Rediscovered plans aid Edsac reconstruction

Edsac chassis and valves Image copyright John Robertson
Image caption The rebuild project had few plans to work with when it began

Detailed circuit diagrams for one of Britain's pioneering computers have come to light during a project to reconstruct the machine.

The 19 large-scale diagrams were drawn up when the first Edsac machine was being built just after World War Two.

Edsac was created at Cambridge University to help the institution's scientists analyse data.

The plans will aid the reconstruction project which started with only scant information about Edsac's innards.

Bug fix

Designed by Sir Maurice Wilkes, the Electronic Delay Storage Automatic Calculator (Edsac) was the first machine created specifically to act as a computational resource for experimentalists.

Unfortunately the post-war effort to create Edsac left relatively little information about how the original design changed as it was put together. Also, relatively little of the original machine's hardware has survived until the present day.

"Very few artefacts of Edsac remain," said Dr Andrew Herbert who is leading the reconstruction project. "However, these papers give a clue as to why a few, such as a chassis, do exist."

The diagrams were handed over to the reconstruction team by John Loker who started work as an engineer in the University of Cambridge's mathematical laboratory in 1959 - soon after Edsac was turned off for the last time.

"In a corridor there was a lot of stuff piled up ready to be thrown away, but amongst it I spotted a roll of circuit diagrams for Edsac," said Mr Loker. "I'm a collector, so I couldn't resist the urge to rescue them."

The diagrams languished until very recently when Mr Loker visited the National Museum of Computing and found out that Edsac was being reconstructed.

Mr Herbert said the diagrams are being used to ensure that the reconstruction is staying faithful to the original design.

"Thankfully, the documents confirm that the reconstruction we are building is basically correct," he said.

"They are giving us some fascinating insights about how Edsac was built and show that we are very much in tune with the original engineers," he added. "Both teams have been exercised by the same concerns."

The diagrams show how Edsac changed as it was put together and tested. During construction, circuitry was redesigned and the instruction set was improved to make the machine more reliable and easier to fix.

The reconstruction of Edsac is due to be completed in late 2015.

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