From Graces Guide
The Plessey Company - an electronics, defense and telecommunications company.
1917. 12th December. Plessey was founded in Marylebone, London as a mechanical an engineering concern, manufacturing jigs and tools, supplying other companies. The company was formed specifically to take advantage of the talents of a German-born engineer, William Oscar Heyne, who had been interned at the start of World War 1. The original shareholders were Thomas Hurst Hodgson, Raymond Parker and his brother Plessey Parker, and C. H. Whitaker, a school friend of Heyne's.
There are various ideas about the origins of the company’s name - it is said to have been taken from the birthplace of Heyne’s wife but Hodgson was brought up close to Plessey in Northumberland, and is thought to have given the name Plessey to one of his illegitimate sons, Plessey Parker, which suggests strongly that the name came from Hodgson.
Heyne was the sole employee initially, and went on to become one of the key figures in the development of the company during the 1920s and 30s. However, for the first few months, Heyne spent most of his time working as engineering consultant for other Hodgson companies including a galvanising company called British Electro-Chemists .
1922, Byron Clark’s son, Allen George Clark, started working for the company.
1922 Byron Clark seems to have been connected to Norman De Maid Watsham and A. H. S. MacCallum who founded British Radiophone Ltd. Watshams won an order from Marconi for 2 types of crystal set and a valve receiver; it is unclear what role British Radiophone Ltd played in this order but the order for crystal sets was sub-contracted to Plessey Co. This was the start of Plessey’s diversification into radio and electronic manufacturing. Plessey’s business model became a combination of mass-production of standard components for the makers of radio sets and sub-contract manufacture but not selling direct to the private customer. The manufacture of electrical components became a key area of growth for Plessey, eventually manufacturing a vast array of different components, many under licence from overseas companies.
1923 Plessey moved to larger premises in Ilford. At the time there were more than 200 employees.
The shareholders of British Radiophone Ltd were persuaded to take shares in Plessey in exchange for Plessey being able to supply Marconi directly. But Marconi then demanded a stake in the growing company, which could not be refused as it accounted for 80% of Plessey's turnover. Marconi appointed 3 representatives to the board of Plessey
1925 15th February. A new company was incorporated with a similar name and larger share capital to take over the original Plessey company formed on the 12th December 1917. 
1925 25th February. 'In the Matter of the PLESSEY COMPANY Limited. At an Extraordinary General Meeting of the above named Company, duly convened, and held at 7, Ely-place, Holborn-circus, E.C. 1, on the 7th day of February, 1925, the following Resolution was duly passed; and at a subsequent Extraordinary General Meeting of the Members of the said Company, also duly convened, and held at the same place on the 24th day of February, 1925, the same Resolution was duly confirmed as a Special Resolution, namely: — "That the Company be wound up voluntarily." And at such last mentioned Meeting, Harold Handley, of 7, Ely Place, E.C.1, was appointed Liquidator for the purpose of the winding-up. Dated this 25th day of February, 1925. (221) W. O. HEYNE, Chairman.' 
1925 Byron Clark became chairman of the new company with Allen Clark and W.O. Heyne as joint managing directors. Allen Clark’s sons John and Michael later both rose to prominent positions in the company.
By September 1925, Plessey was supplying 2,500 sets/week to Marconiphone
1926 Marconiphone bought their own manufacturing company so orders to Plessey Co ceased.
1927 Henry Morgan appointed chairman - Morgan had been one of Marconi's appointments on the Plessey board.
1928 Important contracts included production of telephones for the General Post Office (GPO) and items for the Air Force and the car industry. Production of equipment for professional customers became a growing part of the company’s business.
1929 John Logie Baird’s first production televisions were manufactured by Plessey. The company also produced the first British-made portable battery radio, the “National” which was supplied to Symphony and Columbia. Allen Clark introduced “mass production” which allowed the company to undercut its competitors in component supply.
1934 Review and image of Plessey Model AC.44 Airborne Transmitter Receiver 
1935 The workforce reached 3,000.
1936 Clark and Heyne negotiated a number of key manufacturing licences from American companies, such as Breeze Corporation, for aircraft multi-pin electrical connectors; Federal Laboratories for Coffman starters (an explosive cartridge device used to start aircraft engines); and Pump Engineering Services Corporation for the manufacture of Pesco fuel pumps. Plessey went on to produce large numbers of Pesco fuel pumps for Rolls-Royce Merlin engines, and in 1940 the fuel pump for Britain's first jet engine. Also see Aircraft Industry Suppliers.
1937. 17th March. In order to raise extra capital to fund expansion, essentially to take advantage of opportunities presented by rearmament, Plessey became a public company with a listing of its shares on the London Stock Exchange. The new Machine Products subsidiary in Cardiff received an order for bomb casings from the Air Ministry and another, London Metal Products, received an order for trench mortar bombs.
1939 The workforce reached 5000.
WWII - During the war, Plessey produced many different types of components and equipment for the war effort, including shell cases, aircraft parts, and radio equipment such as the R1155 (receiver) and T1154 (transmitter). Following the bombing of its Ilford site, Plessey converted a tunnel, built as an extension of the London Underground Central Line, into a munitions factory. The company also built a new factory at Swindon, and opened several other shadow factories around the U.K. The small research and development activity was moved to Caswell House, near Towcester. The wartime workforce grew to over 11,000.
They also held many manufacturing licenses for other products associated with the aircraft industry including from Borg Warner for aircraft engine starters, fuel and hydraulic pumps.
1946 With the coming of peace, the company shrank back to its core activities of radio, television and components; the workforce was reduced to 6000. In this year Byron Clark died. At the end of the year, Heyne retired, recognising that he "couldn't go on without Clark", the "American gentleman" as he called him.
1947 EMI placed an order for at least 100,000 radio and television sets.
1949 Plessey entered the hydraulics field. John Allen Clark joined the company.
1950 Michael William Clark joined Plessey and was appointed head of the Electronic Division. The number of research staff at Caswell reached 50, investigating new areas of business including ceramics, piezo-electric materials, ferrites, radar-absorbing materials, and tantalum capacitors.
1951 The new Communications Division was formed, with Michael Clark at its head, and a remit to catch up with its competitors in VHF and UHF equipment. Two senior managers, John Cunningham and Raymond Brown, left Plessey to form Racal. A licence was obtained from the US company Philco for the manufacture of semiconductors; the two companies set up a joint venture.
1952 A plant for making pure Silicon was established but the potential of the new field of “solid state technology” (the name given to it by Plessey) was not fully recognised.
1955 With the growth of the hydraulics business, the business was split between new divisions: Fuel Systems which was moved to Titchfield, Hampshire and Industrial Hydraulics when went to Swindon, Wiltshire. The Swindon business built strong links with its customers in the tractor market and expanded the product range into hydraulic rams, mono-block valves and cam lobe motors for hydro-static steering.
1956 Founded Plessey Research Roke Manor
1957 Manufacture of transistors began at Swindon. Plessey also manufactured the first VHF/UHF airborne radio in the UK.
1958 Royal Radar Establishment placed a contract with Plessey for integrated circuits. A new factory was opened at Cheney Manor (Swindon) for large-scale manufacture of Germanium transistors. Plessey purchased Garrard Engineering and Manufacturing Co, a company to which Plessey had lent factory space at Swindon after a fire. This was the start of a new approach to growth of the company - one of acquisition.
1959 The workforce was 20,000.
1960 At this time Plessey, Phoenix Telephone and Electric Holdings and the Telephone Manufacturing Co were the 3 smaller suppliers in the telephone supply industry. Other suppliers included AEI, Automatic Telephone and Electric Co, Ericsson Telephones Ltd, GEC, Marconi's Wireless Telegraph Co and STC.
1960 A consortium of AEI, Automatic Telephone and Electric Co, Ericsson TelephonesLtd, GEC, Marconi's Wireless Telegraph Co, Plessey Co and STC formed a holding company Combined Telephone Holdings only days after its members had failed in their bid to acquire Telephone Manufacturing Co. Combined Telephone Holdings purchased for cash more than half of the shares in Phoenix Telephone and Electric Works and offered to purchase the rest.
1961 Plessey ceased radio and TV manufacture. Plessey took over two other telephone manufacturers, Ericsson Telephones Ltd and the Automatic Telephone and Electric Co, doubling Plessey’s size to become Britain's largest manufacturer of telecommunications equipment (including the majority of the country's crossbar switches / exchanges). Philco pulled out of the Semiconductors JV, selling its shares to Plessey. The change in nature of the company from manufacturer of components to provider of complete systems took place faster than the evolution of Plessey’s corporate planning leading to problems in later years.
1961 Plessey had 23 subsidiary companies , employing 17,500 persons. These included designers and manufacturers of radio and television apparatus and components, electronic equipment, telecommunication equipment, electrical equipment, electrical instruments and commercial hydraulic power systems, makers of general light engineering products and atomic energy power control equipment.
1962 Sir Allen Clark died. After boardroom disagreements, John Clark became MD; several directors resigned. Plessey were partners in the development of the Atlas Computer.
1963 Plessey took over Ducon (Australian telecommunications equipment and components manufacturer) followed by many other purchases in the following years. Development of an electronic telephone exchange (TXE2) started.
1965 US management consultancy McKinsey was brought in to review the company; the businesses were divided into Groups: Automation, Components, Dynamics, Electronics, Telecommunications, Overseas. Plessey Telecommunications would incorporate Ericsson Telephones and Automatic Telephone and Electric Co.
1966 MOS integrated circuits were produced at Swindon, making Plessey the first European manufacturer of such microcircuits.
1967 The 68,000 employees included 6,500 in research and development with R&D labs at Caswell, Roke Manor, Taplow, Havant and Poole.
1971 Plessey installed its 100th TXE exchange, demonstrating the value of the 1961 merger. Plessey's defence and aircraft business were also prospering and the Swindon silicon chip business made a profit but the Numerical Controls business was losing money and substantial redundancies were required. Worldwide workforce reached 85,000.
1972 Chairman Sir John Clark, needing to tackle the loss-making activities, initiated a reorganisation into 24 businesses, each operating as a separate profit centre, organised in nine divisions.
1974 Plessey Co, Vicarage Lane, Ilford, Essex. Chairman — Sir John Clark; managing director — M. Clark. Group turnover (1972-73) £325 million. UK-based employees 60,000. 
1974 75,000 employees worldwide. Digital telephone exchange (TXE4) ready.
1974 Signed a statement of intent to develop a fibre optics picture transmission system.
1977 Post Office cutbacks hit Plessey’s business with 4800 workers made redundant. Threats to sell the semi-conductor businesses and microchip research centre (Caswell) were reversed by the Board.
1981 First System X electronic exchange delivered to British Telecom.
1981 With the corporate focus on semi-conductor manufacture, radar systems and telecommunications networks, the company divested parts of the business that did not fit, one of which was the Hydraulics business which was sold to the Sundstrand Corporation of Rockford Illinois and became Sundstrand Hydratec.
1984 First drop in profits for 8 years.
1985 Increased semiconductor sales; focus on core businesses; sale of non-core businesses.
In December 1985 GEC launched a takeover bid for Plessey, valuing the group at £1.2 billion. Both Plessey and the Ministry of Defence (MoD) were against the merger as GEC and Plessey were the two largest suppliers to the MoD and, in many tenders, the only competitors.
1986 GEC’s bid was referred to the Monopolies and Mergers Commission (MMC), whose report (August) advised against the merger although did favour a merging of telecommunications interests. The government blocked GEC's bid.
1986 Plessey acquired Ferranti Semiconductors, creating Europe's largest semi-custom chip company. A new state-of-the-art silicon chip production facility near Plymouth was opened.
1987/8 Made 5 large acquisitions including:
- Sippican, a US defence company
- The semiconductor operations of Ferranti
- Bid for GEC's semiconductor activities, which were smaller than GEC's or Ferranti's
- Acquired Leigh Instruments of Canada
- Acquire 49 percent of Elettronica of Italy
- Acquired a controlling stake in Hoskyns, Britain's 4th largest computer services company
1988 Plessey and GEC merged their telecommunications businesses in a joint venture GPT (GEC-Plessey Telecoms). This was the UK's leading telecommunications manufacturer but it had excess production capacity.
1988 Plessey took over various defence suppliers in other countries to lessen its dependence on the UK Ministry of Defence and BT.
1988 30,000 employees (not including GPT)
1988 GEC and Siemens AG set up a jointly held company, GEC Siemens plc, to launch a hostile takeover of Plessey. GEC Siemens' initial offer was made on 23 December 1988 valuing Plessey at £1.7 billion. Plessey again rejected the offer and again it was referred to the MMC. The original proposal envisaged joint ownership of all of Plessey's defence businesses, with GPT and Plessey's North American businesses split in the ratios 60:40 and 51:49 respectively. The level of GEC's involvement in the Plessey defence businesses was not likely to meet with regulatory approval. After reference to the MMC, in February GEC Siemens announced a new bid. The takeover was completed in September 1989.
1989 Hansard recorded that Plessey had nearly 15,000 employees in the UK 
- Plessey Addlestone: Around 1965 Plessey, which had just taken over part of Decca to establish Plessey Radar, had to move out of Decca's premises at Chessington, and relocated to Addlestone. In the early 80's the main business became Plessey Displays, then later merged to become part of Plessey Naval Systems. Other businesses that existed alongside, or spun off, included Plessey Airports.
- Braxted Park: In 1969 this was part of Radio Systems Division; the facility is thought to have been a small one in an old stable block; the main house was the residence of Michael Clark, managing director of the company.. Work included MALLARD, an abortive multi-national project to develop a battlefield communications systems
- Plessey Caswell (Research): During WWII, the small research and development activity was moved from Ilford to Caswell House, near Towcester. In 1945 (31st July) the farm was acquired by The Plessey Company from Major G. C. B. Bramwell. In 1950 the number of research staff at Caswell reached 50, investigating new areas of business including ceramics, piezo-electric materials, ferrites, radar-absorbing materials, and tantalum capacitors.
- Plessey Christchurch: Prior to Plessey's arrival, this site was the Ministry of Defence's Signals Research and Development Establishment (SRDE), until the MoD relocated it to RSRE at Malvern in the mid to late 70s
- Plessey Exchange Works, (Cheapside, Liverpool): In 1959 Automatic Telephone and Electric Co, later Plessey, became the Prime Contractor for a new UK Air Defence System, under the name Plan Ahead and, from 1961, known as Project Linesman. To enable the system to be designed and built without too much information becoming public knowledge, a new factory called "Exchange Works" was built in Cheapside in Liverpool city centre, where young employees were granted exemption from conscription. At the heart of the system was the XL4 computer, based entirely on germanium transistors and using a computer language developed at Exchange Works in the 1950s and 1960s. During this period the company effectively became the world leader in computer design... unfortunately, this fact remained a close secret. The secure status of the factory attracted many other secret contracts and led to it becoming one of the major designers and manufacturers of cryptographic equipment. Exchange Works is now luxury flats.
- Plessey Ilford: 1939 Much of Plessey's production at Ilford was, then, on the Secret list and with war looming Plessey management took the decision to destroy all unnecessary paperwork related to any project that had reached production maturity. The reasoning was that if Germany did invade the UK one direction could well be up the Thames and Plessey was perceived to be a high value target.
This invasion concern was also shared by HM Government and by the beginning of 1940 the Upminster underground railway tunnel had virtually been completed, although no tracks had been laid. The Government therefore decided that it would be sensible for Plessey to go underground. This was put the the management who, without hesitation accepted. Because such a move was going to take up a significant length of the underground, a special 18 inch gauge miniature railway was installed over 5 miles. Also additional entrance and exit points were arranged for the employees. By February 1942 the immense task of moving the Plessey factory underground, without too much loss of production, had been completed. In March 1942 the Luftwaffe carried out a systematic bombing run against Plessey Ilford without the knowledge that by then the factory was empty. The factory was almost totally obliterated, but there was no loss of life and no interruption to production.
Plessey's Ilford activity quickly became the largest underground factory in the World with some 300,000 sq ft of floor space in its 5-miles length of 12ft diameter tunnel. Access was via Wanstead, Redbridge and Gants Hill stations and specially prepared intermediate points. It was obvious that the Luftwaffe had no doubts about the importance of the Plessey factory, which was clearly indicated on a detailed German aerial photograph that was recovered on cessation of hostilities.
Over 4,000 employees were engaged in two shifts down in the tunnel where both employees and material were moved around on the mini gauge railway. Due to the management decision to destroy all 'unnecessary' documentation, there are, sadly, few details of events during the 'tunnel' period other than that provided by staff who worked there of which virtually all are now deceased. In 2005 Denis Dalton was interviewed. Denis joined as an apprentice in 1938 and shortly after commencement of hostilities he was seconded to the RAF as an apprentice engineer but he returned to Plessey Ilford and eventually became the Company's Chief Engineer. He was also able to provide some photographs that were taken both inside the tunnel and of the bombed factory. (copies attached)
What is known is that over 161,500 major assemblies of military equipment were manufactured in the tunnel during the period from 1942 to 1946. They included including shell cases, a wide range of aircraft parts and radio equipment such as the R1155 communications receiver and T1154 transmitter, also the B7/RL66, B71/RL66, RL66A, RL7 and RL135 intercept receivers plus the very special cathode ray tube (CRT) that was used in the FH4 to aid the detection of U-boats.
- Plessey Isle of Wight: Production of radars for military and commercial Air Traffic control (ATC), naval and land based applications.
Products included ACR-430 motion detection radar used to control traffic on airfields, AR-1, AR-5, AR-5D, AR-15, AR-15AB, AR-15D, AR-15M, AR-15/2, DASR-1, DASR-3 then, in cooperation with ITT the AR-320 3D radar system for various ATC and air defence (AD) applications. Plessey also cooperated with Siemens in the production of the Astral and AWS-2, AWS-3, AWS-4, AWS-5, AWS-5A/B/C/D, AWS-6, AWS-6/100/200/300 series, AWS-6D/E, AWS-7, AWS-8, AWS-9(2D) and (3D) all for maritime applications whilst the Guardsman-C, Guardsman-2 and Guardsman-S were widely deployed in both civil and military airfields. The Company also produced the CDR-431 radar for coastal surveillance and the Commander S, a mobile system employed by the Army in 1996. Plessey was variously involved in the production of IFF (Indicate Friend / Foe) equipment including the Condor system and IFF Mk 10 that became a World standard for NATO and commercial IFF. The Company also produced the HPR and MPR signature measurement systems that were used to evaluated radar cross sectional reflection areas of ships and aircraft, which in turn lead to the development of stealth aircraft and ship designs. Over time, with improving experience and skill Plessey designed and manufactured a range of additional specialist radar systems such as MSAM, MTCALS, P-SCAN 2000, Pike, PTR-826, PVS 2000, Sentinel, Sigma, Spectar, the Rapier fire control system for the British Army; the Type 93 and Type 101 for the Royal Air Force while the Types 993, 994 and 996 series were produced for the Royal Navy.
- Plessey Liverpool (Edge Lane): Telephones
- Plessey Nottingham (Beeston): Telephones
- Plessey Plympton: Plessey Semiconductors
- Plessey Poole:
- Plessey Roke Manor: Acquired the Roke Manor estate in 1956 and founded Roke Manor Research. Initially staffed by 28 engineers, the company undertook research into military communications systems.
- Plessey Swindon: in 1940, the company was primarily known as a manufacturer of radio components, when it opened a factory in Kembrey Street, Gorse Hill. By 1952, the Town Development Act led to the building of Cheney Manor Trading Estate which was to become an important part of the Plessey operation as the company chose the estate as the site of its new factories. With the 1955 split of hydraulics into two divisions, Industrial Hydraulics moved to Swindon. By the beginning of the 1960s the total number of Plessey workers in Swindon had grown to more than 5,000, and more expansion was to come with the opening of another Cheney Manor factory in 1962, providing a further 350 jobs in resistors. There were now three Cheney Manor sites - called Building 101, Building 102 and Building 103 - as well as the original and now substantial factory at Kembrey Street. By 1969 there were 1,100 employees in Swindon working on Fuel Systems alone, which was plot into two separate departments - Power (ie pumps and motors) and Control (concentrating on the valve business).
- Plessey Titchfield: The 1955 split of the hydraulics activity into two divisions led to Fuel Systems going to Titchfield. In 1977, the site was Plessey Wound Components and Plessey Aerospace, with well over 1,200 employees. Since then the components side of the site was sold in 1980 leaving just the Aerospace business.
- Plessey Towcester: Plessey Microsystems
- Plessey West Leigh (Havant): in 1969 was part of Radio Systems Division, which also operated at Ilford and at Braxted Park, Witham. Worked on MALLARD, the multi-national battlefield communications project. See GEC-Plessey Avionics
UK Registered Companies
- 00079061 PLESSEY-UK LIMITED - Incorporated 6th November 1903. Last accounts made up in 2000. 
2012/12/27 NC Writes:
You mention 3 computers manufactured at Exchange works being aimed at an Australian telephony market. I worked on those and can add a few more bits of info. The computer hardware design was done at Exchange works. The construction and initial testing to successful execution of all instructions was done at 3 facilities, Exchange Works, Beeston and Poole Dorset. I was at Poole.
This was a crash project. We built and tested the 3 computers in 3 months and they were integrated into a working redundant group on a token ring ready for demo in exchange control in another 3 months.I was pulled of my prior job I think in 1966 and told I was on a new project to build a computer. We were given computer register specs and told to design it using limited range of TTL chips in the a.m. In the p.m. we were given 2 pieces of transparent film with circuit card outline and fixed 1416 pin chip pad positions on it and a roll of black tape. We implemented the morning's design in circuit wiring layout with the tape joining the pads. We gave those in and next day we had actual circuit cards made by using the taped film as masters for the PC etch resist exposure.
Subsequent days we assembled cards, built test equipment, and built the computer. One computer took a 5 ft cabinet. That included a core memory also made by Plessey. All instructions were microcoded. The microcode design was customisable using wire-wrap on cards with extendable ANDOR gates for conditional inputs. Each register bit and each BUS bit was displayed on a light on the side of the cabinet to facilitate debugging. A set of 20 switches allowed data entry to registers and also were switches for address input and a single instruction and single clock step function. We worked 8:30 a.m.to midnight 3.5 days a week with 2.5 days off in teams of 2. There was 0.5 day team overlap and the teams at the 3 locations shared debug information daily and implemented fixes the others found.
A similar process was implemented for the integration phase done with all 3 computers trucked to Taplow Court, Maidenhead, then a Plessey Research Center. We were EXTREMELY relieved when the 3 computers still worked perfectly after their truck rides. We successfully had it making phone calls within the second 3 months and also for the Australian demo made it play Waltzing Matilda on speakers. Sound was implemented by using the NO-OP instruction decode and feeding it to audio. Using this we could hear the actual routines and get a sense of proper or improper operation as well as play tunes.
Subsequently, before I went to the USA in 1970 I worked on the PP250 designing the shared memory to CPU bus. Later I joined the same team at ITT in Shelton Connecticut to help design and implement ITT (then Alcatel's) 1240 telephone exchange (another fascinating project) in a dedicated research center built just for that project.
2015/11/2015 MC Writes:
"My name is Michael Corke (Korky) and I started Plessey Radar in 1967 at the age of 15, myself and 11 others were the first to do the EITB apprenticeship course. Some apprentices I remember were: Terry Nolan, Peter Toone, Peter Joyce, Frank Burns, Graham Maitland, the other just a distant memory, I remember all these old men in the factory who were probably 30 or 40 years old and thinking to my self “I am never going to stick this place” and now I am 63.
In 1967 there were large plates being laid for the 909 radar and in 1971 it was ready to be installed. The radar was fitted on board HMS Sheffield which was sunk at the Falklands. I worked on radars as a qualified wireman AR3D, WF3, ACR430, AR5, AR1, Skynet, 909 and more.
1983 I got married on the Saturday and on Tuesday I was asked to go to Peitersburg South Africa to do the up grade to 3 AR3Ds radars - gave up a honeymoon for the company we loved. We worked 7 days a week with overtime to meet deadlines, and of course with cutting edge technology the equipment was always being modified or upgraded.
In 1985 I was promoted to engineering were we came up with CAP computer assisted planning. I left Plessey Radar in 1990 and went to work for Plessey South Africa where I am today in 2015. Plessey South Africa own the name and the logo of Plessey. Plessey Semiconductors UK is up and running once more, their devices were and are the best, winning many awards. Plessey Semiconductors CEO came to Plessey SA in Cape Town to get permission to use the name Plessey, which they are doing and it’s nice to see Plessey in the UK once more. At Plessey Radar Cowes the wiremen, turners, fitters and design guys were the best, our radars were the best and the family at Plessey were very close, our slogan “PLESSEY THE BEST”……. and we were."
1959 Automatic Telephone and Electric Co became the prime contractor for a new UK air defence system, under the code name Project Linesman, which was led by Plessey after the take-over in 1961.
The best staff from ATE Edge Lane were moved to a new building in Cheapside in Liverpool city centre; to maintain secrecy, this was called Exchange Works. The task was to design and manufacture the data processing system for Linesman, which meant designing the system, designing the computers, developing software (which included a new high level language) and manufacturing the hardware (something like 1,000 racks of equipment). The first computer was developed with RSRE Malvern using neon tubes and valves. This was later developed into the XL2 (the XL standing for Exchange Laboratories) built entirely of germanium transistors and using a computer language developed at Exchange Works in the 1950s and 60s. The finished data processing system was installed at West Drayton; this used the XL4, a more powerful machine, and a few XL6 machines developed later. The radar was by Metropolitan-Vickers and Marconi.
The secure status of the factory later attracted other secret contracts and led to it becoming one of the major designers and manufacturers of cryptographic equipment.
As Linesman was being completed a silicon based computer, XL9, was developed for commercial use, using mini-Coral; this was sold to various users including for a wide area traffic light control in Liverpool, an air defence system for Burma, a message switch comprising five XL9s together with lots of communications interfaces for GCHQ, and an accounting system to the GPO for a West London telephone exchange. This was followed by XL12, a really powerful computer, development of which was halted when ICL ?? was formed. Having determined there might be a requirement for a new telecoms computer, development restarted on a computer aimed at the telephone exchange market, specifically for the Australian market. Three computers, manufactured at Exchange Works, working in a redundant system were successfully demonstrated. These were quite unusual as their instruction sets were designed around the requirements of a telephone exchange.
Immediately after this, the PP250 was designed, aimed at military communications systems. These were manufactured at the new Plessey Christchurch site for the Ptarmigan communications system.
Sources of Information
- Into the Sunrise - a history of Plessey by Berry Ritchie (1989)
- Into the Sunrise - a history of Plessey by Berry Ritchie (1989)
- Private communication from Noel Clark
- The Times, Monday, Mar 17, 1947
- THE LONDON GAZETTE, 6 MARCH, 1925.
- The history of Plessey 
- London Gazette, 13 March 1925
- The history of Plessey 
- The Times, Tuesday, Jan 07, 1930
- The Times, Friday, Nov 25, 1932
- The Times, Friday, Dec 02, 1932
- FLIGHT, MARCH 29, 1934
- Into the Sunrise - a history of Plessey by Berry Ritchie
- The Times, 29 July 1960
- The Times, 29 July 1960
- The Times, 10 August 1961
- The Times, 29 December 1961
- The Times, Jun 12, 1962
- The Times, 25 March 1965
- Plessey history: 
- FLIGHT International, 29 August 1974
- The Engineer 1974/04/18
- The Engineer 1974/06/13
- The Times July 23, 1988
- Hansard 20 January 1989 
- The Times, Oct 10, 1969
- Private communication from AB
- The Times Sep 13, 1973
- The Times, Oct 10, 1969
- Companies House
- Private Communication from Allan Isaacs
- Forty Years of Marconi Radar from 1946 to 1986 
- Webster's Online: Plessey 
-  Wikipedia
- The Engineer of 30th August 1968 p306
- 1961 Guide to Key British Enterprises
- 1961 Dun and Bradstreet KBE
- 1937 The Aeroplane Directory of the Aviation and Allied Industries
-  A résumé of the history of Plessey compiled by Hugh Culverhouse
-  British Telephones web site
- Hydraulics