ISDN power source detection circuit

In an ISDN communications system, a remote station power detector circuit comprises a capacitor connected between center taps of transformers on the receive and transmit pairs of conductors. The capacitor stores energy delivered from a central DC power source. Two serially-connected opto-couplers form a discharge path for the capacitor. The first opto-coupler is periodically driven on to partially complete the discharge path. If the capacitor charge exceeds a predetermined minimum, the second opto-coupler will then be driven on, providing a logic level output usable by the remote station. Power consumption is minimized by the use of large resistors in series with the capacitor while reliable power detection is achieved by detecting accumulated charge on the capacitor. Dielectric isolation is provided through the use of opto-coupler circuits.