ALARM TRANSMISSION LINE SECURITY SYSTEM UTILIZING PSEUDO RANDOM ENCODING

• US 3,735,353 A
• Filed: 10/28/1971
• Issued: 05/22/1973
• Est. Priority Date: 10/28/1971
• Status: Expired due to Term

• Current Assignee: Johnson Controls International Inc (Johnson Controls International plc)
• Original Assignee: Frank J. Esser, John C. Donovan, Ramesh Krishnaiyer
• Inventors: Donovan, John C., Krishnaiyer, Ramesh, Esser, Frank J.
• Primary Examiner(s): Yusko, Donald J.
• Application Number:
  US05/193,450
• Time in Patent Office: 572 Days
• Field of Search: 340/164,167,147,274
• US Class Current: 340/6.11
• CPC Class Codes: G08B 29/08 Signalling of tampering with the line circuit

Abstract

An alarm line security system wherein areas to be protected each include an alarm source having an intrusion detector and an alarm source monitoring circuit including a code generator for generating a code comprised of a pseudo-random sequence of bits for transmission over an alarm transmission line to alarm monitoring circuits at a central monitoring area, and a code complementor circuit controlled by the intrusion detector for modifying the generated code prior to transmission to the central monitoring area whenever an unauthorized entry is detected. A multiplexer circuit interposed between the alarm source monitoring circuits for all of the protected areas and the transmission line enables transmission of one or more of the bits which comprise the pseudo-random codes generated at each alarm source to be transmitted over a common transmission line. The alarm monitoring circuits at the central monitoring area include a reference code generator which generates the same pseudo-random code as the code generators associated with the alarm sources, and a code comparator circuit which compares the code received from each source to the reference code and provides an alarm indication whenever codes from one or more of the alarm sources differ with the reference code.

Claims

1. In a security system including a transmission line carrying alarm information from at least one protected area to a central monitoring area that is remote froM the protected area, means at said protected area for generating a monitor code comprising a selected sequence of code bits for transmission over the transmission line from said one protected area to the central monitoring area, means for modifying the code bits provided by said generating means to indicate an alarm, means at said central monitoring area for generating a reference code comprised of a selected sequence of code bits that is identical to the monitor code sequence, means for comparing the monitor code with the reference code, and means for providing an alarm indication whenever the codes are different.

2. In a transmission line security system including a transmission line for carrying alarm information from at least one protected area to a central monitoring area that is remote from the protected area, alarm source means including intrusion detector means at said protected area, alarm source monitoring means at said protected area including first code generator means for generating a monitoring code comprising a known pseudo-random sequence of bits for transmission to said central monitoring area over the transmission line at predetermined intervals and modifying means controlled by said intrusion detector means in response to the detection of an unauthorized entry to modify the code bits generated by said first code generator means, and transmission line monitoring means at said central monitoring area including second code generator means for generating a reference code that is identical with the monitoring code, and code comparator means for receiving the monitoring code transmitted to said central monitoring area and comparing the monitoring code with the reference code, and register means controlled by said code comparator means providing an alarm indication whenever the codes are different.

3. A transmission line security system as set forth in claim 2 wherein said first code generator means comprises first multistage shift register means, and first feed back means including exclusive OR gating means connected between outputs of certain stages of said shift register means and the input of said shift register means.

4. A transmission line security system as set forth in claim 3 wherein said second code generator means includes further multistage shift register means and second feedback means including further exclusive OR gating means connected between outputs of certain stages of said further shift register means and the input of said further shift register means.

5. A transmission line security system as set forth in claim 4 wherein said first feedback means includes means for altering the feedback connections of the shift register means of said first code generator means whereby said first code generator means provides a further pseudo-random sequence of bits, and wherein said second feedback means includes means for altering the feedback connections of the shift register means of said second code generator means whereby said second code generator means provides a pseudo-random sequence of bits that is identical with the bit sequence provided by said first code generator means.

6. A transmission line security system as set forth in claim 2 including sync pulse generating means for synchronizing the operation of said first and second code generator means to provide a pseudo-random sequence of bits at said predetermined intervals.

7. In a transmission line security system including a transmission line for carrying alarm information from at least one protected area to a central monitoring area that is remote from the protected area, alarm sources means including intrusion detector means at said protected area, alarm source monitoring means at said protected area including first code generator means for generating a monitoring code comprising a known sequence of bits for transmission to said central monitoring area over the transmission line at predetermined intervals and code complementing means controlled by said intrusion detector Means in responsive to the detection of an unauthorized entry for inverting each bit of the bit sequency generated by said first code generator means, and transmission line monitoring means at said central monitoring area including second code generator means for generating a reference code that is identical with the monitoring code, and code comparator means for receiving the monitoring code transmitted to said central monitoring area and comparing the monitoring code with the reference code, and register means controlled by said code comparator means providing an alarm indication whenever the codes are different.

8. A transmission line security system as set forth in claim 7 wherein said alarm source monitoring means further includes alarm converter means controlled by said detector means for providing first and second logic level signals to indicate respectively, that the protected area is secure or that an unauthorized entry has occurred, and wherein said code complementing means includes exclusive OR gating means having a first input connected to the output of said alarm converter means and a second input connected to the output of said first code generator means, the output of said code complementing means being connected to said transmission line whereby the code bits output at said first code generator means are passed over said code complementing means to said transmission line whenever a first logic level signal is provided by said alarm source converter means and the code bits output at said first code generator means are inverted by said code complementing means as they are passed to said transmission line whenever said second logic level signal is provided by said alarm source converter means.

9. A transmission line security system as set forth in claim 7 wherein the bits of at least two successive pseudo-random bit sequences are inverted by said code complementing means whenever an unauthorized entry is detected.

10. A transmission line security system as set forth in claim 7 wherein said code comparator means includes gating means for comparing each bit of the monitoring code received from said one protected area, with each bit of the reference code and providing an enabling signal whenever one or more of the bits of the monitoring code differs with the bits of the reference code and latch means responsive to said enabling signal to provide an alarm output representing an alarm condition said register means being controlled by said alarm output to register said alarm.

11. A transmission line security system as set forth in claim 10 wherein said code comparator means includes counter means for providing a further enabling signal for said latch means whereby said latch means provides an alarm output only when two or more successive bits of the monitoring code sequence differ with corresponding bits of the reference code sequence.

12. A transmission line security system as set forth in claim 10 wherein said code comparator means comprises exclusive OR gating means having a first input connected to said transmission line for receiving the monitoring code transmitted from said protected area and a second input connected to the output of said second code generator means for receiving the reference code, said exclusive OR gating means providing a first logic level output wherever the code bits compared are the same and a second logic level output for enabling said latch means whenever the code bits compared are different.

13. In a transmission line security system including a transmission line for carrying signals from a plurality of protected areas to a central monitoring area that is remote from said protected areas, each of said protected areas having individual monitor code generator means for generating a line monitoring code comprising a selected sequence of code bits, switching means for sequentially connecting the output of each of said monitor code generator means to said transmission line to thereby pass a different bit of the sequenCe of bits provided by each monitor code generator means to said transmission line during successive time intervals whereby a further selected sequence of bits is transmitted to said central monitoring area over said transmission line with successive bits of the further bit sequence being provided by different ones of said monitor code generator means, reference code generators means at said central monitoring area for generating a reference code comprising a selected sequence of code bits, each bit of which is normally identical with corresponding bits of the further bit sequence at any given time, and means for comparing the further bit sequence with the reference bit sequence and providing an indication whenever any of the corresponding bits of the two sequences are different.

14. A transmission line security system as set forth in claim 13 wherein said code generator means all generate a known pseudorandom sequence of code bits and wherein the further bit sequence provided by said switching means is identical to the pseudo-random sequence of bits provided by each of said monitor code generator means.

15. In a transmission line security system including a transmission line for carrying alarm information from a plurality of protected areas to a central monitoring area that is remote from the protected area, each of said protected areas having individual alarm indicating means, monitor code generator means for generating a known sequence of coded bits, and means controlled by said alarm indicating means to modify the code bits generated by an associated monitor code generator means, multiplexer means for sequentially connecting the output of the monitor code generator means at each of said protected areas to the transmission line to enable the transmission to said central monitoring area of a different one or more of the bits provided by each of said monitor code generator means thereby providing a further sequence of coded bits, and transmission line monitoring means at said central monitoring area including reference code generator means for generating a sequence of coded bits that is identical with the further bit sequence provided by said multiplexer means, comparator means for receiving the further bit sequence and comparing each bit of the further bit sequence with the corresponding bit of the reference bit sequence, and alarm registering means controlled by said comparator means for registering an alarm whenever one or more of the bits of the further bit sequence differs with corresponding bits of the reference bit sequence.

16. A transmission line security system as set forth in claim 15 wherein said alarm registering means includes demultiplexer means controlled by said comparator means whenever one or more of the bits of the further bit sequence differs with corresponding bits of the reference bit sequence to identify each bit of the further bit sequence which differs with a corresponding bit of the reference bit sequence.

17. A transmission line security system as set forth in claim 16 wherein said multiplexer means comprises a plurality of gate circuits, each having a first input individually connected to an output of the code generator means of one of said protected areas, and means for selectively enabling the gate circuits to pass at least one bit of the sequence generated by an associated code generator means to the transmission line.

18. A transmission line security system as set forth in claim 17 wherein said demultiplexer means comprises a further plurality of gate circuits including an individual demultiplexer gate for each protected area, said demultiplexer gates having first inputs connected together to the output of said comparator means and means for sequentially enabling each of said demultiplexer gates whereby each demultiplexer gate provides a first logic level output whenever the compared bits are the same, and a second logic level output whenever the compared bits are different.

19. A transmission line security system as set foRth in claim 18 wherein said alarm registering means further includes a separate indicator for each of said protected areas, each indicator being individually connected to the output of one of said demultiplexer gates to be energized whenever the corresponding demultiplexer gate provides said second logic level output.

20. A transmission line security system as set forth in claim 18 wherein said multiplexer enabling means includes multi-stage ring counter means having a separate stage for each protected area and an enabling signal stored in one of the stages of said multi-stage ring counter means, each of said multiplexer gate circuits having a second input connected to the output of a corresponding one of the stages of said ring counter means, and wherein said demultiplexer enabling means includes further multi-stage ring counter means having a separate stage for each protected area and an enabling signal stored in one of the stages of said further multistage ring counter means, each of said further gate circuits having a second input connected to the output of a corresponding one of the stages of said further ring counter means, and sync pulse generating means for providing sync pulses for the ring counter means of the multiplexer and demultiplexer means for shifting said enabling signal from stage to stage of said ring counter means whereby said enabling signals are selectively applied to the second inputs of said gate circuits in sequence.

21. A transmission line security system as set forth in claim 20 wherein the monitor code generator means associated with said protected areas and said reference code generator means each include multi-stage shift register means and associated feedback means for selectively connecting outputs of certain stages of said shift register means to the corresponding input of said shift register means, said sync pulses being extended to the shift register means of said monitor and reference code generator means to shift the signals output from associated gating means through the shift register means of said code generator means whereby said sequence of coded bits is provided at the output of the shift register means of said monitor and reference code generator means.

22. A transmission line security system as set forth in claim 20 wherein the monitor code generator means associated with said protected areas and said reference code generator means each include multi-stage shift register means and associated feedback means for selectively connecting the outputs of certain stages of said shift register means to the corresponding input of said shift register means of said monitor and reference code generator means to shift the signals output from an associated gating means through the shift register means of said code generator means whereby said sequence of coded bits is provided at the output of the shift register means of said monitor and reference code generator means, said multiplexer means and said demultiplexer means each including pulse divider means for dividing the sync pulse rate by a predetermined number of the coded bits of the sequence provided by each monitor code generator means and sequentially transmitted over said transmission line to said central monitoring area and compared with a corresponding number of bits provided by the reference code generator means.