First EDSAC Replica chassis metalwork


EDSAC was built in a fairly modular fashion. It consisted of 12 vertical racks, each of which could hold up to 14 individual horizontal chassis. Each chassis provided all or part of one of the logically distinct functional units, such as the Digit Pulse Generator or the Tank Flashing Unit.

EDSAC had around 70 different chassis variants. There was only a single example of many of these: eg the Clock Pulse Generator. On the other hand, there were 41 identical Store Regeneration Units.

The chassis variants are superficially very similar. They all have two long rows of 14 holes each, for valve-holders, allowing each chassis to hold up to 28 valves. Given that it takes three pentode and two diode valves to build an AND gate, each EDSAC chassis is therefore barely equivalent to even the simplest of modern integrated circuits.

When EDSAC was decommissioned, only three chassis survived – one in the Cambridge University Computer Laboratory (since loaned to The National Museum of Computing located on Bletchley Park), one in the London Science Museum and one in the Museum of Computer History Museum in Silicon Valley.

Luckily, the Computer Laboratory’s chassis was a Store Regeneration Unit, which was one of the most complex chassis in terms of the number of components used. Each mercury delay line memory tank required one of these units to keep stored sequences of bits recirculating through the tank. From the surviving chassis we made detailed measurements of the overall dimensions, the layout of the valve holders, the positioning of the tag strips and also of the shielding plates, specific to that chassis, which enclosed the interference-sensitive parts of the circuit.

The detailed chassis measurements allowed us to recreate the design of the support racks by carefully measuring and scaling high resolution archive photographs of EDSAC. And because the chassis had sections of tag strip on it, we were able to commission almost 1,000 feet of new custom tag strip manufactured by Central Eyelets & Pressings in the West Midlands.

Chris Burton had a first rough prototype chassis made locally in Shropshire and brought this to the first volunteers’ meeting on 24th March 2012. While quite close to the original chassis we wanted something even more accurate.

To make our production-quality chassis, we turned to Teversham Engineering (TE), a small company on the outskirts of Cambridge that specialises in high precision sheet metal assemblies. Its Managing Director, Alan Willis, has become a keen supporter of the project.

Terry Smith, TE’s designer, made detailed measurements of the Computer Laboratory’s chassis and then built a 3D CAD model using SolidWorks software. Chris Burton was then able to review and manipulate this 3D model interactively using SolidWorks’ eDrawing Viewer. Terry then used SolidWorks to ‘unfold’ or flatten the model into its component 2D pieces, automatically applying the necessary bend allowances, ‘nesting’ multiple components on each sheet of steel to minimise waste, and finally automatically generating the tooling schedule and instructions for TE’s Amada Pega CNC sheet metal punch and Amada HFE1030 bending machine.

On 26th July, almost exactly three months after the first volunteers’ meeting, Alan Clarke and Andrew Herbert went to TE to watch the first sheet metal being cut for two prototypes. It took the Amada less than 5 minutes to ‘punch and nibble’ the first two highly accurate main chassis pieces! These were then bent and painted, and we were able to bring the two prototypes to the second volunteers' meeting on 16th August 2012.

Once we had the chassis metalwork, we could then fit valve-holders, shielding plates, tag strips etc. This flushed out some additional problems with the exact design of the shielding plates and the positioning of some holes and so the design has been further refined.

The Store Regeneration Unit chassis will serve as a model for many of the other chassis types, but we will not be able to commit them to production until the circuit for each is designed and verified using modern circuit simulation software so we can be certain of the precise valves and components to be fitted.

It’s fascinating to reflect on the way that the project is using the very latest computer technology to recreate one of the earliest computers – producing circuit drawings, running circuit simulations, producing 3D CAD models and finally automatically programming the punching and bending machines! It’s also sobering the realise that the PC used to design the chassis runs about three million times faster than EDSAC’s 650 instructions a second!

We hope to manufacture the first batch of about 20 Store Regeneration Unit chassis, and a prototype rack, before the end of 2012.