ALARM SYSTEM

• US 3,713,142 A
• Filed: 01/17/1972
• Issued: 01/23/1973
• Est. Priority Date: 01/17/1972
• Status: Expired due to Term

• Current Assignee: Signatron, Inc.
• Original Assignee: Edward H. Getchell
• Inventors: Getchell, Edward H.
• Primary Examiner(s): Habecker, Thomas B.
• Application Number:
  US05/218,360
• Time in Patent Office: 372 Days
• Field of Search: 340/164R,152T,408
• US Class Current: 340/505
• CPC Class Codes: G08B 26/002 only replying the state of the sensor
• CPC Class Codes: G08B 29/02 Monitoring continuously signalling or alarm systems
• CPC Class Codes: G08B 29/046 prevention of tampering with detection circuits
• CPC Class Codes: G08B 29/08 Signalling of tampering with the line circuit

Abstract

This invention relates generally to alarm systems and, more particularly, to an alarm system using a two-way communication link between a monitoring station and a transponder station at a remote location the alarm status of which is being monitored, such system employing encoding techniques based on the generation of a truly random signal at the monitoring station.

Claims

1. An alarm system for monitoring at a first station the alarm status of at least one second station, said system comprising a monitor located at said first station, said monitor including means for generating a random signal;

means for producing one or more simulated signals corresponding to one or more alarm status conditions at said second station;

monitor encoding means responsive to said random signal and to said one or more simulated signals for producinG one or more encoded monitor signals, said latter signals being encoded with a selected code program;

monitor transmitter means for feeding said random signal to a transmission link means for transmission to said second station;

a transponder located at said second station, said transponder including means for receiving said transmitted random signal;

transponder encoder means responsive to said received random signal and adapted to be responsive to a signal representing the alarm status of said second station for modifying said received random signal to produce an encoded transponder signal, said latter signal being encoded in accordance with said selected code program;

transponder transmitter means for feeding said encoded transponder signal to said transmission link means for transmission of said signal to said first station;

said monitor further including means for receiving said transmitted encoded transponder signal;

means for comparing said encoded transponder signal with each of said encoded monitor signals to produce an alarm output signal when the characteristics of said encoded transponder signal are the same as the characteristics of at least one of said encoded monitor signals; and

display means responsive to said alarm output signal for annunciating the alarm status of said second station.

2. An alarm system in accordance with claim 1 further including timing means for timing the operation of said random signal generating means, said monitor encoding means, said transponder encoding means, said comparing means, and said display means whereby said monitor encoding means produces said one or more encoded monitor signals and said transponder encoding means produces said encoded transponder signal in response to the same random signal generated from said random signal generating means for comparison at said comparing means.

3. An alarm system in accordance with claim 2 wherein said monitor encoding means includes first data storage means responsive to said random signal for feeding said random signal data to an encoder in parallel form;

second data storage means responsive to said encoded transponder signal for feeding said encoded transponder signal data to said comparing means in parallel form; and

said timing means includes means for timing the operation of said first and said second data storage means.

4. An alarm system in accordance with claim 2 wherein said transponder encoding means includes first data storage means responsive to said transmitted random signal for feeding said random signal data to an encoder in parallel form;

second data storage means responsive to said encoded transponder signal for feeding said encoded transponder signal to said transponder transmitter means in series form; and

said transponder timing means controlling the operation of said first and said second transponder data storage means.

5. An alarm system in accordance with claim 1 wherein said display means includes means for audibly displaying said alarm status.

6. An alarm system in accordance with claim 1 wherein said display means includes means for visually displaying said alarm status.

7. An alarm system in accordance with claim 1 wherein said random signal generating means includes an unstable oscillator for producing a plurality of pulses at a randomly varying frequency;

gated storage means for receiving said plurality of pulses; and

clock means for producing a series of gating pulses at a fixed frequency for actuating said gated storage means to produce an output stream of random bits.

8. An alarm system in accordance with claim 7 wherein said randomly varying frequency is f1 and is of the form f0 + or -f(t), said fixed frequency is f2, and the width of said gating pulses is Tau , the frequency f1 being much greater than the frequency f2 and said pulse width Tau is much less than 1/f1.

9. An alarm system in accordance with claim 1 wherein said monitor transmitter means includes means for providing a reference voltage;

voltage comparator means responsive to said reference voltage and to the random signal from said random signal generator;

means responsive to the output of said voltage comparator means and connected at its output to said transmission link for producing a controlled current pulse on said transmission link when the value of said random signal exceeds that of said reference voltage and for producing a zero current on said transmission link when said random signal value is less than that of said reference voltage; and

power supply means for supplying power for transmitting said controlled current pulse on said transmission link.

10. An alarm system in accordance with claim 9 wherein said transmission link is a transmission line;

and said current pulse is produced independently of the length of said transmission line.

11. An alarm system in accordance with claim 1 wherein said monitor receiving means includes means responsive to received current pulses on said transmission link for producing a voltage pulse;

filter means responsive to said voltage pulse for producing a filtered voltage pulse;

means for providing a reference voltage;

voltage comparator means responsive to said filtered voltage pulse and to said reference voltage for producing an output pulse representing the received signal data on said transmission link.

12. An alarm system in accordance with claim 3 wherein said first data storage means is a shift register for storing said random signal input data in serial form and for providing said signal data at its output in parallel form;

a plurality of logic networks responsive to parallel signal data for producing encoded parallel signal data;

a code control program network interconnecting the inputs of said logic networks with said output signal data from said shift register and with simulated alarm status information signals in accordance with a selected interconnection program.

13. An alarm system in accordance with claim 12 wherein said logic networks include a plurality of exclusive/or gates.

14. An alarm system in accordance with claim 13 wherein each such logic network includes a first exclusive/or gate having two inputs connected to said code control program network and an output connected to said code control program network;

a second exclusive/or gate having one input connected to the output of said first exclusive/or gate and a second input connected to said code control program network, the output of said second exclusive/or gate thereby providing an encoded data bit.

15. An alarm system in accordance with claim 1 wherein said display means includes a plurality of display units, each being responsive to a different one of said alarm output signals and including a plurality of storage means adapted to store said alarm output signals;

means for feeding said alarm output signals sequentially to each of said plurality of storage means.

16. An alarm system in accordance with claim 15 wherein said sequential feeding means includes means for generating strobe pulses;

gating means for feeding said strobe pulse to said plurality of storage means for sequential operation of said storage means; and

means for actuating said gating means.

17. An alarm system in accordance with claim 16 wherein said gating actuating means includes detector means responsive to said encoded monitor signal for producing an output signal if all the data bits in said encoded monitor signal are in either a zero or a one state, said output signal preventing the actuation of said gating means when present and permitting the actuation of said gating means when absent.

18. An alarm system in accordance with claim 15 and further including alarm gate means connected to the outputs of said storage means for producing a gating output signal if any of said storage means has an alarm output signal stored therein and for producing a zero signal if none of said alarm storage means has an alarm output signal stored therein;

detector means responsive to said encoded transponder signal for producing an output signal if all of the data bits in said encoded transponder signal are in either a zero or a one state;

logic circuit means responsive to said further gate means and to said detector means for producing a first output signal when an output signal is present from said detector means and no output signal is present from said further gate means for producing a second output signal when no output signal is present from said detector means and no output signal is present from said or gate means;

first means for displaying the presence of said first output signal; and

second display means for displaying the presence of said second output signal.

19. An alarm system in accordance with claim 15 wherein each of said plurality of display units further includes averaging means for producing an averaged voltage in response to a stored alarm output signal;

threshold reference means for producing a threshold reference voltage;

voltage comparator means responsive to said averaged voltage and to said threshold reference voltage for producing an output voltage when said averaged voltage is greater than said threshold voltage; and

latching circuit means responsive to the output voltage from said voltage comparator means for actuating a display device for displaying the presence of an alarm output signal.

20. An alarm system in accordance with claim 19 wherein said display device includes a visual display unit;

a source for producing an on-off voltage;

an and gate responsive to the output of said latching circuit and to said on-off voltage for producing a display output signal when a voltage is present from said latching circuit and said on-off voltage is in the on state, said visual display unit being responsive to said display output signal from said '"'"''"'"''"'"''"'"'and'"'"''"'"''"'"''"'"' gate to produce a blinking visual effect.

21. An alarm system in accordance with claim 4 wherein said transponder receiving means includes a signal filter means responsive to said received random signal to provide a filtered received signal;

detector means for detecting the presence and absence of a current pulse in said filtered signal and for feeding said detected pulses to said timing means and to said first transponder data storage means.

22. An alarm system in accordance with claim 21 wherein said timing means includes a timing chain generator for producing a plurality of timing pulses for operating said first and said second transponder data storage means and said transponder encoder;

means for producing clock pulses for the actuation of said timing chain generator;

synchronous detector means responsive to said detected pulses and to a timing pulse from said timing chain generator for producing a reset signal to reset said timing chain generator to its initial condition at the end of each message frame of said received signal;

said timing chain generator producing a strobe output pulse at a selected time in said message frame for stopping the operation of said clock pulse producing means.

23. An alarm system in accordance with claim 22 wherein said synchronous detector includes a monostable multivibrator means for producing said reset pulse in response to said strobe pulse from said timing chain generator;

and trigger circuit means responsive to said received detected pulses for triggering the operation of said monostable multivibrator.

24. An alarm system in accordance with claim 21 wherein said transponder receiving means further includes means connected to said transmission link means for sensing the current in said transmission link means, said current sensing means being connected to the input of said filter means;

and said detector means includes reference voltage source means; and

voltage comparator means having a first input connected to said reference voltage source means and a second input connected to the output of said filter means, said voltage comparator means producing said detected pulses, said pulses including said random signal transmitted from said monitor transmitter means and a synchronous pulse for actuating the timing means of said transponder.

25. An alarm system in accordance with claim 1 wherein said transponder transmitter means includes switching means responsive to said encoded transponder signal for interrupting a continuous current signal on said transmission link means in response to the encoded data of said transponder encoded signal.