Abstract: D.C.-fed ringing signal generator for a group of subscriber lines comprises a cascade arrangement of a digital signal generator controllable by a control signal to produce a signal corresponding either to the ringing signal or to a signal having zero value,a digital-to-analog converter,a capacitively-coupled class-A amplifier, anda direct-coupled class-B symmetrical power amplifier to produce the output signal of the ringing signal generator.

Abstract: An off-hook detector for use in subscriber line interface circuits to indicate status of the subscriber line, or loop, exhibits reduced sensitivity to common mode or longitudinal currents. A central signal from the exchange switches the transmission bridge for the subscriber line from a balanced to an unbalanced condition. In the balanced condition a difference detector indicates loop status. In the unbalanced condition, a comparator compares the voltage on one side of the transmission bridge to a reference voltage to indicate loop status.

Abstract: This test system is intended to test subscriber line interface circuits (1), (2), . . . etc. which are connected by means of one end to subscriber lines and by means of the other end are put in connection with a test apparatus (107). Change-over means (26), (27) or (76), (77) are provided to disconnect each interface circuit such as the interface circuit (1) or the interface circuit (2) from its subscriber line and to connect it to a test bus (112), (113) which is common to one group of interface circuits. Control means are provided in the test appartus (107) for connecting, depending on the test to be effected, one interface circuit or two interface circuits to the test bus. This test system allows remote testing of interface circuits, for example from a telephone exchange by using only the normally existing links between this exchange and the interface circuits.