The TXE4 was a cost reduced development of the TXE3 system and catered for up to 40,000 subscribers with over 5,000 erlangs of both-way 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 and used reed relays as the switching medium which proved reliable in service. Later a few exchanges were also manufactured by Plessey and GEC. 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.