Interlock circuit for preventing corruption of telephone line signals

Abstract From the Canadian patent

Corruption of communication signals on a telephone line by the direct (low impedance) interfacing of test equipment to the line is precluded by a protective interlock circuit which, prevents the test set from seizing the line if data traffic is present. The interlock circuit is coupled between the line circuit to be accessed and the test equipment and includes a, multiple band frequency detector which monitors the line for, signals lying within (in and out-of-audio) frequency bands. Should the detector identify any traffic within its sensitivity range (out of the audio band) during the interval of a timing pulse produced by the closure of a switch the logic level, applied to combinational …