page_title: TXE
section_title: Exchange description
hierarchical_section_title: TXE / TXE4 / Exchange description
context_page_description: TXE, was a family of telephone exchanges developed by the British General Post Office, designed to replace the ageing Strowger systems.
  When World War II ended, the UK telephone exchange suppliers supported the GPO’s decision to stay with Strowger until a viable electronic system became available. The GPO largely did this to protect their success in the export market, but it actually had the effect of ultimately destroying it. This allowed competitors to develop their own improved switching systems ahead of the GPO. In 1960 the situation rapidly changed when the Australian Postmaster-General's Department rejected a system from a consortium of British manufacturers who offered a register-controlled version of a motor-uniselector system in favour of a crossbar system from the Ericsson. Suddenly the rules had changed and the race was on to develop an electronic telephone exchange that could operate with the current GPO telephones used in the UK, including shared service.
context_section_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 was in the format of BUMCLK. When a subscriber lifted their handset it sent a pulse down this wire, which was picked up by a 156 ms scanner, which set up a path through the reed relays to a register.  This register then returned dialling tone to the subscriber and dialling could commence.
  Up to 36 registers were "owned" by each MCU.  The MCU was responsible for looking after all of its Registers and deciding from the dialled information where the call was going to be routed. The local exchange numbering system would be available to the MCU via threadings in the cyclic store so the MCU would be able to read this information and in this way, all exchanges could be configured as required.  If a call was identified by the MCU as internal to the exchange, usually by the first digit, then the MCU would tell the Register to come back when the complete number had been dialled.  If the first dialled digit was a zero then this would normally be routed straight to the Group Switching Centre.  However, if the exchange had an Alternative Available Route then the MCU would have to wait until enough routing digits had been received to make a routing decision.  This AAR information was stored in the cyclic stores. Once the MCU had decided the routing, it sent a command to the Interrogator/Markers to set up the required path and also told the Register what dialled digits to forward on. Then the MCU would move on to the next task.  Once the connection had been established, the Supervisory Processing Unit (SPU) took care of the path and all the call metering tasks. The MCUs had core memory to hold the dialled digits from all the Registers and also had other storage to manipulate call set-up information. There were three scan rates: 156ms for subscribers, 36ms for Registers, Outgoing Junctions and Routing Translations, and 12ms for Incoming Junctions.  The last of these was the quickest scan, to ensure that no incoming digits were lost from the incoming junctions.
  Timing pulses were generating by the Pulse Generator Rack. The generator used a delay line oscillator of 166.7 kHz to produce a basic pulse of six microseconds duration and this was fed to eight ring counters which then multiplied up the basic six microsecond pulses into the various pulse requirements.  There were four generators with a redundancy of one.
  A problem was discovered very late in the development of the TXE4 in that if an equipment number was threaded with the wrong directory number on the cyclic stores, it could clash with the directory number of another equipment number leading to multiple directory numbers. This would cause Number Unobtainable (NU) tone when the incorrectly threaded number was dialled and the double threading number received incorrect calls.  The exchange had no way of detecting this but the programming of one of the MCUs allowed a separate program to run to detect errors and print out the locations of the duplicates.  This had to be done on a regular basis.  Eventually, after many more diagnostic routines had been added, this became known as Tester 299A.
  The MCU executed a program stored in 10 Slide in Units (SIU) located at the bottom of the MCU rack.  These MTWS units (Miniature Threaded Wire Store), were matrices of eight by ten cores through which enamelled wire were threaded. Each MTWS held 500 program steps.
caption: TXE4 Floor Plan
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