Control of the motors used four different systems, one for each six-car train, but all of the systems were compatible, so that any combination of two-car units could be used to form a train. British Thompson-Houston (BTH) provided the only system which was based on previous experience, which became known as the Pneumatic Camshaft Motor (PCM) system. This proved to be the most reliable, and was the system adopted for subsequent batches of trains. The other three systems were new designs, produced by Metropolitan Vickers, General Electric Company, and a collaboration between Crompton Parkinson and Allen West & Co. The PCM system used an air-operated camshaft, to control the switching out of starting resistances. With the motors connected in series, the camshaft rotated in one direction, until no resistances were in circuit. A separate electro-pneumatic switch then controlled the transition from series to parallel configuration, and the camshaft moved back to its starting position, as the resistances were removed again.