Recreating EDSAC


Information about EDSAC was located in the libraries of the Cambridge University Library and the Cambridge University Computer Laboratory library, as well as in the possession of individuals.

The original EDSAC was scrapped in the late 1950s and only three identical examples remain of one specific ‘chassis’ type, out of a total of about 65 different chassis types, and 140+ individual chassis in the original. And no definitive record exists of the final ‘as built’ design, either in terms of the circuitry or the physical layout of the machine. The archive material that we have found so far spans the whole timescale of the original EDSAC project, from Wilkes’ first thoughts to final decommissioning.

EDSAC was repeatedly modified over its almost 10-year lifetime. Our aim is to create a replica of EDSAC as it was sometime in 1951, about two years after it ran its first program (on 6th May 1949) when most of the initial problems had been ironed out, and it was providing a reliable computing service for Cambridge University.

The project has required complex detective work in order to come up with both the logical and physical designs that we can be reasonably confident reflect EDSAC as it was in mid-1951.

Where possible, we have built the replica using authentic materials and components - with one major exception. EDSAC’s memory was constructed from ‘delay line tubes’ filled with mercury; these exploited the relatively slow speed of a sound wave in the liquid metal. Expense and operating concerns will prohibit an authentic reconstruction of these tubes. An alternative is to use magnetostrictive steel wire delay lines, a technology employed in computers that followed EDSAC, and which follow similar physical principles and method of operation as acoustic delay lines.

In the 67 years since EDSAC became operational, technology has changed out of all recognition. Because of this, original components (or even their later replacements) have become very hard to find. In the “You Can Help” section, we will post details of the less common components that we still need to obtain if we are to build a close replica of the original machine.

Location and Construction

The EDSAC replica is being built at The National Museum of Computing (TNMOC) located on Bletchley Park, the UK’s WWII code-breaking centre and undoubtedly the spiritual home of computing in the UK. The new EDSAC gallery was home to a Colossus code-breaking machine during WW2.

It is being built entirely by volunteers, many of them in their 60s or 70s (some even older!) many of whom have experience of old valve-based technology. In time, some of them will train much younger volunteers, so that the EDSAC replica can be maintained in an operational state for decades to come.

As is TNMOC’s policy, it will not be a mere static exhibit. Once completed, it will be powered up and running as often as is practical. Particular efforts will be made to involve and inspire schoolchildren – the next generation of technologists and engineers!

Right now, if you visit TNMoC you may well see our volunteers at work debugging and commissioning EDSAC; we encourage you to talk to them, ask them questions and find out more about the project and the history of this very important computer.

The first meeting of the EDSAC Project volunteers