Private telephone installation architecture

A private telephone installation architecture incorporating a central unit (2), said central unit including a digital time-division switching network (3), controlled by a control unit (4) and carrying signals arranged in frames, said control unit having at least one processor (5), a set (6) of random access and/or read-only memories and a clock (7), said telephone installation permitting communication by a plurality of telephones or terminals (1) by means of said switching network either directly, or indirectly through telephone lines (32) which connect said installation to at least one local central office of a telephone network, the telephones, terminals or lines in the installation being connected to said switching network by interface circuits (30, 31), via at least one bi-directional time-division multiplex link (12) carrying said signals arranged in frames, wherein said installation also includes a specific two-wire signalling link (13) connected to all said interface circuits and connected via a serial-parallel converter to said control unit, the frames for this signalling link having time slots fixedly assigned to the interface circuits.