Patent ID: 6185296
Filing Date: 2001-02-06
Classification: H04M

Abstract:
A ring trip circuit associated with a call signal source electrically connected through a relay unit and a selected subscriber line to a user's terminal for supplying a call signal changed across zero level, comprising:a photo-emitting unit connected to a signal line assigned to said call signal, and producing an optical signal representative of the magnitude of said call signal;a photo-detecting unit optically connected to said photo-emitting unit, and producing a first electric signal changed from a first potential level to a second potential level and vice versa when said call signal is decreased to a certain magnitude close to said zero level;a status discriminator connected to said photo-detecting unit discriminating status of said user's terminal at the change of said first electric signal changed from said first potential level to said second potential level, and producing a second electric signal representative of said status of said user's terminal, in which said status discriminator includes,a first counter counting enabled with said first electric signal of said second potential level for counting pulses supplied thereto and producing a first digital signal representative of the number of said pulses stored therein,a two's complement Generator connected to said first counter for producing a second digital signal representative of two's complement of the binary number of said first digital signal,a second counter enabled with said first electric signal of said first potential level for counting said pulses and producing a third digital signal representative of the number of said pulses stored therein, andan adder calculating the sum of said two's complement and the binary number of said third digital signal at said change of said first electric signal and producing a fourth electric signal representative of a carry bit of said sum, a value of said carry bit being indicative of said status of said user's terminal; anda relay controller connected to said status discriminator and a command source for controlling said relay unit, and supplying a third electric signal to said relay unit depending upon said status for changing said relay unit.