 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 Processing Unit (SPU), which took control of the calls from information supplied to it by the MCU.To prove the enhancements of TXE4 over the TXE3 a test bed trial installation was installed in Tudor exchange in North London in 1969.