TXE, which stands for Telephone eXchange Electronic, were a number of exchanges developed by the General Post Office (GPO), now BT, designed to replace the aging 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. This allowed the competition to develop their own systems ahead of the GPO. In 1960 the situation rapidly changed when the Australian PO rejected a system from the GPO in favour of a crossbar system from the Swedish firm of Ericsson. Suddenly the rules had changed and the race was on to develop an electronic telephone exchange.
Tommy Flowers MBE, a much-underrated and unrecognized genius of the GPO, who had built the first digital computer called Colossus at Bletchley Park that helped break the German codes, had already been working on a fully electronic system before the war. A fully electronic system was built and tested at Highgate Wood, North London in 1962 but it was found to be beyond the technology of the time, in particular the solid state switching technology could not cope with 'nosiy' analogue signals. However, the principles would be used later, as technology improved in the development of digital exchanges the world over including System X. It was decided to use reed relays that were considered to be electronic enough, at the time, for the design of the next generation of exchanges, although Tommy Flowers did not approve. This was when the term TXE was coined.
Jim Warman, a member of the Siemens, later AEI, research team at Blackheath led by John Flood who had worked with Tommy Flowers, headed up the design team. They came up with many of the key principles of reed exchanges i.e. choosing an optimum path through multi stage switching. This all lead to the first exchange of this type namely TXE1, which is described below.
Although the GPO were, as already stated, somewhat late in developing these new systems, nevertheless two major types went into large production and these were the TXE2 and the TXE4.
TXE1
The prototype and the only one was installed at Leighton Buzzard. This exchange type was developed by JERC, Joint Electronic Research Committee. This consisted of ATE, STC, AEI and the GPO. STC built the common control, and AEI the switching and line scanning, whilst ATE took care of the dialling capturing equipment (Registers) and the Out Going Junctions.
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The STC common control consisted of 14 racks and made up a complete suite of the exchange. It was made entirely from discrete components as ICs were not yet in common use. There was much discussion by all contractors as to whether at the time there was a reliable connector so as to provide the ability to withdraw and replace units. STC decided to have the units withdraw and ATE did not. It turned out that the connectors were reliable and STC had a great advantage in fault finding. It also allowed the STC engineers to place a suspect faulty unit in an outrigger so it could be tested in situ.
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One of the jobs of the Common Control was to decide which was the best connection to be used through the switching network and this part was called the Route Choice. The Interrogators would return the available paths and the Route Choice would make a choice and tell the Markers to mark that route.
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The exchange used reed relays as the switching medium, but they were twice the size of the ones in TXE2 exchanges which gave so much trouble later on. It had multi stage switching divided into A, B and C switches and there were devices called links to connect the paths together. A typical local call would go ABC Link CBA. There were two types of links: ones with transmission bridges for local calls and others with no bridges for the out going junction calls. The bridges for the later were contained within the out going junctions.
The exchange had some advanced features at the time i.e. tone dialling was an option as opposed to pulse dialling, with no post dialling delay for own exchange calls. It also had the ability to detect a switching failure and go for a repeat attempt without the subscriber being aware of it. Any repeat attempts were printed on a standard teleprinter. It had a futuristic test console, which monitored all the calls on a digital display.
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A new approach to the inter rack cabling was taken. A ceiling was built above the top of the racks, creating a cable loft. The cables were just pushed through holes in the cable loft and taken to where they were going by the shortest route. The result was a complete mess of a cable loft, but all cables were labelled; it was quicker and easier than the normal way of lacing all the cables.
A novel feature was how the subscriber’s Class of Service information was held i.e. PBX, Shared Service, TOS etc. It was stored on a thin plastic strip, into which could be inserted up to 10 little copper squares which had a capacitance of 10 pico farad. The thin plastic strip was then inserted into the Data Store rack at the position representing the directory number. This can be seen in the photograph together with some plastic strips hanging by wire. Hanging the strips by wire was a common practice for subscribers who were constantly changing their Class of Service i.e. being made TOS. This information was then pulsed by the STC common control Translator and appropriate action taken.
The ATE Registers looked after all the dialling and there were three sorts of Registers:Loop disconnect, MF later called DTMF, and incoming. Loop disconnect and MF (Local Registers) took care of local calls and incoming, incoming junction calls. 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. If it were a local call the Translator would say, "come back when you have the complete telephone number". If not, it would tell the Register what other action to take.
The MF senders/receivers were used when an MF subscriber initiated a call. They were set up to the subscribers line and switching network to an MF register, they converted the MF tones to pulses for the registers to store. They used the X, Y and auxiliary switching planes.
Incoming Registers used a time-shared electronic dial path (TDM) to transfer pulsing information from the incoming junction to the Incoming Register. This feature was necessary to ensure that no pulsing information was lost.
The Registers were made entirely from discrete components. The exchange had about 20 Local Registers and 12 Incoming Registers, each Register containing nine double units, which were hinged and could be lowered for easier access. These units unlike the common control were hard wired. However, a unit could be changed by breaking straps at the rear and then rewiring them, but it was a lengthy process taking about an hour to change just one unit on one Register. A subscriber was connected to the Local Register using the normal reed switching as the Local Registers were connected to the C switches. However, they communicated directly to the common control Translator via hard wiring.
The Translator was a complete rack of the common control (in fact rack 12) and its job as the title implies was to translate information and then give the appropriate action to the required part of the exchange.
In the event of a cable breakdown or other similar event which may result in permanent loops on subscribers lines, after a predetermined time the Register would be forcibly released and the subscriber put into a parked condition. This was possible because each subscriber had a dual armature line relay and in the parked condition the low current armature was operated but the other not. The scanners would ignore any park condition.
The scanners were provided by AEI and as the name implies scanned the subscribers seeking out the ones that had initiated a calling condition. They also detected subscribers in a “parked” condition. The scanners were mounted the racks of the associated switching units and fed back information so that a Register could be switched to the subscriber to provide dial tone.
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The exchange was capable of dealing with 10,000 subscribers but it started out with a capacity of 3,000 and plans were made to double this but this never happened. Tracing of calls within the exchange was instantly displayed digitally on the Test Console.
Occasionally the call trace did not work but the engineers worked out a way of manually tracing a call. What they did was to buy a little compass and glue a piece of magnetic ferrite on the side to pull the compass needle away from North. They would then run this compass along outside of the reed relays and when a relay was operated the needle would move back to North. This was repeated over several sets of the switching path until the trace was complete.
ATE and STC created testers so that parts of the exchange could be taken out of service and the testers connected to these parts. The testers then simulated the signals that the exchange would send to it, and in this way individual parts of the exchange could be tested.
The Out Going (O/G) Junctions were provided by ATE, and yet again were made of discrete components. There were three O/G junctions per shelf and they could be busied by using the keys that can be seen in the photograph.
There was an interesting opportunity using the three contractors as to how the different parts of the exchange were to be connected together, and much time was spent on this.
A display was created next to the test console, which gave a visual indication of the traffic flowing through the exchange, and it was named the Hubblemeter after the instigator Ray Hubble.
The TXE1 required power supplies of -18 V, +50 V and -50 V DC. These were provided in the usual manner by lead acid rechargeable batteries charged from the 240 Volts AC mains supply and backed up by a diesel generator in case of mains power failure.
The exchange went into service in 1968 and proved reasonably reliable although it did have a few outages. It was withdrawn from service in 1977 when it was replaced by a TXE4. Unfortunately the whole exchange was then scrapped.
TXE2
The most prolific and 2-3000 were produced. They were designed to serve up to 2000 customers and about 240 Erlang units. They were therefore mainly used to replace the larger rural Strowger exchanges - usually UAX13s. The first TXE2 was Ambergate near Matlock in Derbyshire and it opened on the 15th December 1966. The last TXE2 closed on June 23rd 1995.
In the summer of 2005 a demonstration rack of TXE2 equipment was transferred to the Connected Earth collection at Milton Keynes Museum. See [1]
For a virtual tour inside a Plessey mobile TXE2 (or MXE2, as they were called) at Avoncroft Museum see [2]
A few details about the MXE2s are given at [3].
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The TXE2 system was a common control design. A call which terminated within the same exchange went through 7 switching stages, whereas a call going out to another exchange went through just 3 switching stages. The switches were designated as A, B, C and D (the paths were ABC for outgoing, ABCDCBA for internal and DCBA for incoming). The common control equipment consisted of A-Pattern Relays, B- and C-Switch Selectors, Supervisory Selectors (a Supervisory relay set stayed in-circuit throughout each call), Register Selectors, Registers and Call Control.
Because of their common-control design, isolation (inability of the exchange to set up calls) of the entire exchange was always a possibility and very occasionally happened. This potential weakness had been at least partially recognised in the design of the exchange type, so the most critical common control units were duplicated into an A-side and a B-side. The exchange automatically changed from one security side to the other every 8 minutes and, in the event of the equipment detecting a serious fault in one of the side-conscious units, all the units in that security side were locked out of service and a prompt alarm was sent to a manned centre to indicate that the exchange needed urgent attention.
As a further security measure, if the first attempt to set-up a path to a Register failed, so that, on an outgoing call, the customer did not get dial-tone, the exchange recognised the failure, stored the details of the equipment in use on the failed call and automatically made a second attempt, using different equipment. This happened so quickly (about 60 milliseconds) that, if the second attempt was successful, the customer would not have been aware of the failed first attempt to get dial-tone.
A security weakness on the rural Strowger exchanges (UAX 13s and smaller) had been the electrical power supply. If the main feed went down, the exchanges relied on back-up batteries, but these could only keep the exchange going for a few hours and after that, service was lost across the whole exchange to typically several hundred customers. The TXE2s were much better equipped. They had a separate power room, itself often bigger than the entire UAX 13, in which, as well as back-up batteries, there was a stand-by diesel generator. This started automatically in the event of a power cut and could keep the exchange going for days. Isolations due to power cuts therefore became a thing of the past, but at considerable initial capital cost.
As a maintenance aid, the exchange was equipped with a Maintenance Data Recorder (MDR). This had a rather primitive printer, which displayed the identities of equipment in use at the time that the exchange had detected a call failure. For example, in the event of a successful repeat attempt to provide dial-tone, the MDR would print. If the repeat attempt failed, then the MDR would print twice in quick succession, giving details of both sets of equipment in use on the failed paths. The prints were not easy to read. All that emerged were short burn marks on the special paper in up to 45 different places in each of two rows. It was necessary to hold a plastic graticule over the paper to find out what the presence of each burn mark indicated. If more than 8 call failures were detected in less than 8 minutes, then the critical common control units would be forced to change from the side in service (A or B) to the other side, the automatic 8-minute changeover would be suspended and a prompt alarm would be sent out.
The picture (left) of the TXE2 equipment in Hullbridge Telephone Exchange gives a good impression of the inside of a typical early TXE2. They were quite spacious (very spacious compared with the Strowger UAX13 which most of them replaced) and with the cleaner generally living locally and taking a pride in the job, they were usually kept very clean and tidy, as in this example. The picture shows the Control Suite in a Plessey exchange. There would typically have been 5 or 6 suites, with plenty more floor space, to allow for growth.
All the TXE2-specific equipment was mounted on slide-in units, mainly single-width, but some double-width. There was a carefully structured holding of maintenance spare units. For those which were likely to be needed frequently or urgently in every exchange, such as a Subscribers Line Unit (a single unit containing the line relays and A-switches for 5 customers), a spare unit was held in every exchange. For those units for which a spare was likely to be needed less frequently or urgently, the spares were held at an Area centre serving perhaps 6-10 TXE2s of the same manufacture. Finally for those units for which a spare was likely to be needed seldom, the spare units were held Regionally. There were 10 Regions in BT at that time.
The switching in TXE2s was carried out by reed relays. A 'reed' was actually two over-lapping ferromagnetic blades hermetically sealed within an inert-gas-filled glass capsule. The blades were gold-plated and separated by just a few thousandths of an inch. The reeds were fast in operation (about 1 millisecond), with a life expectancy of more than 10 million operations. The glass capsules were about an inch (25 mm) in length and about an eighth of an inch (3 mm) in diameter. Four reeds were generally present inside each relay coil. Switching with these reeds held out the prospect of much greater reliability compared with the Strowger system, where switching was carried out by base metal wipers moving through banks of metal contacts. The Strowger switches required routines to be carried out on them to clean the banks: they also required oiling and occasional adjustment. Reed relays required none of this. However, in practice, and particularly in the early years of the system's service, the performance of the reeds proved to be less good than had been expected.
The picture (right) of a TXE2 Supervisory relay-set gives an idea of the technology involved. 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. 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. The "candles" were used throughout the exchange as part of fault-finding.
For security, the GPO had insisted on competitive tendering for the TXE2 exchanges. Production contracts were then awarded to Plessey, STC and GEC. The TXE2-specific equipment in these exchanges differed between the manufacturers, so that spare equipment had to be held for each type of manufacturer. Importantly, each manufacturer made their own reed inserts (GEC actually out-sourced theirs to the Mazda Osram Valve Company) and their performance differed significantly in the first years of production.
This picture shows an STC/GEC SLU (Subscribers' Line Unit - in those days customers were always called subscribers, invariably shortened in conversation to 'subs'). It handled the traffic to and from 5 customers, and had 5 trunks going on to the B switches. There is therefore a 5X5 switching matrix of reed-relays, which constituted the A-switch. Note that the 4 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, 2 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 (PO Type 12 for Plessey and Type 23 for STC/GEC), as the reeds 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.
In the early Plessey exchanges a significantly high proportion of the reed-inserts were contaminated with a high-resistance film and were prone to giving an intermittently high-resistance contact. If this occurred in one of the common-control areas of the exchange it could, and did (despite the A- and B-sides described above), give rise to the exchange becoming isolated (being unable to set up any calls) for perhaps several hours in a worst case. 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.
The STC reeds proved to be more reliable, but, if they failed, they tended to stick or fail short-circuit. This was also a cause of isolations early on, but a simple modification restricted the most serious failure to a small part of the exchange. The GEC/MOV reeds proved to be the most reliable of all.
Once the teething troubles had been largely dealt with, which was not until about 1973, 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 5-6000 customers in total.
TXE3
A cost reduced and improved version of TXE1 designed for exchanges with more than 2000 subscribers. In actual fact it was very similar in design to the later TXE4. A prototype was built at Royal exchange in London and a two year trial from 1968 to 1970 took place where 100 subscribers from the Strowger unit were switched over to the TXE3. However, the TXE3 was still found to be too expensive so TXE4 was developed.
TXE4
A larger type catered for up to 40,000 subscribers with over 5,000 erlangs of bothway 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. It used reed relays as the switching medium which gave little trouble. 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 Control Unit, which took control of the calls from information supplied to it by the MCU.
As stated the switching was via reed relays and multi stage like the TXE1. The difference from the TXE1 design was that an extra switching stage called the D switch was added. So a typical path would be ABC Link DCBA. There was much debate about the D switch and whether it was necessary and it was an idea that Jim Warman had worked hard to avoid, never the less it was used to simplify the growth problems.
The subscriber information was programmed into the exchange in racks called cyclic stores. This was a PTFE wire threaded through magnetic cores. The information was the Class of Service(COS) i.e. PBX, CCB or just a DEL and then 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, which describes to the exchange where this is. In some parts of the exchange equipment the equipment number was used as BUMCLK and this was used by engineers as a pseudo swear word. When a subscriber lifted their handset it sent a pulse down this wire, which was picked up by a 156mS scanner, which initiated a path to be set up through the reed relays so the subscriber could be connected to a Register. This Register then returned dialling tone to the subscriber and dialling could commence.
The Registers were "owned" by the MCUs and each MCU had a maximum of 36 Registers and 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. Once it decided this it sent a command to the Interrogator/Markers to set up the required path and then moved on to the next task. Once the connection had been established, the SPU took care of the path and all the call metering tasks. The MCUs all had 36 Registers and had solid-state 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 and Outgoing Junctions, and 12ms for Incoming Junctions. The last of these was the quickest scan, to ensure that no pulses were lost from the incoming junctions.
As the exchange ran on pulses, unlike the TXE2 design, there was a Pulse Generator Rack, which provided these pulses. The generator produced a basic pulse of 6 microseconds duration and this was then multiplied up into the various requirements. Because of the vital importance of these pulses most circuits in the Pulse Generator were quadrupled.
A problem was discovered very late in the development of the TXE4 in that an equipment number could be threaded with the wrong directory number by mistake. Even worse it could be the directory number of another equipment number leading to multiple directory numbers. The exchange had no way of detecting this but the problem was solved in a very novel way.
The MCU was run by a programme that was stored in 10 Slide in Units (SIU) located at the bottom of the MCU rack. These units were called MTWS, which stood for Miniature Wire Threaded Store which was a misnomer as there was nothing miniature about them. The MTWS was a matrix of eight by ten cores through which enamelled wire could be threaded. Each MTWS held 500 programming steps and so the whole exchange had to be run using 5000 steps, which was a remarkable feat. It was even more remarkable because it only used the first 8 MTWS as the last two were reserved for special routines.
The 5000 programming steps were named with the letters A-E and then three numbers e.g. B525. Each step consisted of 8 pieces of information and these were of value from 1 to 10 depending on what core the wire was threaded through. The first three digits told the MCU what program step to go to next e.g. 891 and the first letter being decided usually by a decision. I.e. whether some information was present A=yes, B=no resulting in either A891 or B891. The next two pieces of information gave what the operation was e.g. 55 compare two pieces of information and the last three told the MCU where to store the result i.e. 020 put this information in MFS (Main Ferrite Store) 10(see next paragraph for more information). So the whole programme step would be 89155020. Each step took 12 microseconds to execute. This program, the software of the exchange, could easily be changed as was developed throughout the life of the TXE4 design.
The MCU contained a non-volatile data store, which used core store. There were three types of data store, Main Ferrite Store (MFS), Special Ferrite Store (SFS) and Register Ferrite Store (RFS). The MFS was used by the MCU itself to hold data for various reasons and the SFS was used for manipulating data. An example of this was that SFS2 could take the data stored is positions 1-5 and swap them with data stored in positions 6-10. It can be seen from this that each store had 10 positions and each one could hold values from 1 to 10. Incidentally this was stored in 2 out of 5 code. I.e. 11000 = 1, 10100=2, 01100=3. The RFS held data from each of the MCU’s associated registers e.g. dialled digits. There were 20 MFS, 4 SFS and up to 36 RFS.
The MCU was in control of setting up switching paths and it knew when a path had not been successful. In this case the MCU would instigate a repeat attempt to set a new path. The details of the failed path were sent to the printer.
The TXE4 had two standard tele-printers, which were used to give fault indications and other information. The difficulty of manually spotting trends brought an attempt to take the paper tape that the tele-printer produced, as well as the print, and analyse it. It was called PATE4, which stood for Print Analysis TXE4 and was a software programme that read the paper tape looking for common fault patterns.
The first TXE4 opened at Rectory near Birmingham in February 1976, although a test-bed exchange had been installed prior to this at Tudor, in North London but this never went into service. These exchanges were designed for a Mean Time Between Failures (MTBF) of 50 years and, although this was never quite achieved, it could be argued that the design requirement had been met if you discounted human interventions!
TXE4A
The last in the line of TXE exchanges, it was an improved version of the TXE4 design. 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. The first TXE4A to enter service was Belgrave, Leicester on 28th February 1981. Altogether over 500 TXE4 and TXE4A exchanges were installed and they were around for over 20 years. 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 the 11th March 1998 when Selby and Leigh-on-Sea were replaced by digital exchanges.
TXE5
This type number is believed to have been reserved for an improved version of the TXE2. Such a version was never produced.
TXE6
An electronic exchange that was designed to extend Strowger exchanges, and known as the Electronic Reed Selector System. Only two were built: one in London and the other at Leighton Buzzard. The one in London was moved and combined with the one at Leighton Buzzard, incidentally the Leighton Buzzard one was coloured cream and the London one grey so when they were combined it was easy to tell where each unit came from.
It was never used for its intended purpose but merely acted as the front end to incoming junction calls and directing them to the appropriate part of the exchange decided by the first dialled digit. As the name suggests it used reeds as its switching medium.
The TXE6 basically consisted of two parts; a unit for receiving digits at 10 pulses per second followed by a two stage reed cross point switch.
References
The Leighton Buzzard Electronic Telephone Exchange - S. H. Sheppard IPOEE Journal January - March 1967.
The Leighton Buzzard Electronic Exchange - T. J. Shiplee IPOEE Journal April - June 1972.
Electronic Exchanges: The Steps Leading to TXE4 - C. A. May IPOEE Journal October - December 1972
TXE4 Electronic Exchange System Part 1 - J. V. Goodman, J. L. Phillips IPOEE Journal - January - March 1976
TXE4 Electronic Exchange System Part 2 - J. L. Phillips, M. T. Rowe IPOEE Journal - July - September 1976
Acknowledgements
Tom Shiplee for pictures of the TXE1.
Tom Shiplee for supplying actual TXE1 reed inserts.
BT Archive for supplying most of pictures of TXE2 and TXE4.
Christopher White for TXE4A pictures.
Harvey Berry for additional TXE4 pictures.
For actual pictures source double click the actual picture.
Thanks to The Communications Network for further background information on Tommy Flowers.