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Intrusion-alarm-cable-supervision-system

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Intrusion alarm cable supervision system

Details

Inventors: Galvin, Aaron A.; Edson, James B.; Bean, James O. C.;
Assignee: American District Telegraph Company (New York, NY)
Primary Examiner: Habecker; Thomas B.
Assistant Examiner: Crosland; Donnie L.
Attorney, Agent or Firm: Weingarten, Maxham & Schurgin

A cable supervision system especially adapted for use in an intrusion alarm installation and which is capable of monitoring at a central control unit all lines of an interconnection cable for proper signal levels and balance. Under intended cable signal conditions a predetermined sensible impedance is provided which is monitored at the control unit. A monitoring circuit is coupled to the end of a system cable remote from the control unit, this circuit being operative to monitor the signal conditions of all lines of the cable. In response to proper signal conditions on the cable as determined by this monitoring circuit a predetermined impedance is provided which is sensed at the control unit to detect a change in impedance which would signal a tamper or cable failure alarm.

DETAILED DESCRIPTION OF THE INVENTION An intrusion alarm system employing a cable supervision system according to the invention is shown in diagrammatic form in FIG.
1.
A control unit 10 is connected to a multiconductor cable 12 which extends to remote points of a system installation at which respective transceivers 14a, 14b and 14c are located and connected to cable 12.
Each of the transceivers includes a transmitting transducer connected to the transmitter lines 16 and 18 of cable 12, and a receiving transducer and associated preamplifier connected to the receiving line 20 and ground line 22 of cable 12.
The control unit includes signal processing circuitry for receiving signals returned by the respective transceivers and providing an output alarm indication upon detection of intruder presence.
The signal processing circuitry also usually includes circuitry for discriminating between valid intruder signals and noise or other spurious signal conditions to minimize false alarms.
The alarm system itself is not the subject of this invention, but can be of many different known configurations, a typical alarm system being shown in U.
S.
Pat.
No.
3,665,413, assigned to the assignee of this invention.
According to the present invention, a monitoring circuit 24 is connected to the several lines of cable 12 at the end of the cable remote from control unit 10.
A cable tamper detection circuit 26 is located at control unit 10 and is connected to the ground line 22 and the tamper line 23 of cable 12.
The monitoring circuit 24 monitors the signal conditions on the lines of cable 12 and provides in response to proper signal conditions a predetermined impedance between the tamper and ground lines which is detected by circuit 26.
Circuit 26 is operative to provide a tamper or cable failure alarm indication upon detection of change in the predetermined impedance beyond selected limits.
In usual installation, circuit 24 is contained in the transceiver connected to the end of cable 12, while circuit 26 is contained in control unit 10

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