TXE4A
TXE4A was the last in the line of TXE exchanges and an improved version of the TXE4 design. This was again developed by STC after BT commissioned them, in 1975, to produce a 15% reduction in cost and provide more customer facilities. It had the same switching as TXE4 but an upgraded common control, using integrated circuits (including some of the first microprocessors) to achieve significant size and cost reductions.
TXE4A dispensed with Dimond rings and used solid state memory. This allowed for changes to exchange data i.e. customer information to be made by keyboard instead of by laboriously threading jumpers through Dimond rings.
The TXE4 MCU program was stored in EPROM with capacity for up to 32k 16 bit instructions which were structured in a similar manner to mini-computers of that era. Each instruction took 2 microseconds to execute except those accessing Registers which were stretched to 6 microseconds. The higher performance enabled the maximum number of registers per MCU to be increased.
The first TXE4A to enter service was Belgrave, Leicester on 28 February 1981. Altogether over 550 TXE4 and TXE4A exchanges were installed and they were around for over 20 years serving 8 million lines. During its life the TXE4/A system proved to be highly successful and reliable until eventually replaced by System X. The TXE4 era came to an end on 11 March 1998 when Selby and Leigh-on-Sea were replaced by digital exchanges.
Incomplete List of TXE4A Exchanges
| Exchange Name | Region | Opening Date | Closing Date | Exchange Name | Region | Opening Date | Closing Date | |
|---|---|---|---|---|---|---|---|---|
| Belgrave | Midland | 20/02/1981 | NA | Hampden Park | NA | NA | NA | |
| Ascott | Northern HC | NA | NA | Hayling Island | SE | NA | NA | |
| Bicester | Eastern | NA | NA | Horley | SE | NA | NA | |
| Bognor Regis | SE | NA | NA | Isle of Man | NW | NA | NA | |
| Chelmsford | Eastern | NA | NA | Leigh-on-Sea | Eastern | NA | 11/03/1998 | |
| Clevedon | Wales&West | NA | NA | Queensmere Slough | Northern HC | NA | NA | |
| Colchester | Eastern | NA | NA | Shoreham-By-Sea | SE | NA | NA | |
| Colchester (High Woods) | Eastern | NA | NA | Stanford le Hope | Eastern | 28/11/1982 | NA | |
| Dunstable | Eastern | NA | NA | Stevenage | Eastern | NA | NA | |
| Eastbourne Neville | SE | NA | NA | Summertown | Eastern | NA | NA | |
| Eastwood (Southend) | Eastern | NA | NA | Tilehurst Reading | Northern HC | NA | NA | |
| Fleet | Northern HC | NA | NA | Waterlooville | SE | NA | NA | |
| Grays Thurrock | Eastern | NA | NA | Windsor | Northern HC | NA | NA |
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