DIGITAL SUPERVISORY CIRCUIT FOR A TELEPHONE SYSTEM

The appts. includes a circuit responsive to an impact image signal representing ring trip, ring present, on-hook, off-hook and/or dial pulses. It gives an output signal indicating the difference in time. The input signal is above and below a reference potential. A second circuit digitally integrates the output signal. A third circuit has a predetermined digital threshold to provide a ring-present supervisory signal when the threshold is exceeded. A fourth circuit has a second predetermined digital threshold to provide a switch hook supervisory signal when its threshold is exceeded. A fifth circuit prevents response of the second and fourth circuits to line transients.