18 Jun
2020
18 Jun
'20
4:21 p.m.
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). The CCITT equivalent
standard is 1200/75 baud, FSK both ways, and in the 1980s and early 90s
that was used for Prestel and similar systems, including Micronet,
Telecom Gold email, Packet SwitchStream (PSS), BT Tymnet and some online
banking systems, here in the UK. It was also used for Minitel in
France, BTX in Germany and later for Telidon in Canada. Some of the UK
banking systems like HOBS survived using viewdata that way up to the end
of the 1990s, and I still have at least a couple of 1275 modems.
The idea was to use 1200 for the transmission from central computer to
consumer, and the back channel for user responses/commands. Not many
people type faster than 7.5cps.
--
Pete
Pete Turnbull