Selby and Leigh-on-Sea were replaced by digital exchanges. Incomplete List of TXE4A exchanges TXE4E TXE4E (Enhancement) was developed by STC and introduced in the late 1980s to update both TXE4 and TXE4A exchanges to provide similar features to those available on System X exchanges, including 'Star Services', CCITT 7 common-channel signalling and itemised call-logging. Analogue signalling on transmission circuits was replaced with the SS7 signalling system that was used on System X and AXE10. This provided the call gapping control that stopped exchanges being overloaded by too many incoming calls (an example being when newspaper competitions misprinted winning numbers). Exchange description The enhancement features were implemented on additional dedicated processing modules interconnected by an Ethernet backbone with the MCU and SPU processors. The enhancement software was implemented on Intel 8080, 286 and 386 processors running on RMX. TXE4A MCUs and SPUs had been designed from the outset with provision for the addition of a communication port to interface to a back-end system, whereas TXE4 MCUs and SPUs had not. 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. This doubled the program store with an additional bank-switched 5,000 program steps and provided the communication port to interface to the enhancement processors. The Cyclic Stores threading fields (12mS x 156mS and 3 x 36mS) were commoned and all new subscribers, ceases or change of Class Of Service, which were previously threaded, were now done via a terminal. The system also had the capability to busy equipment and reset alarms remotely. The cyclic store gates, where all the subscriber information was held, were replaced by solid-state devices as phase two of the enhancement. Due to the improved electronics TXE4A and TXE4E (or TXE4RD/IW interworking) were able to receive downloadable updates for tariff data at bank holidays etc. This downloadable capability enabled the centralised control of a lot of the manual functions that previously had to be carried out manually at each exchange. Tariff changes for the eight million customers could be built and implemented by one person following the introduction of a centralised data management tool. At the time this gave a similar capability as available on the System X and AXE10 exchanges. TXE5 TXE5 is believed to have been reserved for an improved version of the TXE2. Such a version was