Abstract: A telephone loop signalling circuit, which automatically replicates either a loop start or a ground start configuration under virtually all operating conditions of the telephone line including those which have heavily induced power line voltages. The circuit utilizes a bias network connected to the central office side of the line together with a plurality of detectors which coact with logic circuitry to differentiate between the two configurations under these widely varying conditions.

Abstract: A circuit configuration for a telephone loop signalling circuit, for detecting a ground start condition on a telephone line. The circuit operates without modification under both loop start and ground start conditions in the presence of heavily induced power line voltages. The circuit configuration utilizes two line current detectors, one driven via a low pass filter to control a D-type flip-flop which controls a ground start condition applied to the line. The other detector resynchronizes a normally free-running clock to the start of each half-cycle whenever current is detected on the line. The clock in turn also controls the set/reset times of the D-type flip-flop.