Programmable selective ringing circuit and method for party telephone

Abstract

An apparatus and method for decoding ringing signals on a party telephone line coupled to the telephone line and a telephone instrument, according to the polarity of a d.c. voltage level present across the telephone line upon which ringing signals are superimposed and according to the grounded state of the telephone line. The apparatus comprises a discrimination circuit which supplies a selected one of the ringing signals to the telephone instrument via a ringing signal path to actuate the ringer of the telephone instrument. The apparatus further comprises a ringing signal rectification circuit connected to the telephone instrument via a voice signal path for preventing the ringing signal present on the telephone line from activating a ringer of the telephone instrument via the voice signal path. A switching circuit selectively connects the telephone instrument to the ringing signal path and the voice signal path. The apparatus further preferably comprises a circuit for eliminating bell taps and a balancing circuit for balancing the impedance across telephone line when the d.c. voltage level on the telephone line falls below a preset threshold, thereby forcing semiconductor devices on the line which could contribute to noise problems when only in a partly conductive state, into a conductive state, thereby eliminating the noise.