design and led to the development of the TXE4. Gallery TXE4 The TXE4 was a cost reduced development of the TXE3 system and catered for up to 40,000 subscribers with over 5,000 erlangs of both-way traffic and was normally staffed by several Technical Officers (TO). This was developed purely by STC to a specification from the GPO. It was built at the STC Southgate factory in north London and used reed relays as the switching medium which proved reliable in service. Later a few exchanges were also manufactured by Plessey and GEC. It had a programmable common control called the Main Control Unit (MCU) and each exchange had at least three MCUs for security and a maximum of twenty, but in theory, could operate with just one. It had a unit called the Supervisory Processing Unit (SPU), which took control of the calls from information supplied to it by the MCU. To prove the enhancements of TXE4 over the TXE3 a test bed trial installation was installed in Tudor exchange in Muswell Hill, North London in 1969. After a successful two-year trial a contract was placed with STC for the provision of £15million of TXE4 equipment in June 1971. The first production TXE4 was installed in 1973 at Rectory, a Birmingham area exchange at Sutton Coldfield, and brought into service on 28 February 1976. TXE4 is sometimes known as TXE4RD where the RD stood for Rectory Design. Rectory opened with 4,300 subscribers and had a maximum capacity of 8,000. In 1983 there were 350 TXE4s in service serving four million customers. The last TXE4s were taken out of service (at Selby, Yorkshire and Leigh-on-Sea, Essex) on 11 March 1998. Exchange description The switching was via reed relays and multi-stage like the TXE1. The difference from the TXE1 design was that an extra switching stage to simplify growth problems. Thus, a typical path would be A-B-C-Link-D-C-B-A. Subscriber information was programmed into the exchange in racks called cyclic stores which used PTFE wire threaded through magnetic cores known as 'Dimond ring' (see TXE2 section for more information). The information stored was the class of service (COS) i.e. PBX, coin collecting box (CCB) or single line, followed by the directory number. The subscribers derived an equipment number from the position on the cyclic store rack. This was a six-digit number and referred to as the MUCKBL. In some parts of the exchange, equipment