Monolithically integratable telephone circuit for supplying ringing

Abstract

A telephone circuit, which may be monolithically integrated, for supplying ringing signals to a subscriber's telephone line and for detecting an off the hook condition during ringing, including a logic control circuit connected to exchange components which control the supply of the ringing signals and determine their rhythm. The logic control circuit is connected to a circuit for detecting direct current on the line and a current comparator for generating a signal when the value of the line current exceeds a predetermined current value. This signal is also supplied to the exchange components via a transfer circuit to inform them that an off the hook condition has taken place. The logic control circuit is connected to a timing signal generator which synchronizes its functions with times in which the ringing signal has a zero amplitude, and is connected to a signal amplifier circuit which supplies the ringing signals to the line and to the transfer circuit. When the circuit for the detection of direct current signals a probable off the hook condition, the logic control circuit disables the amplifier circuit from supplying ringing signals to the line and inhibits the transfer of any signals to the exchange components, and checks after a predetermined time interval that an off the hook condition has actually taken place.