Rediscovered EDSAC diagrams reveal secrets

Post this page to popular social media

Some of the earliest diagrams of a computer have been rediscovered more than sixty years after they were drawn and are giving the EDSAC team at The National Museum of Computing fresh insights into their ongoing reconstruction of one of the world's first general purpose computers.

EDSAC, the Electronic Delay Storage Automatic Calculator, was originally built in the University of Cambridge immediately after World War II. It was the first practical general purpose computer and transformed research possibilities for many academics and even helped three in their Nobel-prize winning work. The EDSAC design was later developed to create LEO, the world's first business computer.

The 19 very detailed circuit diagrams that have just been rediscovered have been given to the EDSAC team by a former engineer in the University of Cambridge Mathematical Laboratory. Many of the diagrams, which date from between 1949 and 1953, were drawn after EDSAC had been constructed, probably as some sort of aid in refining the original machine and in designing the next. They seem to have been part of a much larger set numbering at least 150 and are in remarkably good condition.

The diagrams form an important extra source of information for today's EDSAC team which has been reconstructing the computer using incomplete evidence. The documents confirm that the team has been correct in most of its re-engineering assumptions, but the drawings have thrown up a few surprises.

John Loker, who has brought the diagrams to light, explained how he came across them: "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. 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 Project."

Andrew Herbert, leader of the EDSAC Project, said: "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!

"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."

Elements that were modified even after the machine was up and running in May 1949 include redesigning the circuitry to obtain stronger signals and improvements to the instruction set and error correction so that programming errors could be distinguished from machine malfunction.

The most significant discrepancy between the original and the reconstruction that the papers reveal is in the "initial orders" (boot ROM in modern terminology). In the absence of fuller information, today's reconstruction team had considered and rejected one possibility which was in fact the one that was used by the original engineers. That will now be rectified in the reconstruction.

Herbert concluded: "Very few artifacts of EDSAC remain. However these papers give a clue as to why a few such as a chassis do exist. We think that the existing artifacts were discards from a very early version of the machine!"

The reconstruction of EDSAC is due for completion late in 2015 and can already be seen as a work in progress by visitors to The National Museum of Computing. The ongoing reconstruction story featuring videos of progress at different stages can be seen at

Notes To Editors


EDSAC, the Electronic Delay Storage Automatic Calculator, was built immediately after World War II by a team led by Sir Maurice Wilkes in the Mathematical Laboratory of the University of Cambridge. It was one of the first practical general purpose computers and was used by scientific researchers across the University. The EDSAC design was later developed to create LEO, the world's first business computer.

• EDSAC was based on the ideas of John von Neumann and others who in 1945 suggested that the future of computing lay in computers which could store sets of instructions (programs) as well as data in a memory.
• EDSAC was over two metres high and occupied a ground area of four metres by five metres.
• Pre-dating the transistor, its 3000+ thermionic valves / vacuum tubes used as logic were arranged on 12 racks containing just over 140 chassis in total.
• Mercury-filled tubes were used for the main memory, comprising 512 words initially, later 1024 word (equivalent to 2KB/4KB of PC storage).
• It performed 650 instructions per second, effectively computing more than 1500 times faster than the mechanical calculators it replaced.
• EDSAC read in programs from paper tape and printed its results on a teleprinter.
• EDSAC ran its first program on 6 May 1949 and soon began nine years of regular service for scientific users across the University of Cambridge and other institutions, ending in July 1958 when it was dismantled to enable the re-use of precious space. By then it had been superseded by the faster and much larger EDSAC 2.

2 The EDSAC Reconstruction Project

The EDSAC Reconstruction Project which began in 2011 is expected to be completed in late 2015. The reconstructed EDSAC, which will occupy 20 square metres, is already being built at The National Museum of Computing, where visitors can see the work in progress. The EDSAC Replica Project has been funded by a consortium led by Hermann Hauser.

The ongoing reconstruction story featuring videos of progress can be seen at

3 About The National Museum of Computing

The National Museum of Computing, located on Bletchley Park, is an independent charity housing the world's largest collection of functional historic computers, including the rebuilt Colossus, the world’s first electronic computer, and the WITCH, the world's oldest working digital computer. The Museum enables visitors to follow the development of computing from the ultra-secret pioneering efforts of the 1940s through the large systems and mainframes of the 1950s, 60s and 70s, and the rise of personal computing in the 1980s and beyond.

A recent pledge by an individual benefactor of £1 million if matched funding is found means that every pound or dollar donated to the Museum will count double. Previous funders of the Museum have included Bletchley Park Capital Partners, CreateOnline, Ceravision,, Google UK, PGP Corporation, IBM, NPL, HP Labs, BCS, and 4Links.

The whole Museum is currently open to the public on Thursdays, Saturdays and Sundays from 12 noon, spring and summer Bank Holidays and increasingly during school holidays. Colossus and Tunny galleries are open almost every day. Guided tours are available at 2pm on Tuesdays. There are often additional opening times for the public -- see the website or the iPhone app for updates. Educational and corporate groups are very welcome and may be on any day or evening by prior arrangement.

For more information, see and follow @tnmoc on Twitter and The National Museum of Computing on Facebook and Google+. A TNMOC iPhone App is also now available from the iPhone App Store.

Support us

The Museum has not received government or Lottery funding, so your help is needed.

Become a member »
Make a donation »
Become a volunteer »
Sponsor us »

Latest Tweets Follow