GB2285157A - An alarm system - Google Patents

Abstract

An alarm system (1) includes a main controller (2) which is connected to detectors via alarm line encoder modules (5) in a multiplex link (3). The main controller (2) is also connected to output devices (4(a), 4(b)) via command line encoder modules (6) also in the multiplex link (3). The main controller (2) uses a control channel to provide for control of return signals from detectors and also for activation of the output devices. The line encoder modules, both for the detectors and for output devices, may be connected at any desired position on the multiplex link (3), thus providing flexibility in installation of the system. The main controller (2) transmits dummy pulses for which it does not expect to receive a return data signal and it also monitors the slope of return data signals for capacitive damping to ensure that the system may not be easily defeated. <IMAGE>

Description

"An alarm system" The invention relates to an alarm system.

There are many alarm systems which are relatively simple such as those for domestic houses. With such systems, installation is relatively straight-forward, particularly if carried out while the house is being built. However, for larger buildings such as business premises, financial institutions, etc., alarm systems which have a relatively complex wiring arrangement suffer from the problems of being difficult and expensive to install, and also from the fact that some of the wiring may provide a weak link for the alarm system which would allow circumvention or defeating of the system by a thief.

British Patent Specification No. GB 1579152 (Denver Fire Reporter and Protective Co.) describes a multiplexed interrogation system which has a transponder for each zone which returns a reply to a controller under control of multiplex signals. A number of replies are stored in the controller for comparison before an alarm signal is transmitted. This helps to avoid false alarms due to spurious signals. Because multiplexing is used, the amount of cabling required is reduced in comparison with the situation where each zone is individually wired to the controller. However, some problems remain such as additional wiring being required for output devices and controllers for these devices and also in monitoring of return signals to ensure that the system cannot be defeated.For example, if the system were tampered with electronically, correct return signals could be transmitted to the controller on the multiplex line from an auxiliary source in order to defeat the system and provided return signals are transmitted at the right time, they would not be identified as false return signals. In addition, the wiring which would be required for various types of alarm detection states, such as tampering and intruders, appears to be quite extensive.

United States Patent Specification No. US 5150363 describes a multiplexing apparatus in which a remote module has an interface for receiving signals from a number of input devices coupled to it and in which the remote module and a local module include serial to parallel converters. This apparatus appears to be quite complex.

British Patent Specification No. GB-A-2217125 (Digital Audio Limited) describes an alarm system in which a detector includes a performance monitor switch and a separate voltage supply is used when this switch is monitored. British Patent Specification No. GB-A-2233486 (Digital Audio Limited) describes an arrangement whereby there is an alarm loop and an anti-tamper loop for alarm system detectors, both loops being connected to a control unit at four terminals, only two of which must be monitored by an analog to digital converter. Both of these arrangements are quite complex.

One object of the invention is to provide an alarm system which requires relatively little wiring so that it may be easily installed, particularly in large buildings.

Another object is that the system be configured so that it is extremely difficult for a thief to defeat the system by connection of an auxiliary signal source. A still further object is that the system be very flexible in the manner in which it allows installation and expandability by addition of further components and detectors.

According to the invention, there is provided an alarm system comprising a main controller, a plurality of detectors, an alarm output device, and a multiplex link connecting the main controller with the detectors, wherein the system comprises line encoder modules for connection in the multiplex link and for connection by a separate cable to a plurality of detectors, each link encoder module comprising means for monitoring control signals transmitted on a control channel of the multiplex link by the main controller to determine a data return time frame, and a means for connecting a data channel in the multiplex link to each of its associated detectors during an appropriate multiplex time frame;; wherein the main controller comprises means for monitoring capacitive damping in returned data signals on the data channel, and means for transmitting an alarm signal on the control channel if the signal is not damped, being indicative of electrical tampering with the system; and wherein the alarm output device is driven by an output controller connected in the multiplex link and comprising means for recognising an alarm signal transmitted in an associated time frame on the control channel and means for activating the alarm device in response to the alarm signal.

In one embodiment, the main controller comprises means for transmitting control signals for which there is no corresponding detector time frame and for monitoring the data channel for presence of a return data signal, presence of such a signal indicating electrical tampering with the system.

Ideally, the main controller comprises means for monitoring the slope of the return data signal to filter out spurious interference signals.

In one embodiment, each line encoder module is connected to each detector by a single loop in which there is a common line for use with other detector loops.

In another embodiment, a detector loop includes tamper and alarm switches, opening of the switches providing different loop resistances, and preferably, opening of the tamper switch provides an open circuit in the loop. In these latter embodiments opening of the alarm switch preferably provides an increased loop resistance by inclusion of a resistive element with which it is connected in parallel.

In a further embodiment, the output controller comprises means for monitoring for detection of an alarm signal in successive multiplex cycles and a means for activating the associated alarm device only on detection of alarm signals in at least two successive multiplex cycles.

In a still further embodiment, the output controller comprises a J-K flip-flop arrangement for monitoring the alarm signals in successive cycles.

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic view showing an alarm system of the invention; Fig. 2 is a diagram showing operation of a detector of the system; Fig. 3 are plots showing control and data signals of the system; Fig. 4 is a circuit diagram showing a line encoder module of the system in more detail; and Fig. 5 is a circuit diagram showing a command line encoder module of the system in more detail.

An alarm system 1 of the invention is shown in Fig. 1.

The system 1 comprises a main controller 2 which includes a microprocessor and various read only memory (ROM) and random access memory (RAM) circuits. Interfacing with the microprocessor is through an analog to digital converter.

The main controller 2 is connected to all of the other parts of the system by a multiplex link 3. The multiplex link 3 may be four or six core unscreened alarm cable.

The length of the link 3 may be up to 2 kilometres. An important aspect of the invention, however, is the fact that only three cores are required, the possibility remaining for the additional cores being used for auxiliary signals where there are special requirements.

The three cores which are used include a data core, a control core and a ground core providing what may be regarded as a control channel for transmission of signals including multiplex signals from the main controller 2 and a data channel for return of information to the main controller 2. However, as is described below, the channels are not always used for these functions, thus providing versatility and simplicity in installation and operation of the system 1.

The system 1 also comprises alarm output devices including a buzzer 4(a) and a bell 4(b). The output devices are controlled by an output controller, namely, a command line encoder module (CLEM). A CLEM may be connected at any position on the multiplex link 3 and there is, within reason, almost no limit to the number which may be connected.

In addition, the system 1 comprises a number of alarm line encoder modules (ALEM) 5 connected in the multiplex link 3. The ALEM's 5 may be placed anywhere on the multiplex link 3 simply by breaking the cable at a suitable point and connecting the cable to each end of the unit.

Continuity of all of the cores is provided through the circuit of the ALEM 5. There are spare terminals on each ALEM 5 for the additional cores and these may be used, for example, to provide 12 V DC to detectors which are connected to the ALEMs 5. Each ALEM 5 is connected to two detectors 18 shown in Fig. 2. Other line encoder modules referred to as QUADLEM's may be used for connection to four detectors. The function of each ALEM 5 is to temporarily connect each of its associated detectors to the data channel on the multiplex link 3 at an appropriate time frame. This is achieved by monitoring a multiplex control signal transmitted by the main controller 2 and connecting the detector circuits onto the data channel at the appropriate time frames.As is described below, the main transmitter 2 is capable of transmitting data signals in the form of activation signals for an alarm output device on the control channel. The system 1 further comprises a power supply unit 7 connected to a remote keypad 8(a), there being another remote keypad 8(b) connected to an ALEM 5. There is also a line encoder module (LEM) 9 which provides for continuity in the multiplex link 3 and filters out distortion in signals.

The system 1 further comprises an auxiliary power supply unit 10 connected to a CLEM 6 and to the bell 4(b).

Referring now to Fig. 2, each ALEM 5 has a number of terminals indicated generally by the numeral 15 and the terminals TS, T6 and T7 shown in Fig. 4 are the only ones shown in Fig. 2 as these are for monitoring of detectors.

The terminal TS is for an even line, the terminal T7 for an odd line and the terminal T6 for a common line. The terminal TS is connected to an even line 16 which together with a common line 17 provides the loop for a detector 18.

There is a second detector connected to the ALEM 5 and this is shown diagrammatically only and is connected between the common line 17 and an odd line connected to the odd terminal 7.

As shown in Fig. 2, the detector 18 comprises a tamper switch 19 which is in the normally closed position. This is in series with a 2K2 resistor 20. There is then a parallel combination of a 2K2 resistor 21 and a relay 22 to complete the loop. The relay 22 is in the normally closed position so that in a normal condition, the current flows as indicated by the arrows Q. Because the resistor 21 is shorted by the relay 22, the total resistance is 2K2 Ohm and thus, in the normal situation, the total resistance in the loop is 2K2 Ohm. If the tamper switch 19 is opened such as by a thief tampering with a detector, the loop is open circuited thus presenting an infinite resistance. If there is an alarm condition such as detection by an infrared sensor of an intruder, the relay 22 is opened so that the resistor 21 is no longer bypassed, thus providing a total resistance of 4K4 Ohm for the loop.Finally, the detector 18 or the line 16 may be short-circuited. These states are indicated in the drawings. It will thus be appreciated that in an extremely simple manner with relatively little wiring, the detector 18 provides four different resistance values in the loop, depending on the condition.

Referring to Fig. 4, the ALEM 5 is described in more detail. Parts similar to those described with reference to the previous drawings are identified by the same reference numerals. The control channel of the multiplex link 3 is connected to the control terminal T3 and the data channel is connected to the terminal T4. Thus, the terminals T4, T3 and the detector terminals T5, T6 and T7 and the ALEM ground terminals T1 and T9 are the only ones which are used. The other terminals are redundant, however, they could be used for transmission of DC power to a detector. An incoming control signal at the terminal T3 passes through power supply circuitry 40 and then to a noise control active filter provided by a Schmitt Trigger 41. The control signals are transmitted to a pin 1 of a counter 43 and are transmitted to a reset pin 9 of the counter 43 via a pulse width discriminator 42.The counter 43 is connected to a set of dipswitches 44. An output pin P14 is connected via the various gates to SCR switches 45 and 46 to provide control of these switches.

As is clear from the circuit, the even and odd lines at the terminals T5 and T7 are connected to the outputs of these switches so that the switches 45 and 46 may connect the terminals T5 and T7 sequentially to the data terminal T4. Thus, in summary, the switches 45 and 46 act as switches which, under control of the counter 43, effectively connect the terminals T5 and T7 to the multiplex link data terminal T4. The output of the counter 43 is determined by the input control signals at the terminal T3 which are transmitted into the pins P1 and P9. As is shown in Fig. 3, the control signals may take the form of a long high signal 31 which provides a reset for the counter 43 and this is inputted at the pin P9.

This signal resets the counter 43 to-the values set by the dipswitches 44. There are then various pulses 42 which are inputted at the pin P1 and each pulse downcounts the reset value of the counter 43 until it reaches zero.

Thus, if the decimal value of the binary reset value set by the dipswitches 44 is ten, then ten pulses 32 transmitted on the control line to the terminal T3 are sufficient for counting down of the counter 43 to zero, causing output of an appropriate signal at the pin 14.

This is the manner in which multiplexing is achieved.

When the counter 43 downcounts to zero, the signal at pin 14 goes low and activates both of the gates. The other inputs to these gates are the control signals and therefore, in succession, the gates 45 and 46 route the signals appearing at the terminals T5 and 7 to the data terminal T4. At a later stage, another reset signal 31 is transmitted to start the next multiplexing cycle and the process is repeated. The reset signal 31 resets all of the ALEM's 5 and indeed all of the other devices which are described below which are connected in the multiplex link 3.

Referring now to Figs. 5 and 3, the CLEM 6 is now described in detail. The data core of the multiplex link 3 is not connected to the CLEM 6, only to the control line and the ground at terminals T2 and T3. A Schmitt Trigger 60 provides the appropriate noise control active filter operations and a circuit 61 provides a discriminator for long pulses, also for resetting a counter 62, the reset value of which is set by a set of dip switches 63. The received control signals are also outputted by the Schmitt Trigger 60 to a discriminator 64 detection of long low signals. The CLEM 6 also comprises a pair of JK flipflops 65 which form a pair of latches. A long low signal which is received is transmitted to the pins P3 and P13 of the latches. The Q and Q bar outputs of the first latch indicate the memory state and effectively store an indicator of a long low signal in the previous multiplexing cycle. If the next signal is also a long low in the appropriate time slot as indicated by the counter 62, then there would be a different input into the second latch 65 and there is no output change. However, if the second multiplexing cycle also provides a long low in the time slot for the appropriate CLEM 6, then an output relay 6 is activated to activate an alarm output device such as the bell 4(b) or the buzzer 4(a).

It will thus be appreciated that each CLEM 6 is activated by a long (4 ms) low signal transmitted from the main controller 2, this signal being embedded in the multiplexing control signals without affecting any of the other devices. Because the activating signal for a CLEM is a long low signal, it does not register in any of the other devices such as the ALEM's, which devices are monitoring long high signals (resets) and pulses. In this way, a single multiplex link may be used not only for monitoring detector outputs but also for controlling output devices such as the CLEM's 6 to provide the output alarm signals. Of course, each CLEM 6 may be connected at any desired position on the multiplex link 3, thus providing for a large degree of versatility and flexibility in installation and operation of the system.

Referring again to Fig. 3, the various signals are shown and an alarm signal 33 is shown as being a long low voltage level.

The data signals shown in Fig. 3 also indicate four possible outputs from the detectors which are linked with the data channel at the appropriate multiplexing time frame. The voltage levels recorded for 2K2 resistance in each detector are shown by the curves 36. It will be noted from these drawings that the level drops gradually, due to capacitive damping. The main controller 2 is programmed to monitor the level at at least two positions during the duration of the return data signal (which is approximately 1.5 ms long). In this way, the main controller can monitor the capacitive damping and if the slope is not correct (within a preset tolerance) the signal may be discarded as a spurious signal. This is more important for an alarm detector signal which is indicated by 37 in Fig. 3, which is a signal returned for a 4K4 resistance in the detector loop.Again, the manner in which the capacitive damping is monitored is extremely important. There is no drop in voltage level for when a tamper occurs as indicated by the numeral 38 in Fig. 3.

This is because there is infinite resistance in the detector loop.

An important feature of the invention is the fact that there is not an exact correlation between the number of pulses and the number of units on the multiplex link 3.

Accordingly, the first two pulses 32 shown in Fig. 3 may be regarded as dummy pulses for which there is no corresponding return data signal. Thus, the correct and normal signal is that shown in Fig. 3. If a thief were attempting to defeat the system by inserting a good" signal representing a 2K2 detector loop resistance, the main controller would detect tampering by receiving a return signal where there should be none corresponding to the dummy pulses. It has been found that the combination of monitoring capacitive damping generally and the slope of the return signal, together with monitoring of the return data slots for dummy pulses, sufficient precautions are taken to ensure that the system may only be defeated with extreme difficulty.

In general, it will be appreciated that the invention provides an extremely simple arrangement whereby only three cores are all that is required in a primary link for both monitoring and output devices. There is versatility in the location of these devices as they can be connected at any desired position on the multiplex link. The output devices are controlled in an extremely simple manner using the multiplex link and the controller is programmed to ensure that the system may not be easily defeated.

The invention is not limited to the embodiments hereinbefore described. For example, the circuitry on the various line encoder modules may be configured differently to provide similar functions. The circuits described are extremely inexpensive as they include low-cost components, however, it is envisaged that with increased circuit integration and lower cost of microcontrollers, that these may be used instead.

The invention is not limited to the embodiments hereinbefore described, but may be varied in construction and detail.

Claims (10)

CLAINS

1. An alarm system comprising a main controller, a plurality of detectors, an alarm output device, and a multiplex link connecting the main controller with the detectors, wherein the system comprises line encoder modules for connection in the multiplex link and for connection by a separate cable to a plurality of detectors, each link encoder module comprising means for monitoring control signals transmitted on a control channel of the multiplex link by the main controller to determine a data return time frame, and a means for connecting a data channel in the multiplex link to each of its associated detectors during an appropriate multiplex time frame;; wherein the main controller comprises means for monitoring capacitive damping in returned data signals on the data channel, and means for transmitting an alarm signal on the control channel if the signal is not damped, being indicative of electrical tampering with the system; and wherein the alarm output device is driven by an output controller connected in the multiplex link and comprising means for recognising an alarm signal transmitted in an associated time frame on the control channel and means for activating the alarm device in response to the alarm signal.

2. An alarm system as claimed in claim 1, wherein the main controller comprises means for transmitting control signals for which there is no corresponding detector time frame and for monitoring the data channel for presence of a return data signal, presence of such a signal indicating electrical tampering with the system.

3. An alarm system as claimed in claims 1 or 2, wherein the main controller comprises means for monitoring the slope of the return data signal to filter out spurious interference signals.

4. An alarm system as claimed in any preceding claim, wherein each line encoder module is connected to each detector by a single loop in which there is a common line for use with other detector loops.

5. An alarm system as claimed in claim 4, wherein a detector loop includes tamper and alarm switches, opening of the switches providing different loop resistances.

6. An alarm system as claimed in claim 5, wherein opening of the tamper switch provides an open circuit in the loop.

7. An alarm system as claimed in claims 5 or 6, wherein opening of the alarm switch provides an increased loop resistance by inclusion of a resistive element with which it is connected in parallel.

8. An alarm system as claimed in any preceding claim, wherein the output controller comprises means for monitoring for detection of an alarm signal in successive multiplex cycles and a means for activating the associated alarm device only on detection of alarm signals in at least two successive multiplex cycles.

9. An alarm system as claimed in claim 8, wherein the output controller comprises a J-K flip-flop arrangement for monitoring the alarm signals in successive cycles.

10. An alarm system substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.