The first tentative steps into electronics for telecommunications began in 1952 with the world’s first electronic switching equipment in the form of an experimental electronic director or register translator at Richmond (London) Exchange. In passing, perhaps it should be mentioned that this was not the first foray of the British Post Office into the electronics field, as it was only revealed as recently as 1975 that during the war the Research Department had developed a series of programmable electronic digital computers code-named Colossus—arguably the first electronic computers in the world.

Reverting to telecomms, arising out of the Richmond experiment and the research that had been going on since 1947, the Joint Electronic Research Committee (comprised of the Post Office and the manufacturers) was set up in 1956. From this was produced a 2,000-line all electronic director exchange opened at London’s Highgate Wood exchange in 1962. It was recognised at the time of its inception that it did not necessarily herald the immediate introduction of electronic exchanges generally and in fact by the following year the problems of time division multiplex (TDM) scheme used at Highgate Wood were recognised as being unlikely to be  solved quickly and attention was concentrated on the development of reed relay systems under electronic control.

The version for small exchanges, TXE2, was first field tested at Peterborough and Leamington in 1965. It was a register controlled multi-stage network of co-ordinate reed-relay switches. The first production exchange was opened by the Postmaster General at Ambergate on 15th December 1966. This exchange marked a milestone in Post Office history as it was also the first production electronic system of its kind in Europe and the first in the world in its particular field of use. Further developments have allowed wider applications of TXE2, such as a satellite exchange in a linked numbering scheme, provision for up to 7,000 subscribers and improvements in service security.

The prototype for the large electronic exchange, TXE1, again using sealed reed relays under electronic control was opened at Leighton Buzzard in 1968, but even before this opened it was recognised that it could be improved (and made cheaper). This later version, known as TXE3, was produced in a 200-line laboratory model only, but a limited public service trial for two years from 1968-76 was conducted using 100 lines of the model exchange connected to a nearby public exchange.

Even though TXE3 had been cheaper than TXE1, detailed economic studies showed that production costs would still be slightly too high to form the basis of a major step forward from Strowger, which together with the recently introduced Crossbar exchanges were still dealing with very nearly al the switching requirements at that time.
Therefore from these experimental exchanges was developed TXE4, which was first tried at Tudor (London) exchange from 1969-72 , and the first production exchange opened at Birmingham Rectory in 1976. Some idea of the difficulties of summarising this equipment here today may be gauged by the fact that the initial manufacturing contract alone was about two inches thick and was supported by an even greater volume of requirements and specifications!

Briefly then, TXE4 which is complementary to TXE2, comprises a control area consisting of a number of identical special-purpose real-time processors with ferrite-core storage, each containing an operating program in a non-destructive read-out store. The actual switching is carried out on a smaller reed insert which enabled significant savings on overall system size to be achieved. A line scanner is used to connect the processor to the calling line. A cyclic store is used, which in addition to driving the line scanner, provides an electronic store of all the necessary information relating to every subscriber or other termination on the exchange

Plug-in units are used which apart from obvious maintenance advantages, mean that installation and exchange extensions are simple and that new facilities, certain to be required during the exchange life, can be added easily. The TXE4 system can be used as an integrated extension of a Strowger exchange permitting replacement or extension of Strowger exchanges, which is an important contribution to the UK exchange modernisation programme.

Such is the pace of modern technology developments that even as the first TXE4 exchange was being ordered a cost cutting space saving version called TXE4A was being developed. It is not a new system, it is just that the control area of the exchange (the cyclic stores, the main, control units and the supervisory processing units) have been re-designed using integrated circuit technology instead of the single circuit elements using discrete components only, which at the time of the TXE4 development were sufficiently proven to be incorporated into designs. The integrated circuit package contained several thousand interconnected transistor cells capable of providing the circuit functions of complete logic elements. Because of the compactness of large-scale integrated circuit devices, the redesigned functional areas occupy far less space than the equipment replaced and work faster, so that less equipment is required to do the work.

A Stored Program Control (SPC) and DTMF (touch-tone) dial receiver front end was a later enhancement to these TXE4 exchanges.


It will be recalled that Highgate Wood (a time division multiplex exchange) seemed at the time to be somewhat of a dead end. However with the practical introduction of pulse code modulation (PCM)—also a time division multiplex system—network it was realised that the principles of TDM switching by high speed electronic gates could be applied directly in a tandem exchange, as the PCM line signal is already time-divided. In order to demonstrate the technical feasibility of PCM tandem switching of PCM junctions serving Strowger exchanges it was decided to construct a model of such a digital exchange at the Research Station in 1967 and this was installed at London Empress exchange in 1968, where it carried live traffic on an experimental basis, until 1975 during which time  it had handled nearly nine million calls and undoubtedly fulfilled its primary purpose of proving the technical feasibility and economic viability of digital switching.

Central control of a switching system is not a new concept, but in the 1960s world-wide interest grew in the control of switching network by computer so that the facilities provided in the exchange could be modified, not by wiring changes but by altering the programming of a computer. Such a system. is termed stored program control (SPC). Although computers using solid state devices were much more reliable than their valve predecessors, a computer for exchange control requires a different approach than that for processing data such as payroll calculations. Such computers should never produce an undetected error, whilst a complete breakdown, although embarrassing, is not catastrophic. The processor controlling a telephone exchange, however, must have outstanding resistance to failure. To test whether computer-industry components would be usable in an exchange (so gaining the benefits of larger production) an experimental exchange  was build at and installed in 1976 in the PO Research Centre and called Pathfinder.

The success of the Empress experiment, the reliability of the equipment, the fact that it was looked after by exchange maintenance staff with no previous experience of digital electronics, was most encouraging and led to digital switching becoming one of the main principles of System X.. [Stan Roberts]

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