Subject: Old 7181 terminals

From: "Allen, Fred" <[email-masked]>
Date: Mon Jul 21 17:25:33 2003

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