Ring trip detection system

Abstract

Ring trip is detected when a telephone set which is being rung goes off-hook, through the use of a comparator in which the ringing signal superimposed on battery feed voltage is compared with a fixed reference voltage closer to ground than the battery feed voltage. Inasmuch as the duty cycle of the ringing signal with respect to the reference voltage is less than or equal to 50% under pre-trip (on-hook) conditions and will increase under trip (off-hook) conditions, the comparator output is a square wave form of corresponding duty cycle. An up-down counter is caused to count up when the comparator output wave form is of one level and down when the comparator output wave form is of another level during each cycle of the ringing signal. The counter counts up during the portion of the comparator wave form which determines the duty cycle and down during the rest of the period of the cycle. Underflow and lack of underflow states of the counter respectively indicate nonring trip and ring trip conditions. Logic responsive to these conditions causes deactuation of a ringing relay to enable disconnection of the ringing signal from the ring lead going from the line circuit to the telephone set. The line circuit has a calling bridge which provides an output which is used to prevent the ringing relay from reoperating unless the telephone set remains on-hook after ring trip detection.