@classiccmp.org>
> wrote:
>
> On 18/06/2020 21:31, Paul Koning via cctalk wrote:
>
>> I did see something vaguely similar. Bell 202 modems are 1200 baud FSK, so
>> on a voice channel they normally are 1200 bps half duplex. They can also be
>> hooked up to 4-wire fixed circuits. But they have a reverse channel, good
>> for 150 baud if I remember right.
>
> IIRC the Bell standard allows for only 50 baud and the back channel uses ASK
> (basically switching a carrier on and off).
That rings a bell (so to speak). Chances are those specs were all pretty
lenient and the 126 bps used by PLATO was non-standard but not a real problem.
I don't know how common those connections were or what distance was needed. A
large number of PLATO terminals were fed via a microwave video signal. That
was a pretty neat setup: all 1008 terminal lines, TDM muxed into what looked
like a video signal (so 60 frames per second). Devices called "site
controllers" would receive that and extract the data streams for 32 terminals
from that data. They'd also accept the 126 bps terminal->host data stream and
stat-mux it into a single data stream going to the host, I don't remember
seeing the format for that.
The data center had a TV monitor that showed the outgoing signal. It looked
vaguely like a punched card with a lot more than the usual number of holes in
it.
paul
