translator. AT&E and STC created testers so that parts of the exchange could be taken out of service and connected to testers. The testers then simulated the signals that the exchange would send to it, and in this way individual parts of the exchange could be tested. Gallery TXE2 The prototype for the exchange which the GPO called TXE2 was a system called Pentex (the Plessey trademark for all non-GPO sales), which, starting in 1963, was developed by Ericsson Telephones, as part of Plessey. The first field trial of Pentex started in the Peterborough Telephone Area in 1965. There was another trial site at Leamington. The system was designed to serve 200–1,200 customers and about 240 Erlang units. It was therefore mainly used to replace the larger rural Strowger exchanges – usually UAX13s. The first TXE2 was installed at Ambergate, some 20 miles from the Plessey factory at Beeston, and opened on 15 December 1966. Although the system had been developed by Plessey, the GPO had insisted on competitive tendering for the TXE2 exchanges. Production contracts were awarded simultaneously to Plessey, STC and GEC. Some 2–3,000 TXE2s went into service with the GPO, the last one being withdrawn from service on 23 June 1995. The Pentex system, which evolved beyond TXE2, was exported to over 30 countries and was largely responsible for Plessey winning the Queen's Award for Exports in 1978. Exchange description Because of their common control design, isolation (inability of the exchange to set up calls) of the entire exchange was always a possibility and very occasionally happened. This potential weakness had been at least partially recognised in the design of the exchange type, so the most critical common control units were split into three sections and each section was duplicated into an A-side and a B-side. In the event of the equipment detecting a serious fault in one of the side-conscious units, all the units in that section were locked onto the side which was giving good service and a prompt alarm was sent to a manned centre to indicate that the exchange needed urgent attention. In normal service, the exchange automatically changed all three sections from one side to the other every eight minutes. If Call Control detected eight failures to set up calls within that eight minutes, then it force-changed all the side-conscious units onto the other side, locked that side in service and raised