Exchange switching system

Claim 1

A two wire telephone exchange switching system including, for monitoring on ON-HOOK or OFF-HOOK (linelooped) condition of an associated subscriber''s telephone instrument, a line circuit of the kind employing a scanner driven pulseplus-bias gate for continuous interrogation of the line condition and including means responsive to the line-looped condition for applying a priming potential to the bias input of said gate, and further including means associated with the supervisory apparatus of the exchange for applying to said line circuit, via line switches when connected to the line wires, a further potential which also serves as a priming potential for the bias input of said gate, there being provided between said line circuit and the bias input of said gate a potential controlled filter means comprising a noise quenching diode connected from a tapping between a line wire and the bias input of said gate to a point of datum potential, in the ON-HOOK condition said diode being reversebiased and the arrangement being such that in response to application of said further potential from the exchange said diode is forward biased to provide a low impedance path to the scanner noise currents and to divert scanner noise currents from the line wires.