Process for Protecting a Remote Monitoring System Against Sabotage and a System Using This Process

The invention relates to the protection of a remote monitoring system against sabotage intended to neutralize it, that is to say to make it inoperative while keeping it in an apparently normal operating condition. For example, a remote monitoring system for detecting an intrusion in premises is neutralized if the system is modified so that the sensors give a normal response whereas in fact they should signal an intrusion.

To avoid sabotage, each sensor of a conventional remote monitoring system is connected to the central station of the system by a line having four conductors: two forming a protection loop for detecting sabotage of the line, and two forming a detection loop charged with conveying alarm information. This line is possibly completed by other conductors for effecting a remote test of the sensors. It is known to detect sabotage of such a line by detecting a current, voltage or impedance variation. These processes are simple and can be easily neutralized by anyone having a little time and a minimum of technical knowledge at his disposal. On the other hand, several sensors are generally connected to the same line and it is not possible to distinguish which sensor transmits alarm information.

To remedy the disadvantages of these conventional processes, it is known to associate with each sensor a series or parallel resonating circuit connected to a bus line, to send successively over this line periodic signals of increasing frequency and to detect the impedance variations corresponding to the resonance of each of the resonating circuits. In such a process, neutralization of the system is much more difficult to perform and the response of each sensor is individualized since it corresponds to a different frequency value for each one. The implementation of this process is however delicate for the selectivity of the resonating circuits and their tuning frequency are affected by the characteristics of the line, which limits the number of sensors usable in the same line. On the other hand, in order that the circuits for analyzing the response of the sensors may be simple, it is necessary to carry out fine on-the-spot adjustments.

The process of the invention has as object to remedy these drawbacks by using simple means.

The invention provides then a process for protecting a remote monitoring system against sabotage, this system comprising a central station connected to a plurality of sensors, consisting:

in testing the state of each sensor of the system;

in transmitting, from a tested sensor to the central station of the system, a signal of variable amplitude formed from a wave-form synthesized from data stored in the sensor;

in authenticating this signal, when it arrives at the central station, by sampling it and comparing the value of each sample with a reference value.

The invention will be better understood and other features will appear from the following description and the accompanying figures in which:

FIG. 1 shows the block diagram of one embodiment of a remote monitoring system;

FIGS. 2a and 2b show the timing diagrams of an example of signals exchanged between the sensors and the central station of this remote monitoring system; and

FIG. 3 shows the block diagram of one embodiment of a sensor.

The remote monitoring system shown in FIG. 1 is formed by a central station 6 and sensors n.degree. 1, n.degree. 2, n.degree. 3 . . . , each having an input terminal 2 connected to a bus line 4 and an output terminal 3 connected to a bus line 5, bus lines 4 and 5 being connected to the central station 6. In this example, each sensor is connected to a switch 1 whose state is transmitted when the central station 6 interrogates the sensor. This switch 1 allows, for example, the opening of a door to be detected. In this example the system may comprise up to 16 sensors. To detect the change of state of a switch 1, or an abnormal operation of a sensor, each sensor is tested cyclically by interrogating it periodically from the central station 6.

Central station 6 comprises a signal generator 8, a synchronizing signal and clock signal separator 7, a binary counter 9, a read only memory (ROM) 10, a digital-analog convertor 11 and an analog comparator 12. The signal generator 8 supplies a periodic binary signal V.sub.1 which is shown in FIG. 2a. To scan the whole of the sensors of the system, signal generator 8 supplies a synchronizing pulse 22 between times t.sub.i and t.sub.o, then 256 periodic pulses, of a period very much less than the time interval t.sub.i -t.sub.o. The output of signal generator 8 is connected to the bus line 4 and supplies then this signal to the input terminal 2 of each sensor. FIG. 2b shows the signal V.sub.2 supplied to bus line 5 by the whole of the outputs of the sensors of the system. During the time interval (t.sub.i,t.sub.o) the voltage present on line 5 is zero, then for a time interval (t.sub.o,t.sub.l) sensor n.degree. 1 supplies a variable voltage 19 formed of a succession of constant levels. During this time, the outputs of the other sensors supply no voltage and present a high impedance. During the time interval (t.sub.1,t.sub.2), the output of sensor 2 delivers to bus line 5 another signal 20 whose voltage is variable, whereas the outputs of all the other sensors are at a high impedance. During the time interval (t.sub.2,t.sub.3), the output of sensor n.degree. 3 delivers a variable voltage 21, having a different form from the two preceding ones, whereas the outputs of the other sensors present a high impedance. In turn, each sensor supplies a signal to the bus line 5 for a time interval corresponding to 16 periodic pulses of the signal generator 8. Thus, 16 sensors may respond during each cycle of interrogation of the whole of the sensors. The signal transmitted by each sensor has a complex form, different for each of the sensors. If switch 1 connected to a sensor changes state, the shape of the signal transmitted by this sensor is modified to transmit this information. For example, the response signal V.sub.2 may be replaced by a signal of zero value.

FIG. 3 shows the block diagram of one embodiment of a sensor, such as sensor n.degree. 1. The sensor comprises a synchronizing signal and clock signal separator 13, a binary counter 14, a ROM, a digital-analog convertor 16 and an analog gate 17. The input terminal 2 is connected to an input of separator 13 which supplies at a first output a logic signal when the central station sends to the input terminal 2 a synchronizing signal characterized by its duration t.sub.i -t.sub.o, and which supplies at a second output a clock signal formed by the periodic pulses which follow the synchronizing signal fed by the central station 6 to bus line 4. These signals are applied respectively to reset input and a clock input of the binary counter 14. This counter 14 comprises eight stages whose outputs are connected to eight address inputs of the ROM 15. The ROM 15 comprises eight data outputs, seven of which are connected to seven inputs of the digital-analog convertor 16, and an eighth one of which is connected to a first control input of the analog gate 17. An output of the digital-analog convertor 16 supplies an analog value to an input of gate 17. The output of gate 17 is connected to the output terminal 3 of the sensor.

At each scanning cycle of the whole of the sensors, the input terminal 2 receives, first of all, a synchronizing pulse, which is transmitted by separator 13 to the binary counter 14 for resetting same, then 256 clock pulses which are transmitted to the binary counter 14 so that it supplies successively 256 address values to memory 15. Memory 15 supplies at its eighth output a logic signal of value 1 for 16 consecutive address values corresponding to 16 consecutive clock pulses and thus enables the analog gate 17. Thus, for sensor n.degree. 1, analog gate 17 is enabled for the time interval (t.sub.o,t.sub.l). The other seven outputs of memory 15 supply successively 16 binary words of seven bits to the digital-analog convertor 16 which thus synthesizes a waveform composed of 16 plateaux whose amplitude may assume 128 values. When the state of switch 1 changes, gate 17 is disabled, the absence of response of the sensor thus triggers off an alarm.

The signals delivered successively by sensors n.degree. 1, n.degree. 2, n.degree. 3, . . . are transmitted by the bus line 5 to the central station 6 where they are authenticated to check whether there is fraud and change of state of one of switches 1. In the central station 6 (FIG. 1), the output of signal generator 8 is connected to an input of the synchronizing signal and clock signal separator 7, identical to separator 13 of the sensors. Separator 7 provides a reset signal and a clock signal to the binary counter 9, identical to counter 14 of the sensors. The binary counter 9 has eight outputs which deliver, during each interrogation cycle, 256 binary words of eight bits to the address input of the ROM 10, which stores the whole of the data stored in the ROMs 15 of the sensors. This data is read into successive addresses in the order of interrogation of the sensors. The output of the ROM 10 supplies 256 binary words of 7 bits to the inputs of the digital-analog convertor 11, one output of which is connected to a first input of the comparator 12. A second input of comparator 12 is connected to the bus line 5, and an output of this comparator forms the output terminal 18 of the central station. The output of the digital-analog converter 11 delivers an analog signal whose waveform is formed by the succession of the waveforms of the signals expected as response from the sensors. Comparator 12 compares the succession of waveforms supplied by the analog converter 11 and the succession of the waveforms delivered by the sensors and generates an alarm signal at output terminal 18 if these two successions of waveforms are not identical. This occurs when there is modification of the state of the switch 1 or else when there is sabotage of a sensor or of one of the bus lines.

It is within the scope of a man skilled in the art to construct a complementary device for counting the number of clock pulses generated between the synchronizing time and the alarm time so as to identify which sensor has transmitted a response different from the expected response; and for checking the alarm over several interrogation cycles before delivering an alarm signal to the output terminal 18. Moreover, it is within the scope of a man skilled in the art to increase the number of sensors which may be used by increasing the number of clock pulses transmitted during each interrogation cycle, or to modify the number of amplitude levels of the plateaux of the synthesized waveforms. It is also possible to carry out differently individual interrogation addressing of each sensor, for example by replacing bus line 4 by a multi-conductor bus transmitting a binary word in parallel form.

The above-described interrogation mode has the advantage of great simplicity since a single bus line is sufficient to transmit addressing information, synchronization information and a clock signal. It is also possible to make the addressing more complex, to make neutralization of the system even more difficult, for example by generating addresses in a pseudo-random order.

It is also possible to use the process of the invention in a remote monitoring system where the sensors take the initiative of transmitting information without being previously interrogated by the central station.

Instead of using an analog comparator 12, it is also possible to digitize the wave form received by the central station 6 and to compare the values obtained with data stored in memory 10.

The waveforms transmitted in response by the sensors may be very complex, and are thus difficult to simulate, neutralization of the system being then practically impossible to achieve. The sensors comprise simple logic means which may be readily integrated in a hybrid circuit taking up little room, which may be situated in the immediate proximity of switches 1 or other means generating an alarm to be transmitted.