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It is now over 10 years since the first production electronic exchange was opened by the BPO. Since this first TXE2 exchange came into service at Ambergate, Derbyshire, in 1966, more than 800 TXE2 exchanges have been installed, and there has been a progressive development of the original TXE2 concept to cater for the changing needs of the expanding BPO network. The evolution from the original (Mark 1) TXE2 exchange to the current (Mark 3) system is also described in this issue.

INTRODUCTION 

The reed-elecCronnc local-exchange system, designated TXE2, was introduced into the British Post Office (BPO) telephone network in 1966 and, to date, more than 800 exchanges of the same generic type have been installed. Continued expansion of the national telephone network has necessitated the progressive development of the original TXE2 concept. Today, Mark 3 TXE2 exchanges offer greater telephone traffic capacffy, an increased number of subscriber connexions and enhanced system security, when compared with the Mark 1 exchanges. This article outlines the operaiona! requirements that led to the progressìon from the original (Mark 1) TXE2 to the current (Mark 3) exchange, and examines the differences between them. Earlier articles1’2 have described the design philosophy and physicaa realization of the Mark 1 and 2 exchanges, but, to facilitate an appreciation of the Mark 3 exchange, a brief recapffulation of the early TXE2 development phases is given. 

MARK 1 TXE2 EXCHANGES Mark 1 exchanges, the first of which was installed at Ambergate, Derbyshire, were the result of a successsul collaborative development involving the BPO and the principal exchange equipment suppliers.* The manufacture and supply of TXE2 exchanges to the BPO is currently undertaken by the General Electric Company Limited (GEC), Plessey Telecommunications Limited (PTL) and Standard Telephones and Cables Limited (STC). Mark 1 TXE2 exchanges provide for up to 2400 subscriber connexions and were designed to replace small locaa non-director exchanges functioning within seefcontained numbering schemes.^ Early in the TXE2 installation programme, modification of the exchange system became necessary to allow wider use as a satenite exchange in linked numbering schemees* Modifications to the common-control equipment and register were necessary to allow TXE2 exchanges to function in linked as well as seef-contained numbering schemes. The modified exchange, providing for up to 4200 subscriber connexions, was designated the Mark 2 TXE2. 

THE MARK 3 TXE2 SYSTEM The Mark 3 TXE2 system has been designed to carry approximately 360 erlangs of offered traffic; equivalent to 450 erlangs of A-B link traffic with an average call-holding time of 2 min. 

with larger stages; that is, a l-out-of-8 primary stage and a l-out-of-9 secondary stage. This method enables a Mark 3 TXE2 exchange to be equipped with a maximum of 72 registers, and is shown in Fig. 3(a). The second method used to increase the register provision is to add a third stage (known as pre-seeeetor and pre-finder) of l-out-of-3 to the existing l-out-of-5 primary and l-out-of-6 secondary register selectors and finders. This method enables a Mark 3 TXE2 exchange to be equipped with a maximum of 90 registers. Fig. 3(b) illustrates this method of register selection. To generate the higher traffic, it must be possible to connect more subscribers to the exchange. This required redesign of the CNG rack to increase the subscriber’s line circuit capacity from 3000 to 3500 and enable a second CNG rack to be added. A Mark 3 TXE2 exchange can accommodate a maximum of 7000 subscriber’s line circuits and 440 incomingsupervisory relay-sets. The additional subscribers and incoming junctions require the exchange to have a larger number capac^y. The number capacity of the ciass-of-service rack has been increased from 4000 to 5000 by redesignmg the rack layout to accommodate 10 additional final decoder units; one unit decodes 100 numbers. A Mark 3 exchange, with 2 classof-srrvier racks, can provide a maximum of 10 000 directory/ engineering numbers. 

TXE2 SYSTEM SERVICE SECURITY IMPROVEMENTS Prioir to devdopment that enhanced the TXE2 system’s capabilities, some 10 years of operational, planning and maintenance experience of the syssem had been gained. Improved knowledge and understanding of red-relay contact performance under varying circuit conditions enabled a searching rerpprrlsal of the system design to be considered. Assimilation of system fault reports and detailed invessigation of circuit performance showed that the TXE2 syssem security could be improved by (a) reduecng the effect of single component failures by circuit rearrangement to alleviate component stress and prevent unwanted feedback paths, (b) rdditional monitoring of pulse and control highways, and (c) more precise automatic fault detection and indication, to prevent good equipment appearing faulty due to reflected faults. 

Examples of Security Improvements Some detailed examples of the seenrity improvements are given below. (a) During call set-up to or from a subscriber’s line circuit, the call control interrogates the line circuit to determine its state: free, busy or parked. This is achieved by a detector, which recognizes a voltage representing the appropriate line circuit state. Part of the circuitry is shown in Fig. 8(a); the KT(A) and KT(B) leads are connected to the cal control, KT(A) to security ride A and KT(B) to security side B. A 

condition can occur whereby a welded (short circuited) MIK reed contact severely restricts outgoing and own-exchange traffic. The problem has been minimized firstly by reducing the likelihood of the MK contact becoming short circuited and, secondly, by preventing any effect on the exchange if it becomes short circuited. Switching excessive currents (typically 1A) causes arcing between the reed contact blades, which tends to weld them. MK contacts can be welded by large back-e.m.f.s produced when relay K releases. Quenching relay K with a diode reduces the back-e.m.f.s to less than DOV, and thus reduces the risk of welding of the MK contacts. If an MK contact still welds, its effect on the exchange is minimized by providing blocking diodes on leads KT(A) and KT(B) to the call control. This prevents unwanted feedback to other subscribers’ line circuits. The revised circuit is shown in Fig. 8(b). (b) Three methods of changing over the security sides in a TXE2 exchange are possible: automatically by normal exchange operation every 8 min, manualy by a local engineer, or remotely from a test desk. Fig. 9(a) shows part of the circuitry. To prevent inadvertent change-over from a remote test desk (due to the effect of junction capacitance, typically greater than 2 pF), a 400 ms delay had to be inserted in the test access supe'visory relay-set. This eliminates transient operation of relay AA, a contact of which provides the security change-over signal. The modified circuitry is shown in Fig. 9(b). (c) A redesign of the register fault logic has eliminated a possible source of exchange service ressriction. Under certain conditions, some or all exchange registers could lock out simultaneously. The busy limit circuit, which restricts the number of register lock-outs, is effective only when sequential fault signals occur. To cater for the simultaneous lock-out situation, a change in the register signaHing sequence to the busy--imit circuit was necessary. The redesign allows a register to notify the limit circuit of its pending busy state before expiry of the pulse time delay; previous:/, the register had to wait until after locking out to notify the busy-limit circuit. The net effect is that the busy limit operates if a number or all registers are waiting to time out to the same pulse (that is, a simultaneous lock-out situation) and, on the arrival of the timed delay pulse, not more than the limited number of registers lock out. (d) The TXE2 exchange uses S and Z time pulses in the detection of faults, the release of equipment held by a fault, and the removal from service of faulty equipment. To improve their security, the Mark 3 TXE2 exchange has been equipped with 2 pulse generators, known as alarm-delay relay-sets, one the main and the other the stand-by; the earlier TXE2 exchange design was equipped with only one generator. A monitor and change-over unit has been designed to detect failure of S or Z pulses and to change over from the main to the stand-by alarm-delay relay-set (see Fig. 10). S and Z pulses are checked and the checking circuit is reset: periodically by S and Z pulses of a lower frequency. The presence of 2'5 s and 20 s pulses is checked every 30 s by the 30 s Z pulses, and the checking circuit is reset by the 30 s S pulses (see Fig. 11). Similarly, 30 s S and Z pulses and the 3 min S and Z pulses are checked and the checking circuit is reset every 8min by pulses from the exchange change-over clock. In addition to checking for complete pulse failures, the monitor also detects the following types of pulse faults: (I) S and Z pulses of the same frequency overlapping by more than 400 ms, (ii) continuous S or Z pulses, and (Hi) short S or Z pulses; pusses must persist for longer than 10 ms. The security modifications, some of which have been mentioned above, have greatly improved the reliability of the TXE2 system and reduced the posssbility of exchange service restrictions. It is not practicable to apply fully all of the modifications retrospectively to all marks of TXE2 exchanges; however all of the improvements will be applied to Mark 3 exchanges. Development will be continued to improve further the TXE2 system. 

References 1 Long, R. C., and Gorringe, G. E. Electronic Telephone Exchanges: TXE2—A Small Electronic Exchange System. POEEJ, Vol. 62, p. 12, Apr. 1969. 2 Stacey, R. R. Teletraffic Studies of the TXE2 Electronic Exchange. POEEJ, Vol. 67, p. 73, July 1974.