Telephone subscriber's line circuit - detects open or closed loop using potential dividers and comparator with threshold switched by controller

摘要

The condition of a subscriber's line, i.e. loop open or closed, is monitored at an exchange by the potential difference between two potential dividers, one across the line, the other from the b-wire to ground. The potential difference is detected by a comparator whose threshold is set by the central controller to one of three values, depending on whether the transmission bridge is switched to high or low resistance and whether calling signal is being sent. One comparator circuit serves a group of 16 lines via two multiplexers which sample the two potential dividers every 4ms. The comparator threshold is switched by a selector in synchronism with the multiplexers. A hysteresis effect is introduced to prevent hunting in borderline cases, by storing the comparator output in a shift register over one sampling cycle, and using it to offset the threshold.