US3713142A

US3713142A - Alarm system

Info

Publication number: US3713142A
Application number: US00218360A
Authority: US
Inventors: E Getchell
Original assignee: SIGNATRON
Current assignee: SIGNATRON Inc A CORP OF DE
Priority date: 1972-01-17
Filing date: 1972-01-17
Publication date: 1973-01-23
Anticipated expiration: 1990-01-23

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.

Description

ALARM v 23 COMPARATOR/ DECODER ALARM OUTPUT 9 I0 DISPLAY SYSTEMS I United States Patent 1 [1 11 3,

Getchell [4 Jan. 23, 1973 [5 ALARM SYSTEM 3,145,380 r, 8/1964 Currie ..3 40/-l52 T 3,587,051 6/1971 Hovey.... ..340/l64 R [75] Invent l' Gmhd" carlsle Mass 3,678,512 7 1972 Fergus ...340/40a [73] Assignee: Signatron, lnc., Lexington, Mass.

Primary Examiner-Thomas B. Habecker [22] Flled: 1972 Attorney-Robert F. OConnell et al. [2]] Appl. No.: 218,360

[57] ABSTRACT [52] CL 340/498, 340/152 T 340/164 R This invention relates generally to alarm systems and, 51 Int. Cl. .Q. "Gosh 26/00 Particularly System "Sing a wmway 58 Field of Search ..340/l64 R -152 '1' 40s link between a and a transponder station at a remote location the alarm [56] References cued status of which is being monitored, such system employing encoding techniquesbased on the generation UNITED STATES PATENTS of a truly random signal at the monitoring station.

3,021,398 2/1962 Barnett ..340/408 25 Claims, 24 Drawing Figures 20 zszg ifg TRANSMITTER DATA RECEIVER A INPUT SIGNAL l i i l TRANSPONDER CODE TRANSMISSION ENQODER PROGRAM CODE LINK PROGRAM s in TRANQMTTER Pmmmmzs ma 3.713 142 sum 03 or 15 f =f IAHI) GATED RANDOM BIT STORAGE STEAM PATENIEB JAN 23 I975 ,713,142 SHEET OR 0F 15 /5| s I RECEIVED l POWER 4 I TRANSMISSION v DATA I SUPPLY R ls POWER LINK 4- AMPLI- F a l FIER --54 50 l l L 63 FILTER H I I r 52 VOLT' 62 I 53 I Com REFERENCE (I I I I l I I I I POWER.

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SHEET 08 [IF 15 TO OTHER ALARM OUTPUT AUDIO AND DISPLAY SUBSYSTEMS CIRCUIT LATCH ALARM INTE- ING AND vIsUAL STORAGE GRATOR CIRCUIT I GATE DISPLAY 7 0 86 l a; 82 FROM COMPARATOR BLINK SIGNAL DECODER 23 r MANUAL I RESET ALARM M STORE STROBE FIG-9 I I I I' I BITTIME ITBITSTIOITOI I I l I I I ALARM DATA ILI I I I l I I I ALARM STORAGE N0 ALARM cowlTIoNl ALARM CONDITION OUTPUT I I I I I l I I ALARM STROBE I I I I SIGNAL L I l I In I To OTHER ALARM OUTPUT CIRCUITS TO AUDIO A cIRcUIT V FROM 88 o ooMPARAToR PUP \I coMRAR- 94 DECODER 23 FLOP ATOR NAND 8| 92 AND 93 .Vth 85 ALARM I 9 STROBE SIGNAL I FIGQB MANUAL BLINK RESET SIGNAL PATENIEnJmamTs 3,713,142

SHEET 1UOF 15 29 TIMING CIRCUIT I 97 REsET 99 I I SYNC CLOCK 98 TIMING CHAIN i l DETECTOR osc CIRCUIT T T I 1 STROBE l- PULSE l m TRANS- i LM'SSION. 1L SIGNAL DATA I3 l DATA RECEIVE f I77 CODE CONTROL i TRANSPONDER PROGRAM ENCODER Q T I ALARM LINE LATCHING ENCODER GATE TRANSMITTER CIRCUIT I .00 1 Km,

MANUAL REsET t OUTPUT DATA sToRE sl ALARM INPUT FROM SENSORS PATENTEDJAH 23 I975 3.713. 142 SHEET 11 [1F 15 v 95 i TO SYNC DETECTOR TRANSMISSION NOISE LINE l3 FILTER TO RECEIVER DATA STORAGE I02 28 REF 0 V| (1) VI, MAX

TIME

'PAIiIIIinIIIIza I975 3.713; 142

SHEET 12DF 15 I TRANSPONDER CLOCK I TO ZERO MONSTABLE I MULTI- ,-I

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V DETECTOR Id I8 I FIGI?) I I "1 II II II II II II II II II I I I I I I I I 1e I I l I II I I I I I I I I I -I I I I I n IL n n n n n n II fl I I I I I I I l l I I I l I I I I I I I I PAIEIIIEIIJIm I975 3.713. 142

' SHEET 114 0F 15 STORE DATA TIMING T INPUT DATA FROM INPUT DATA sToRE TRANSPONDER SHIFT REGISTER RECEIVER CODE I J30 CONTROL l N D R PROGRAM 1 E Co E I H 1 H3 J H, x -32 I ENCODER GATE ENCODE TIMING REsET LATCH l l i i II4 ALARMS OUTPUT DATA STORE oNE SHIFT REGISTER TRANSMIT TIMING OUTPUT DATA TO TRANSMITTER 2| FIG.I4

PATEN Enmzama 3.713.142

SHEET lSUF 15 22 [SIGNAL LINE RECEIVER T ONDER f INPUT DATA T R ANSMISSION LIN E ;Q= TRANSPONDER OUTPUT DATA TRANSMISSION Ll NE --'W\, OUTPUT DATA FIG. [5A

ALARM SYSTEM BACKGROUND OF THE INVENTION Alarm system installations, particularly those which are used to protect relatively large industrial institutions having a highly valued inventory such as money or other valuable materials, as in banks, industrial plants, government installations housing classified documents, and the like, should be such that it is substantially impossible for an intruder to thwart the operation of such a system and, thus, enter the premises undetected.

In simple alarm systems in present use, intruders often are able to take appropriate action to prevent the annunciation of an alarm. Even in the more complicated of such systems using selective coding and communication techniques for informing a central monitor station of the alarm status of a station at a remote location, a sophisticated intruder may be able, for example, to break the system code using appropriate electronic techniques and thereby produce a simulated signal indicating an all clear status for insertion into the communication link between the stations and, accordingly, avoid detection.

Further, it is desirable that an alarm be able to detect and display other undesirable conditions which may exist in the system, as, for example, the presence of an open or short circuit in the communication link, or of other troubles which may arise in the system, such as malfunctions in the e'quipment being used or tempering with the system by an intruder.

DESCRIPTION OF THE PRIOR ART Alarm systems of the prior art for installations which require a relatively high degree of protection usually utilize a one-way communication link for transmitting alarm information from a remote location to a central monitoring station. In such systems, for example, an appropriately encoded signal which contains the alarm status information is entirely generated at the remote location and is transmitted to the central monitor station where it is decoded and the alarm status information displayed in some appropriate audible or visual manner. Such systems may utilize either signals of a non-random nature which are suitably encoded at the remote station with the alarm status information, or, alternatively, they may utilize signals of a psuedo-random nature for encoding. The complementary decoding process utilized at the monitor station detects and identifies the alarm status information which is carried by the remotely generated signals.

Such systems, however, can often be defeated by an intruder through appropriate electronic techniques wherein the intruder, for example, can use his own equipment to produce a simulated signal corresponding to the coded signal which signifies a normal or allclear condition and can insert such artificially produced signal onto the communications link in place of the signal generated by the system itself. In that way the monitor station is not alerted to the presence of the intruder and the true nature of the alarm status at the remote location remains undetected.

Thus, where the encoded signal is generated at a remote station, whether using a non-random signal or a pseudo-random signal in the encoding process, an intruder normally need only record the encoded signal in the all clear state and, by suitable statistical analysis thereof, fashion a correctly synchronized simulated signal having the same characteristics as the true encoded signal, without the need for a knowledge of the specific coding scheme which is being used therein.

Even in systems where the code scheme being used can be changed periodically, the number of different code schemes available is usually quite limited in the prior art systems and the capability of an intruder to reproduce the desired simulated all clear signal is still relatively high.

While more elaborate coding systems may be devised by the prior art to defeat intruders in certain applications, such as for extremely sensitive military security purposes where cost is often no object, the more complex equipment required for such purposes increases the costs thereof to a point where the use of such elaborate schemes in the applications in which the system of the invention is intended becomes prohibitive.

As discussed more fully below, the system of the invention is designed so that its costs are substantially the same as the costs of prior art systems for use in the same applications and are well below the cost of the more complex coding systems used in other highly sensitive security applications. However, despite the relatively low cost of the system of the invention, the system is much less susceptible to the recording and analysis techniques discussed above and the possibility that an intruder might defeat the system is considerably reduced in comparison with such possibility in connection with presently known systems of comparable cost.

DESCRIPTION OF THE INVENTION The above discussed disadvantages of prior art systems are overcome by the system of the invention, which permits the use of an extremely large number of different code schemes for use therein. While the techniques used in the invention are somewhat more complex than those used in prior systems, the increase in complexity does not require an unwarranted increase in equipment costs while, at the same time, the ability of an intruder to fool the system is considerably reduced in comparison to present systems.

In this system of the invention, a two-way communication link is utilized between a central monitoring station and one or more remote locations which are to be protected. Further, the system utilizes a basic signal which is truly random in nature and which is generated at the monitor station for transmission to a transponder station at a remote location. The transponder appropriately encodes the random signal with alarm status information using a specifically selected code program network to form specific code words representing the different alarm status conditions. The random signal is simultaneously encoded at the monitor station so as to produce a plurality of code words representing a plurality of different anticipated alarm status conditions which may be present at the remote location, the monitor encoder using the same specifically selected code program network as that used at the transponder.

The encoded alarm status signal transmitted from the transponder to the monitor station uses, in a preferred embodiment, the same communication link as that used to transmit the random signal from the monitor to the transponder station, through appropriate time-multiplexing techniques. The encoded signal from the transponder is compared at the monitor with each of the plurality of encoded alarm status signals generated at the monitor. So long as the encoded word from the remote location matches the encoded all clear" word of the monitor with which it is being compared, an all clear output signal is generated at the monitor. However, when the encoded word received from the remote location is the same as that of one of the plurality of encoded alarm signals generated at the monitor, an alarm output signal is produced to indicate the particular alarm status involved, such output signal being thereupon used to activate an audible and/or a visual display unit. As used herein the term alarm status is used to include an indication of an all clear status at the remote location.

Moreover, in the system of the invention, appropriate logic circuitry in the comparator unit at the monitor is used to detect and display the presence of a line fault, that is, a fault occurring in the communication link itself, e.g., a transmission line which is either opened or short-circuited. Further, should any other abnormal condition occur, resulting from a malfunction of the equipment at the remote location, for example, or from an attempt by an intruder to substitute an incorrect bit stream on to the communication link, such a trouble condition is also appropriately detected and displayed at the monitor.

As mentioned above, the basic random signal is generated at the monitor station and is transmitted to the remote location for encoding and then retransmitted in encoded form to the monitor. The fundamental encoder/decoder design used in the invention permits an extremely large number of different codes to be used in the system in conjunction with the randomly generated signal. Such codes can be changed in a periodic, or non-periodic, manner known only to the user of the system. With such a large selection of codes available, even were an intruder somehow able successfully to analyze the signals in the system of the invention for one specific code scheme in order to simulate a counterfeit all-clear signal, a difficult enough task in itself, the fact that a user can readily change the code program greatly reduces the chances that an intruder can successfully insert the simulated signal without detection. Thus, the ability of an intruder to defeat the system becomes in a practical sense effectively negligible.

The detailed operation and configuration of the system of the invention can be described best with the help of the accompanying drawings, wherein:

FIG. 1 shows a block diagram of the overall system of a preferred embodiment of the invention;

FIG. 2 shows a more detailed block diagram of the system of FIG. 1;

FIG. 3 shows a more detailed block diagram of the random signal generator of FIGS. 1 and 2;

FIG. 3A shows a more detailed circuit and block diagram of the generator of F IG. 3;

FIG. 4 shows a more detailed block diagram of the monitor transmitter and receiver of FIGS. 1 and 2;

FIG. 4A shows a more detailed circuit and block diagram of the transmitter and receiver of FIG. 4;

FIG. 5 shows a more detailed block diagram of the monitor encoder of FIGS. 1 and 2;

FIG. 6 shows a chart illustrating the operation of a circuit configuration of FIG. 6A;

FIG. 6A shows a typical circuit configuration of a portion of the encoder of FIG. 5;

FIG. 7 shows a more detailed diagram of one form of the code control program unit of FIG. 5;

FIG. 8 shows a more detailed block diagram of a portion of the monitor system of FIGS. 1 and 2;

FIG. 9 shows a more detailed block diagram of the alarm and display units of FIGs. 1 and 2;

FIG. 9A depicts certain waveforms to illustrate the operation of the units of FIG. 9;

FIG. 9B shows a more detailed block diagram of the units of FIG. 9;

FIG. 10 shows a more detailed block diagram of the transponder of FIGS. 1 and 2;

FIG. 11 shows a more detailed block and circuit diagram of the transponder receiver of FIG. 10;

FIG. 11A depicts certain waveforms to illustrate the operation of the receiver of FIG. 11;

FIG. 12 shows a block diagram of the timing circuitry of FIG. 10;

FIG. 12A depicts certain waveforms to illustrate the operation of the timing circuitry of FIG. 12;

FIG. 13 shows a more detailed block diagram of a portion of the timing circuitry of FIG. 12;

FIG. 13A depicts certain waveforms to illustrate the operating of the circuitry of FIG. 13;

FIG. 14 shows a more detailed block diagram of the transponder encoder of FIG. 10;

FIG. 15 shows a more detailed block diagram of the transponder transmitter of FIG. 10; and

FIG. 15A shows a more detailed block and circuit diagram of the transmitter of FIG. 15.

FIG. 1 shows a simplified block diagram of the overall system of the invention wherein a centrally located monitor station 10 includes a random signal generator 11 which provides an output signal in digital form, such signal having a truly random nature, i.e., a signal having aperiodic characteristics. The random signal is fed to an appropriate transmitter 12 where it is supplied to a transmission link 13, which in a preferred embodiment may be a two-wire transmission line, for example. The output of random signal generator 11 is also simultaneously fed to an appropriate monitor encoder 14 which operates in accordance with a specifically selected code program 15 to produce a plurality of encoded signals, each one of which represents a different alarm status condition which may exist at a remote location which is being monitored.

The remote location is identified in FIG. 1 as including a transponder station 16 which utilizes an appropriate data receiver 17 for receiving the random signal transmitted by the monitor station from transmission link 13. The received signal is fed to a suitable transponder encoder 18 which functions in accordance with code program 19. Encoder 18 is adapted to be responsive to one or more different alarm input signals received from one or more sensors (not shown), one of which is activated in accordance with the alarm status of the remote location. Accordingly, the encoder 18 produces an encoded signal which represents the particular alarm status of the remote location, the encoded signal being thereupon transmitted back to monitor 10 via data transmitter 21 and transmission link 13.

At monitor the encoded alarm signal transmitted from transponder 16 is received by receiver 22 and is thereupon fed to an input of an alarm comparator/decoder 23 which also has fed to it the encoded signals from monitor encoder 14. The received signal from receiver 22 is then compared in sequence with the plurality of encoded signals from encoder l4 and produces an output signal only when the two signals being compared represent code words which are identical. The output from decoder 23 is connected to a plurality of alarm output and display systems 24 which are used to display the alarm status of the remove location when an output signal indicating a matching of the coded characteristics of the input signals to the comparator/decoder occurs.

The operation of each of the above subsystems shown in the simplified overall block diagram of the invention is discussed in more detail below.

FIG. 2 shows a more detailed block diagram of the overall system shown in FIG. 1. The monitor 10 is typically located at a central alarm receiving facility, such as a central privately operated station, a police station, or the like. In general, the monitor station generates a digital signal of a truly random nature and transmits such signal to the remotely located transponder whose alarm status is to be monitored. The monitor further provides for the encoding of the digital random signal at the monitor to produce a plurality of encoded signals for comparison with the encoded alarm signal received from the transponder. Accordingly, the received alarm signal is compared sequentially to each of the encoded comparison signals generated at the monitor, the alarm status of the remote location being extracted as a result of the comparison. The monitor further provides for an audible and/or visual annunciation or display of the alarm status.

In FIG. 2 the digital random signal from generator 11, which is fed to transmitter 12 for transmittal to the remote transponder location, is simultaneously stored in a data storage unit 25 for subsequent use in encoder 14 in accordance with a timing signal from a timing circuit 26 which operates in response to a suitable clock 27. Timing and data storage units are required at various positions in the system because the processing of the data is performed sequentially and because the transmitted and received signals are carried on the same two-way transmission link at different times using known techniques for time division multiplexing.

The encoder 14 at monitor 10 utilizes a selected code which is determined by code control program unit which is responsive to a plurality of simulated signals each representing a different anticipated alarm condition (including an All Clear condition) which may arise at the remote location which is being monitored. Encoder 14 thereby operates upon the random digital signal stored in data storage unit 25 and produces a plurality of encoded signals, each one of which represents a different encoded alarm signal or All Clear signal.

The transponder unit 16 receives the digital random signal transmitted from monitor 10 and stores the received signal in data storage unit 28. This received signal also includes appropriate synchronizing information in the form of a suitable timing pulse for actuating timing circuitry 29 in the transponder subsystem l6.

Encoder l8 encodes the signal from data storage unit 28, at an appropriate time, in accordance with code control program unit 19 the code program of which is selected to be the same as that used for code control program unit 15 in the monitor 10. As discussed more fully below, an extremely large number of code programs may be selected for use in the system of the invention. In each case, the same code control program is selected and used in both the monitor and transponder units during operation at any one time.

One of a plurality of alarm signals which indicates the alarm status at the remote location where the transponder is positioned is suitably inserted into the signal which is encoded by encoder 18, so as to produce a coded alarm status information signal which again is appropriately stored in a data storage unit 31, the operation of which is suitably timed to produce an encoded alarm information output signal for transmission via transmitter 21 and transmission link 13 back to monitor 10.

The receiver unit 22 at monitor 10 then feeds such signal to data storage unit 32, the operation of which is appropriately timed so as to produce a signal for feeding to comparator/decoder unit 23 which also receives the signals from encoder unit 14. Decoder unit 23 thereupon compares the received encoded alarm information signal from data storage unit 32 with each of the plurality of encoded signals from encoder 14 in a sequential manner to determine which of the latter signals has the same characteristics as the coded alarm information signal from transponder 16.

The comparator/decoder 23 is connected to a plurality of alarm display subsystems 24. As discussed more fully below, decoder unit 23 produces an output when the signals being compared have matching characteristics and such output activates one of the alarm display subsystems at a time depending on the alarm status information contained in the encoded signal from the transponder 16. In addition the decoder 23 is also arranged to produce output signals for indicating an All Clear status, a Line Fault status, or a Trouble status which represents a malfunction in the system which may have resulted from incorrectly operating, or non-operating, units of the system or from tampering with the system by an intruder. The operation of all of the above subsystems and units therein is described more fully with reference to the remaining figures.

Monitor Random Signal Generator The purpose of random signal generator 11 is to produce a random sequence of digital bits (i.e., ones and zeros) which are non-periodic in nature, so that the sequence has truly random characteristics. The random bits are then grouped in series to form random digital groups or words which form the basic message signal for communication between the monitor and the transponder subsystems. Although not limited thereto, the invention is described herein with reference to the use of 4-bit groups in each digital word for transmission from the monitor to the transponder and return.

FIG. 3 shows a functional block diagram of one embodiment of random signal generator 11 which includes a noisy oscillator 33 which is an oscillator designed to be relatively unstable so as to produce in effect a signal having a frequency f, which is made up of a basic frequency f and has superimposed thereon a relatively rapid frequency jitter of noise, represented byiAflt) about the average or basic frequency f,,. The frequency f is designed to be substantially higher than the bit rate from the random signal generator 11 and is also statistically independent of the bit rate. The oscillator output (which by appropriate limiting means provides either a digital one or a digital zero oscillation) is sampled at a bit rate determined by stable clock 34 which produces a gating signal comprising bits with a width 7 at a frequency f where f, f and r l/f,. The gating signal is applied to an appropriate gated storage unit 35 which produces a random bit stream at the output thereof. The output random bit stream has essentially no bit-to-bit correlation and no message-to-message (i.e., bit group to bit group) correlation.

FIG. 3A illustrates a specific implementation of the system shown in FIG. 3. In that figure the oscillator comprises three digital inverters, 40, 41 and 42 interconnected as shown with a resistor 43 connected between

inverters

40 and 41 and a resistor 44 connected between the input of the inverter 40 and the output of the inverter 42. A capacitor 45 is connected in parallel with inverter 40 and resistor 43 as shown. The values of

resistors

43 and 44 and capacitor 45 determined the time constants of the circuitry and, accordingly, determine the oscillation frequency. The oscillation, however, is highly unstable in frequency so that the output signal effectively has an average frequency on which is superimposed a relatively rapid jitter, or noise, frequency signal, as discussed above. Relatively short strobe pulses at the desired bit rate (i.e., l lf are supplied fr om stable clock pulse generator 34 to a well-known JK flip-flop circuit 36, the J and 1 iinput terminals of which are connected to the output of the unstable frequency oscillator and the timing strobe pulses being connected to input terminal t. The

v flip-flop circuit samples and stores the oscillator output and produces an output v, therefrom an output terminal O which changes only when the strobe pulse occurs, such output representing the most'recently sampled oscillator output state. Accordingly, the output of flip-flop circuit 36 is a sequence, or stream, of bits (i.e., ones" or zeros) which are truly randomly sequenced. Monitor Line Transmitter and Receiver A block diagram of the monitor transmitter receiver circuitry is shown in FIG. 4, the transmitter 12 (shown by dashed line 50) being in the form of a switchable current source connected amplifier having a power supply 51 producing a voltage V,, as the power source therefor. Transmission of data bits on transmission link 13, which in a preferred embodiment is a two-wire transmission line, is achieved by the presence or absence of current flow I, in the transmission line. The current flow is controlled by the state of the logic input signal v, which is received from random signal generator l l, where v, a v, 0v) or a one" (v, +v). THe magnitude of the current flow is controlled by a reference voltage v from a suitable reference source 52. The logic input signal v, and reference voltage signal v are fed to operational amplifier 53, the output of which is fed to a power amplifier 54 with a current feed back loop 55 producing a current dependent feed back voltage across a resistor 56, as shown. A specific circuit configuration for transmitter 12 is shown in FIG. 4A wherein the logic input voltage v, representing the random data to be transmitted is fed through a diode 57, to one input of amplifier 53 via input resistor 58, the voltage reference v being fed to the other input thereof. The output of amplifier 53 is fed via diode 59 to the base of a transistor across resister 61. Current feed back is supplied from the transistor 60 to the input of amplifier 53 across resistor 56.

If the logic input voltage v, is high (+5v), diode 57 conducts and the output of amplifier 53 is negative so that diode 59 is non-conducting and the base of transistor 60 is at ground potential. The transistor is thereby cut off with no collector current thereby inhibiting the flow of transmission line current.

If v, is low (0v.), diode 57 is off and the output of amplifier 53 rises so that diode 59 conducts and transistor 60 is turned on, thereby allowing the flow of transmission line current 1,. Current I, flows through resistor 56 to produce a feed back voltage v proportional to 1,, i.e., V =1, R The feed back control is such that I, V,,,/R, v /R A feature of the transmission line signaling system as shown in the FIGS. 4 and 4A is that the signal current will remain approximately constant (I, v /R regardless of the transmission line length up to some maximum length. The transmission line, for example, offers a resistance R which is proportional to its length. The maximum transmission line resistance R,,, max for a given signal current I, and power supply source voltage V L is approximately V /I, While 59 is not essential to the operation of the circuit shown in FIG. 4A, its use prevents the reverse base-emitter voltage breakdown of transistor 60 from being exceeded and also protects amplifier 53 if the collector-base voltage breakdown of transistor 60 is exceeded as, for example, because of a spike of voltage noise on the transmission line.

THe monitor receiver circuitry 22 is also shown in FIGS. 4 and 4A as enclosed in dashed line and includes a low pass filter circuit 61 having a received voltage signal, v,., supplied thereto across resistor 62, the output of the filter being supplied to one end of a voltage comparator circuit 63 via resistor 64. The other input of voltage comparator 63 is a voltage reference v' from a suitable reference source 66. Tile signal to be detected is the presence of absence of current on the transmission line 13 as received from transponder l6. Resistor 62 is a current sensing resistor having a value R, in series with the transmission line so that v, I, R, When line current is flowing, v, is greater than v' and the comparator data output is a logic zero" signal. When no line current is flowing, v' is greater than v,, the latter being approximately equalto zero, and the comparator data output is a logic one signal. In this way, the on-off signal current between the monitor and the transponder is converted to appropriate logic level signals by the monitor line receiver. The low pass filter 61 prevents noise spikes and other highfrequency noise typically encountered on long transmission lines, such as long wire lines, from causing false data to be supplied at the output of comparator 63.

Encoder

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.