TNMOC's EDSAC rebuild uncovers unique circuit diagrams

June 25, 2014 // 10:05 a.m.

Tags: #andrew-herbert #bletchley-park #cambridge #computer-history #edsac #edsac-rebuild-project #history #john-loker #leo #tnmoc #vintage-computing

The National Museum of Computing (TNMOC) is celebrating today following the discovery of some of the earliest diagrams of a computer, drawn 60 years ago in an effort to document the world's first general-purpose computer EDSAC.

The National Museum of Computing announced its intentions to host a rebuilt project for the Electronic Delay Storage Automatic Calculator (EDSAC) system back in 2011, and in 2013 began to produce the first parts. EDSAC has a special place in computing history: built immediately after World War II by a Cambridge-based team led by Sir Maurice Wilkes, EDSAC was the world's first practical general-purpose digital computer. It also served as the blueprint for the Lyons Electronic Office I (LEO I), the first business computer and forerunner to today's highly-computerised world.

Sadly, EDSAC itself is no longer with us. The system's more than 3,000 thermionic valves - having pre-dated the transistor - spread over 12 racks were dismantled and spread to the wind in 1958 after nine years of service. Its successor, EDSAC 2, was an improvement in almost all respects, but the historical impact of EDSAC is not to be underestimated - and it's this impact that the Museum wishes to highlight with its rebuild project, to occupy a whopping 20 square metres of the facility when it is finished in late 2015.

To aid the team in its efforts, former University of Cambridge Mathematical Laboratory engineer John Loker has donated 19 detailed circuit diagrams detailing various parts of EDSAC after saving them from landfill. 'I started work as an engineer in the Maths Lab in 1959 just after EDSAC had been decommissioned. 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,' explained Loker of his discovery. 'I'm a collector, so I couldn't resist the urge to rescue them. It wasn't until I visited TNMOC recently and learned about the EDSAC Project that I remembered I had the diagrams at home, so I retrieved them and gave them to the [EDSAC Rebuild] Project.'

'Thankfully, the documents confirm that the reconstruction we are building is basically correct, but 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: both teams have been exercised by the same concerns,' added Andrew Herbert, leader of the EDSAC Project, of the diagrams. 'Importantly, the drawings clearly show that the aim of EDSAC's designer, Sir Maurice Wilkes, was to produce a working machine quickly rather than to create a more refined machine that would take longer to build. The refinements could come later - and many did as the sequence of diagrams over the five-year period shows.'

Many of the diagrams were created after EDSAC had been constructed in an apparent effort to document the build and aid in the creation of its successor, and show modifications from the original design including circuit improvements that boosted the signal strength and error correction algorithms capable of distinguishing errors in programming from malfunctions of the machine itself. Coverage of 'initial orders,' the equivalent of an initial program loader (IPL) or boot ROM in modern systems, also highlighted that an approach rejected by the rebuild team was in fact the one chosen by the original creators - something the team will now correct.

The rebuild project is due to complete in late 2015 with a fully-working recreation of EDSAC as close to the original as possible, which will be open for public viewing at the Museum. Currently, visitors to the site can enjoy a work-in-progress exhibit also documented in detail on the official website.
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