G'day Guys,
just came across your query about 7181s. If you're still
interested, here's some info:-
These were an ICL-manufactured 2000 character text video terminal used for a variety of
purposes in the early seventies. They were a replacement for the Cossor DIDS units.
While later modules used new-fangled MOS memory, earlier models used a circular wire
accoustic delay-line as the data storage element. In theory, this unit was in synch with
the CRT scanning such that, as the electron beam reached a particular spot on the screen,
the data for the character at that spot would just be appearing at the delay-line output.
The speed of the data through the delay line was notoriously temperature sensative - a
real problem in Australia if the air-conditioning broke down in summer!
The spot scanning method was a little bizarre also - there were only 25 horizontal scans
per frame but there was a small high frequency vertical scan component superimposed on the
normal vertical scan, such during each of the 25 horizontal line scans, the beam
'painted' each character in that line character-by-character as it traversed the
screen. ( Very different from the ~500+ pixel-line scan universally adopted by just about
everyone else!. )
The keyboards were a parallel data design, and those fitted to the early 7181s were fitted
with Hall-effect switch key modules ( I have a logic diagram of one of these somewhere! )
- a real "find" for the hobbyist!
The 7181s on which I worked ( around 1972 ), talked across a synchronous V24 link to ICL
System 4 mainframes. They used ICL's proprietary C03 protocol where multi-dropped
terminals on one comms link could be individually polled for outstanding messages. Thus
the operators were effectively entering data 'off-line' until they pressed the
'send' key, whereupon the next poll to that terminal would result in the date
being transferred to the mainframe.
Cheers
Fred
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