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
Received on Mon Jul 21 2003 - 13:15:31 BST
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