Plessey Exchange Works

See the workers a 1978 Personnel Listing a 2020 update

a Leaving Card.

 The factory was built in the 1950s in order to hide classified design and manufacturing of the UK Air Defence System away from prying eyes. It was an extension of the ATE (Automatic Telephone & Electric Company) based nearby in Edge Lane, Liverpool. Alas, with the end of the Cold War the British defence industry collapsed and the building was turned into luxury flats. The site was alongside Cheapside at the front and Cunliffe Street at the rear. Adjacent to the site across Cheapside Alley was a small factory that made candles. Looking at the picture below, the far end of the factory housing laboratories, some manufacturing, and the factory canteen, before being bought by ATE, made margerine and (so I'm told) during WW2 bombing a direct hit resulted in a stream of molten margerine running down Cheapside.




 Looking a bit gloomy, a view along the old Plessey building to the Liverpool Municipal Building in Dale Street.

See the new plans for the building when the apartments were being advertised for sale.

What went on in the building where I worked for 20 years? I've managed to find some rare pictures of the equipment (thanks to Terry Pate who was my boss for quite a few years before moving to the radar division on the Isle of Wight where he was the Managing Director).

The first picture is the XL4 computer of which a couple of dozen were manufactured. This was a 64 bit machine that worked asynchronously rather than being governed by a master clock. As you can see it has five cabinets and each was stove enamelled with chrome-plated fittings.

The XL4 was developed from the XL2 (awaiting a picture). An early XL2 handled the wages for Exchange Works staff and had about twice the number of cabinets of the XL4.


 I understand the XL4 was designed by Ron Threadgold and Dave Hartley (who together also designed the XL2); and Terry Pate (who supplied these pictures) worked on the XL4 processor. Linesman was known as PLCS to muddy its purpose.

Each cabinet housed a number of plug-in units carrying about 25 printed circuit boards whose connections were wired to a frame by gun-wrapping. The circuit boards carried standard logic functions constructed from germanium transistors and diodes with their interconnections made in the wiring of the unit frame. This method made the design quite complicated but allowed for standardised logic functions equivalent to todays integrated circuits ie. each circuit board, say for example six inverters was equivalent to a modern logic chip such as a 7404. Like a modern PC the XL4 needed memory, and this was 32K x 64 bit x 2uSec, and occupied a couple of 7 foot cabinets.(nowadays, double this amount of RAM would occupy just a few square millimetres within a microprocessor chip)



Next a rare view inside RAF West Drayton computer room... Note the "wiring" carrying power to the computers.. enormous copper busbars in the ceiling driven by a number of hundred amp power units backed up by lead acid batteries. and the trunking carrying cooling air to keep the fragile germanium transistors safe. There were several power supply voltages and it was vital to turn these on/off in the correct order to avoid thermal runaway in the germanium transistor junctions. The computers operated in real time and were responsible for tracking and predicting the course of enemy aircraft. Radar sites around the UK coast picked up echoes from all aircraft in the vicinity of the UK and transmitted data to West Drayton where it was processed by the Linesman computers with results passed to display consoles.

If I recall correctly, in the room above this were something like 70 radar display consoles.


 If you'd like to see an outstation, RAF Neatishead, just as it was when Linesman was scrapped visit the Radar Museum close to Horning in Norfolk.

The next major development was the XL6 computer (designed by Terry Pate), a 24 bit machine. Left the computer and right its memory. This was supplied as a message switch for ICI.


 Then the XL7.


 Next was the development of the XL9 computer (awaiting pictures)

This was a commercial venture designed by Terry Pate and Dave Paculabo, and was used in a number of projects including an air defence system for Burma, a telephone exchange in London, a message switching system based at GCHQ Oakley, a wide area traffic control systems in Liverpool and West London (awaiting pictures).

 Following up from the XL9, a venture into a more advanced machine was the XL12 which unfortunately fizzled out... see below... but Exchange Works did design and develop a new computer for the management of telephone exchanges. This was known as "Sprat" and three computers were manufactured and installed at the Plessey site of Taplow near Heathrow. These computers were again based on ideas from XL9 and XL12 and were designed to be fault tolerant. A loss of one or two machines from three would result in the remaining system of two or even a single machine picking up the workload without any hesitation. To demonstrate this to the customer it was arranged that test software was written and an audio amplifier connected to part of the computer to produce tones. Using the test software a musical tune was produced, that was "English Country Garden". During playback one computer was turned off and then a second with no hesitation in the tune. Then the computers were re-introduced and again no hesitation in the tune.


 Meanwhile, sharing the top floor of Exchange Works with the Drawing Office, and even more secret than Linesman, took place the design and development of crypto equipments, a few of which are pictured here. Left is the BID610.

Besides manufacturing complete equipments, Exchange Works developed the small unit forming the most important part of most UK cryptographic equipments, the noise generator; based on a semiconductor junction or zener diode which was able to provide completely random binary code used to produce the key for encrypting messages..






Below, newer machines.. first BID1000..


 Then, below the more compact BID950.


 When Margaret Thatcher decided to sort out troubles in Northern Ireland, Exchange Works won contracts for the design and development of specialised military equipment. These included P6 (a mine detector), P7 (a device for detecting command wires for things like A mines hidden in trees), IA2 (a device for recording sounds from terrorst activities), IA5 (a sniper detector), 3PS (an intruder detection system) and others (awaiting pictures).

 At roughly the same time, Exchange Works entered into the satellite business. Monitoring and control equipments were supplied to support the ground station handling the GEOS and METEOSAT satellites and others, managed by ESOC in Bretigny and Darmstadt. This area of expertise ended once the design, development and manufacture of equipment for RAF Oakhanger (the anchor station for Skynet) had been completed and installed (awaiting pictures).

 Because of expertise from previous air defence systems, Exchange Works provided the communications cabins for a number of 3D-radar based air defence systems including those for South Africa (1981 Project Rodent), Egypt (1981 Project Lion), Ecuador and Qatar (Project Falcon), plus helping out when the Falkland Islands were invaded by diverting to there one of the systems for Qatar (1983 Project Penguin). Below.. a comms cabin for Project Rodent. Because of their large physical size these cabins were manufactured on the second floor of the building because lower down the building support pillars were too close together. A doorway needed to be cut into the side of the building through which a crane could deliver empty containers and collect equipped cabins for transit, usually to Bournemouth Airport.

 A Project Rodent communications cabin

for DoD in South Africa


 Is that everything? Not quite.. Exchange Works also supported other Plessey businesses by designing and manufacturing all sorts of interesting things. These included parts of radar jamming equipments (for Syria would you believe...), message heading generators for NATO (because we had experience with ACP127 for GCHQ), the Store and Forward Message Switch for Ptarmigan, studies for Ptarmigan and Wavell for the British Army and specialised equipment for Eurocontrol. This last equipment comprised a pair of 128-input multiplexers using lots of microprocessors developed before those from Intel. The design of these was based on the type of work that had gone into the XL12 computer which was being built for commercial systems but had been cancelled when Plessey and others formed ICL. The pair of multiplexers were installed at Karldap and used for combining peripheral equipments into Telefunken computers for the control of middle airspace over Europe.

Air Defence Systems 

Project Lion (Egypt) 

Project Falcon (somewhere in a desert)


Project Penguin (somewhere down south)


 Not easy finding a record of what used to be highly confidential military projects but here are some snaps from old pals Norman Fung and Allan Marshall.

Click on these to see more and click the link below to see the radar history of Plessey (thanks to Allan Marshall who commissioned the systems seen by clicking the pictures here).

Below, a scene from a quiet Sunday morning with a communications cabin being lifted from Exchange Works en-route to Bournemouth Airport for onward delivery to a customer. Further down a view inside the cabin (pictures from Chris New).



A view south down Cheapside towards the Liverpool Municipal Offices 

This is a Project Rodent communications cabin en-route to South Africa. On the right is the old margarine factory housing the canteen, and upstairs the Accounts Department, Purchasing and laboratories.

Below, views to the north with the Liverpool College of Commerce on the right in Tithebarn Street.



To see how these equipments fitted together into systems look in the brochure  

Plessey Radar brochure from the 1980s

Then finally with the cold war coming to an end and the defence industry collapsing the Exchange Works factory was no more... click the picture to see the buyers plans.


 Under the building was a garage which had reserved spaces. The general principle was you had to wait for someone to die or retire before you could get a space and then only if your status was high enough to qualify. Working shifts could sometimes earn you a temporary space and I recall one day in the late 60s trying to get to street level after working all night on a project called "Sprat". The exit was fairly steep and my car an old 1953 Rover 75. It was when attempting to drive out I discovered all was not well with the car. I could almost reach street level before the engine cut out and I had to roll backwards. After several unsuccessful attempts I managed to race up the ramp before the engine spluttered out. Many months later at home on an extremely cold morning the car refused to start and it was then I found the fuel filter glass bowl was full of ice. Ordinarily driving flat and level petrol would pass over the water filled bowl, but meeting even a slight hill caused water to enter the carburettor and stop the engine. Once unclipped and emptied of water no more problems...

 Anyone with more information or corrections please contact me.

Read about HDRS which thankfully was never deployed for its purpose!

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