Apparatus for remotely determining hook status conditions

Abstract

An apparatus for effectively monitoring the hook status of a remotely stationed telephone, and a sub-audible tone associated with a corresponding alarm reporting device, monitors variations in impedance in the subscriber loop, preferably from the central facility which is monitoring the remote location. To this end, a disturbance (voltaage signal) is introduced onto the subscriber loop by an appropriate source. Steps are then taken to measure potential variations in this voltage signal, and to determine the hook status of the subscriber loop based upon these variations in potential. To make sure that the ancillary equipment which is being connected to the telephone network will not adversely affect basic telephone services, care is taken to make sure that the signal which is introduced onto the subscriber loop will neither create audible noises on the telephone line, nor interfere with the operation of the switching equipment of the telephone company. A low-frequency AC sine wave is used for this purpose, preferably the low tone which is produced by the remotely stationed alarm reporting device. The selected signal is inductively coupled to the monitoring apparatus to prevent unacceptable interference with the equipment of the telephone company, and to promote line balance and isolation.