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 in positions 1-5 and swap them with data stored in positions 6–10. Each store held 10 decimal digits, represented in 2 out of 5 code. The RFS held data from each of the MCUs associated registers, e.g. dialled digits. There were 20 MFS, 4 SFS and up to 36 RFS. The MCU was informed by the Supervisory Processing Unit if the setting up switching paths had failed. In this case the MCU would instigate a repeat attempt to set a new path. The details of the failed path were printed. The TXE4 had two standard teleprinters, which logged fault indications and other 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. PATE4 (Print Analysis TXE4) was an experimental program used that read the paper tape looking for common fault patterns. The TXE4 exchanges were designed for a mean time between failures of 50 years. Incomplete List of TXE4 exchanges TXE4A TXE4A was the last in the line of TXE exchanges and an improved version of the TXE4. It was developed by STC after Post Office Telecommunications commissioned them, in 1975, to produce a 15% reduction in cost and provide more customer facilities. It had the same switching as TXE4 but an redesigned common control, using integrated circuits (including microprocessors) to achieve significant size and cost reductions. TXE4A dispensed with Dimond rings and used solid state memory. 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 TXE4 MCU program was stored in EPROM with capacity for 32k 16-bit instructions. Each instruction took 2 microseconds to execute except those accessing Registers which took 6 microseconds. The higher performance enabled the maximum number of registers per MCU to be increased. The first TXE4A to enter service was Belgrave on 28 February 1981. Over 550 TXE4 and TXE4A exchanges were installed and were in use for over 20 years serving 8 million lines. 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 11 March 1998 when