High security communication line monitor

Claim 1

In a security or fire detection system of the type including a central alarm station and a plurality of remote sensors coupled to said central alarm station by a communication line comprised of a pair of lines, said system being selectively operable from said central alarm station to establish said communication line in a secure state wherein all remote sensors are activated or an access state wherein at least one of said remote sensors is deactivated, a communication line monitor for monitoring the condition of said communication line, comprising: a remote transmitter coupled to said communication line at the remote end thereof, said remote transmitter being assigned one unalterable preassigned address out of a predetermined number of different addresses and including means for transmitting a series of electrical signals over said communication line corresponding to said one preassigned address and to the secure or access state of said communication line; monitor means located at said central alarm station and being coupled to said communication line for indicating an alarm condition when the electrical signals received from said remote transmitter fails to correspond to said one preassigned address or to the state of said communication line established by said central alarm station; means for maintaining said pair of lines opposite in polarity; and means for reversing the polarity of said pair of lines when changing said communication line from one of said states to the other said state.