Telephone exchange call tone verifying circuit - has buffer register storing signal from loop current indicators

摘要

The circuit detects a loop current at the subscriber appts. via indicators (I) inserted in the subscriber circuit. The signal (S) from the indicators (I) is stored via a buffer register (SR) during emission of the call current, the buffer register (SR) controlled by a clock signal coinciding with the zero crossing points of the AC call current supplied to the subscriber line. After several such zero crossing points a signal (SA) appears at the output of the buffer register (SR), which is combined with the direct signal (S) from the indicators (I) via a logic gate. The control clock signal (T) for the buffer register (SR) is pref. obtained via two anti-parallel optical couplers associated with a resistor.