Collection of manuals for British Post Office Monarch 120 system digital PABX (private automatic branch exchange)
Monarch was developed from 1976 by a Post Office research team, working at first at Dollis Hill and later at Martlesham, together with the telecommunications industry, at the same time that the System X digital exchange was being developed for the public network. The first trial local System X exchange went into service at Woodbridge, Suffolk, in 1981 and the first System X main exchange was installed in 1983. By 1987 virtually all the main switching units had been converted to digital, comprising some 55 main exchanges. By 1988 local exchanges were being converted to System X in large numbers. (System Y, an alternative digital exchange system, was also being installed in many local exchanges.) The British network was entirely digital by 1998. System X was a great success technically, although export orders were few, largely owing to political issues arising from management of its production development among several manufacturers. The Monarch 120 PABX system first went into service in 1980 and aspects of its circuit design had a crucial influence on System X development. It was one of the first all-digital PABX systems in the world.