TXE

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.

This lab was headed by John Flood, who had been a founder member of Tommy Flowers' electronic switching team at Dollis Hill.

At this time, in the US, Bell Labs were developing a system based on electronically controlled reed relays, and this looked promising.

STC built the common control, AEI the switching and scanners, line scanning and test console, and AT&E the dialling capturing equipment (registers) and the incoming and outgoing junctions.

A local register would provide dial tone to the subscriber, wait for the first dialled digit, and then apply to the translator to see what action was required.

Periodically over some weeks, the mercury would migrate to the point of contact of the blades, leaving a mercuric bead giving “ON” and both A and B sides into service.

In periods of very low traffic, there would typically be fewer than eight call set-up attempts on the exchange in eight minutes and this would have prevented the above security system from working.

Unlike the preceding rural Strowger exchanges (UAX 13s and smaller) the TXE2s were equipped with an uninterruptible power supply with auto-starting diesel generators.

As the TXE2 call-set-up time was some 50 milliseconds, this design requirement was just met, but even so, the overall capacity of the system was determined by the probability of an incoming call being delayed too long in its initial connection to a register.

These faults were very difficult to locate and in the end, the problems were only resolved by a fairly substantial re-reeding programme carried out on the common-control units of the early Plessey exchanges.

Once the teething troubles had been largely dealt with, which was not until about 1974, the TXE2s realised more of their expected benefits and it was eventually not uncommon for one Technical Officer to maintain the operation of three of these exchanges, serving perhaps some 5,000–6,000 customers in total.

It did, however, take a lot of subsequent work by the switch maintenance Technical Officers to get the exchange up to an acceptable standard of service, as it had been standing idle for several years.

The takeover bid was successful and GEC decided that they preferred the crossbar system to TXE3 and promptly cancelled the contract to supply TXE3 to the BPO.

The MCU operated in accordance with an instruction program stored in the form of a number of wires threading a bank of magnetic cores.

This information was returned to the Route Choice Unit, which then identified those link circuits, which were available to both peripheral terminals, and selected the most suitable, according to predetermined rules chosen to make maximum use of the network.

In every case the MCU would decide, in accordance with its program instructions, what connection pattern was appropriate in the circumstances indicated and issue orders for setting the paths.

Within each MCU information was handled in a "two-out-of-five" code which enabled errors to be detected, and the output of the program store was duplicated to give additional protection.

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).

It was built at the STC Southgate factory in north London and used reed relays as the switching medium which proved reliable in service.

The information stored was the class of service (COS) i.e. PBX, coin collecting box (CCB) or single line, followed by the directory number.

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.

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.

The difficulty of manually spotting trends brought an attempt to take the paper tape that the teleprinter produced, as well as the print, and automatically analyse it.

It had the same switching as TXE4 but a redesigned common control, using integrated circuits (including microprocessors) to achieve significant size and cost reductions.

This allowed for changes to exchange data i.e. customer information to be made by keyboard instead of by manually threading jumpers through Dimond rings.

The enhancement features were implemented on additional dedicated processing modules interconnected by an Ethernet backbone with the MCU and SPU processors.

The TXE4E replaced the ten Miniature Threaded Wire Stores (MTWS) of the TXE4 with two units, each containing six chips which were removable and re-programmed with a separate computer.

Tariff changes for the eight million customers could be built and implemented by one person following the introduction of a centralised data management tool.

It was never used for its intended purpose but merely acted as the front end to incoming junction calls at Leighton Buzzard and directing them to either the TXE1 or one of the three TXE2 exchanges, which was decided by the first dialled digit.

unit was the interface, converting information from dialled pulses, in Strowger form, to fast parallel signal conditions for the reed group-selector registers.

TXE1 Reed Relay
A TXE1 switching rack capable of dealing with 1,500 subscribers
Half of the racks making up the TXE1 common control
A unit removed from the TXE1 common control; this was the only part of the exchange where units could be removed, the rest being hard-wired
TXE1 reed relay inserts, which rarely failed
A TXE1 test console without the rack covers in place. The teleprinter can be seen to its right.
The TXE1 cabling loft
An engineer examining an outgoing junction
The TXE1 data store and translator, the twelfth rack of the common controller
Ambergate the first TXE2 Telephone Exchange
Alarm Panel on a preserved TXE2 at Avoncroft Museum . The red lamps show that all three sections are alarmed. The 8-minute changeover will have been suspended and the white lamps show that all three security sections are locked on to side B.
TXE2 Register just before call set up
MDR printer
MDR graticule
Control Suite in Hullbridge Exchange, a typical early TXE2 installation. It was spacious in comparison to the Strowger UAX13 which it replaced.
TXE2 Subscribers' Line Unit (SLU) manufactured by STC. Each SLU contained the line relays and A-switches for five customers. It handled the traffic to and from five customers and had five trunks going on to the B switches. There is, therefore, a 5×5 switching matrix of reed relays, which constituted the A-switch. Note that the four reeds in each of these reed relay were in-line, whereas in Plessey reed-relays the reeds were in a square formation. The SLU also contained 10 electro-mechanical relays, two for each line. They were the Line Relay (LR), which was operated when the customer picked up the handset and which generated the calling signal, and a K relay which gave the correct tones and prevented spurious calling conditions. These two relays both provided change-over contacts and therefore had to be electro-mechanical because the reed-relays only gave make-break contacts. The face-plate of the unit is to the right: at the other end, one can see the edge connector . It was feared that this type of connector would cause problems after a relatively low number of removal/re-insertion operations, but in practice, they proved to be more than adequately robust.
TXE2 Ambergate Calling Number Generator (CNG) and Class of Service (COS) rack
TXE2 trunking
TXE2 Supervisory Relay Set. Designed in the 1960s, it consisted of discrete components mounted on circuit boards. These relay sets were of double width. On the face-plates there were two built-in lamps (for call-tracing and fault indication) and a block of test points, which gave test access to the circuits inside. All TXE2 units had such test points. Three "candles" can be seen protruding from units: these were simple indicator-bulbs which were used as required to show when the relay sets were in use. These "candles" or "busy indicators" were used throughout the exchange as part of fault-finding.
TXE3 racks see top of the racks for description. The MCU is different from the TXE4 and includes the cyclic store
TXE2 (left) and TXE4 (right) switching matrix SIU in the London Science Museum
These racks contained the subscriber COS in the first four levels and their telephone number the last 5 levels. Note clipped on white cores which provided TOS (Temporary Out of Service) status
This is the panel of the MCU which was the processor of the exchange
Two engineers Dave Atkins (left) and Tim Walker (right) examine a MTWS
TXE4 Floor Plan
The building that held the Felixstowe TXE4
The building that held the Headington TXE4 near Oxford
The Outgoing trunking from part of the handwritten documentation of the TXE6 at Leighton Buzzard. It shows the unique environment of having a TXE1, three TXE2s and a TXE6 in service in the same building all at the same time.
The Rack Layout from the handwritten documentation of the TXE6 at Leighton Buzzard.