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Leo III Digital Clock

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A digital clock that would have been installed in a LEO III computer. This example in our collection is from LEO III/39 which was installed at the Ever Ready factory in London in 1965.

'The Saxby clock on the Standard LEO III (not the IIIF where it was on packages) was inserted under the table shelf on the Engineers' Control Desk so it not easily seen, It took up channel 7, the lowest priority on the Store Access Control.' - Tony Morgan
'In reality it is a digital timer, counting in Hours, Mins. & 1/10th of Mins. (6 seconds). ...it was rack mounted, with a front panel of 25" x 4.1/2 ... Judging from the two plug sockets, there were a lot of inputs to it, maybe outputs as well, with 25 pins each, on C371 & C372. Mains input was separate, although just chopped wires remain, with a plug retaining clip (no plug or socket)'
- Mike Storey

The first LEO III was completed in 1961. This was a solid-state machine with a ferrite core memory. It was micro-programmed and was controlled by a multi-tasking operating system. LEO III was a new development, using new technology like transistors and core storage, allowing it to be much larger in capacity than LEO I and II. It had no magnetic drums or discs. It was a parallel machine with 40 bit adders, data paths and routes to autonomous input / output channels that avoided loading the central processor. Programs were loaded from magnetic tapes and there was an operating system, called the Master Routine, to allocate resources to programs, control the key facility of multi-programming (several programs in store but only one active at any instant in time with each program protected from interfering with another) and to provide an interface for the operators to deal with job initiation and any error conditions.

In 1963 LEO Computers Ltd was merged into English Electric Company and this led to the breaking up of the team that had inspired LEO computers. English Electric Company continued to build the LEO III, and went on to build the faster LEO 360 and even faster LEO 326 models, which had been designed by the LEO team before the takeover.
All LEO IIIs allowed concurrent running of as many as 12 application programs through the Master program operating system. Some were still in commercial use in British Telecom until 1981. Many users fondly remember the LEO III and enthuse about some of its quirkier features, such as having a loudspeaker connected to the central processor which enabled operators to tell if a program was looping by the distinctive sound it made.

Manufacturer: LEO Computers LTD
Date: 1961

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Other Systems Related To Leo III Digital Clock:

Item Manufacturer Date
Leo II Valve Rack LEO Computers LTD June 1959
Leo III Logic Board LEO Computers LTD early 1960s
Leo III Board LEO Computers LTD 1961
55625 Program Wiring Subset Unit from LEO III LEO Computers Ltd c1965

This exhibit has a reference ID of CH70741. Please quote this reference ID in any communication with the Centre for Computing History.


Leo III Digital Clock

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