GB2051438A

GB2051438A - Security alarm systems

Info

Publication number: GB2051438A
Application number: GB7919784A
Authority: GB
Original assignee: Price-Smith I C D
Current assignee: Price-Smith I C D
Priority date: 1979-06-07
Filing date: 1979-06-07
Publication date: 1981-01-14
Also published as: GB2051438B

Abstract

For fault monitoring in a multi- line security alarm system, a circuit (46 and 52) detects high frequency bursts of pulses received by the transponder (36) in an out-station (or transmitted by the transponder) to light an indicator lamp (48). The test circuit may be in a probe or mounted permanently in the out- station with switch means (38) for supplying either the output signals from the receiver or the signals from the transponder output to the circuit. Logic circuit means (64, Fig. 5) may generate a pulse corresponding to the period between the end of a received call signal and the beginning of a transmitted reply signal to flash a lamp (68), and second circuit means inhibit a second lamp (86) when the transponder is producing output signals. <IMAGE>

Description

SPECIFICATION Improvements in and relating to security alarm systems Field of invention This invention concerns alarm systems typically installed for security purposes and is particularly applicable but not exclusively so to multi-line security alarm systems in which alarm signals are transmitted from different out-stations to a central control station. The invention is particularly concerned with the monitoring of the various circuits so as to indicate at an early stage that a fault has developed.

Background to the invention It has been recognised for some time that it is an important requirement of any security alarm system that each of the lines should be monitored either continuously or at least regularly for line faults. Obviously only genuine line faults should create an alarm situation although it is obviously better for a fault monitoring system to occasionally detect a non-genuine line fault than to fail to detect a fault condition.

One such monitoring system is described in British Patent Specification No. 1391905 reference to which is made for general background information to typical multi-line security alarm systems. This patent specification deals with a system in which the lines are continuously monitored for line faults and requires a line fault indicator responsive to the absence of an answer signal to be located at one end of the line to be tested and a second line fault indicator responsive to the absence of a call signal to be located at the other end of the line so that the line is continuously monitored for line faults at both ends.

It is an object of the present invention to provide an alternative and simpler method of line fault detection not requiring two line fault indicators one at each end of the line.

The invention In a typical data signalling system for monitoring the status of a number of out-stations via a communication channel such as a telephone circuit, the central station produces an intermittent series of selective call signals and during quiescent periods between the transmission of the intermittent series of selective call signals, the central station receives reply signals from the out-stations by virtue of transponders located in the out-stations which receive selective call signals and transmit reply signals a short time afterwards.Typically each of the out-stations is addressed in turn by coding or otherwise identifying the call signals so that only one of the out-stations is responsive to an outgoing call signal and provided an appropriate reply signal is received by the central station during the following quiescent period the system is deemed to be working correctly. However this check can only be confirmed at the central station.

The signals which are transmitted and received in such a data signalling system are of relatively high frequency and normally comprise distinct bursts of high frequency signal the frequency and/or duration of which can be used to code the different signals.

The invention is applicable to data signalling systems of the type just described and a signalling system of this type will be referred to as a data signalling system of the type described.

According to one aspect of the present invention in a data signalling system of the type described circuit means is provided at an out-station for converting the high frequency bursts of pulses received by the transponder in the out-station or transmitted by the transponder in the out-station into an electric current and an indicator lamp is provided operable by the current derived from the circuit means to indicate the presence of the high frequency signal to which the circuit means responds.

The circuit means may be located in a test probe adapted to be connected into the outstation so as to pick up the appropriate signals and indicate whether the out-station is receiving signals from the central station and whether the transponder is responding appropriately thereto. Alternatively the circuit can be mounted permanently in the out-station and switch means provided for supplying either the output signals from the receiver or the signals from the transponder output to the said circuit means.

The circuit means most conveniently comprises a rectifying circuit which may be a single diode and an appropriate electrical lamp which responds to direct current pulses from the rectifying circuit. Where a continuous indication is required, a large capacitor may be connected across the output from the rectifying circuit. In order to allow a charge to develop across the capacitor a resistor will normally be required in series with the lamp and an appropriately lower rated lamp used than would otherwise have been the case.

According to another aspect of the invention, in a data signalling system of the type described first circuit means including logic circuit means for generating a signal pulse corresponding to the period between the end of a received call signal and the beginning of a transmitted reply signal from the transponder of an out-station, and indicator means is provided responsive to this signal or signals so as to indicate the correct functioning of the out-station, second circuit means being provided for generating an electrical signal from the transmitted output signals from the transponder and the signal so generated is used to inhibit an output signal to a second indicator lamp so that the latter is prevented from lighting up all the time that the transponder is producing output signals for transmission back to the central station correctly but this second indicator lamp will immediately become illuminated in the event that the transponder output signals cease or become of insufficient amplitude and/or duration as to generate the inhibiting signal within the second circuit means.

Conveniently both the first and second circuit means are permanently connected to the out-station transponder so as to provide a ready system check on the operation of the transponder. However it will be appreciated that the system does not have to be permanently connected to the transponder and can be constructed in the form of a test probe adapted to be connected to appropriate junctions within the circuits of the out-station transponder.

According to a preferred feature of the invention the output from the transponder receiver and the output from the transponder transmitter are supplied as inputs to a logic circuit having a NOR function and the output of this circuit is inverted and supplied as a control signal via a current limiting resistor if required to the system monitor indicator lamp.

According to another preferred feature of the invention the second circuit means comprises a rectifying circuit for producing from the transmitter output pulses a D.C. signal which is supplied as an input to an inverting amplifier the output of which is connected to a fault indicating lamp if necessary through a current limiting resistor. By providing a sufficiently long time constant in the rectifying circuit so a D.C. voltage is maintained in the output of the rectifying circuit from the intermittently received pulses from the transmitter output so that the inverter amplifier is continually forced to produce a zero output.In the event of a failure of the transmitted output pulses from the transponder or in the event that these pulses become too short or of insufficient amplitude the rectifying circuit will fail to produce the required D.C. signal and the inverting amplifier output will rise and the fault indicator lamp will be illuminated.

Latching circuit means may be provided so that in the event that the inverter amplifier output rises only intermittently for short periods as a result of insufficent amplitude or duration of the transponder transmitter output pulses so the indicator lamp will be latched into an ON condition. Preferably means is provided for overriding the latching circuit so that an engineer can see whether the transponder transmitter circuit is producing any signals (in which case the indicator lamp will flash on and off when the latching circuit is overridden) or is failing to produce any signals for transmission in which event the fault indicator lamp will remain illuminated permanently due to the high output from the inverting amplifier even when the latching circuit is overridden.

The invention will now be described by way of example with reference to the accompanying drawings.

In the drawings Figure 1 is a diagram illustrating a multiline data signalling system, Figure 2 illustrates the call and reply signals transmitted and received from the central station to the out-stations, Figure 3 is a part schematic part circuit diagram of a signal check facility for connection to an out-station of a system such as shown in Fig. 1, Figures 4(i), (ii) and (iii) illustrate the signals received and transmitted by the out-station of Fig. 3, Figure 5 is a part schematic part circuit diagram of another signal checking facile'ivy adapted to be connected to a transponder of an out-station of a system such as shown in Fig. 1 and Figure 6 illustrates signals received and transmitted by the transponder out-station and testing circuit of Fig. 5.

Detailed description of drawings In Fig. 1 a central station 10 is connected via a communications channel 12 and branch channels 14, 1 6 and 1 8 to out-stations 20, 22 and 24. Additional branch communication channels such as 26 serve to connect the communications channel 1 2 with further outstations (not shown) if required.

The communications channel 1 2 is adapted to be capable of transmitting high frequency electrical signals in the form of a chopped carrier which may be at constant or different frequencies and either the frequency or the duration of the bursts of pulses are used to code the signals transmitted from the central station 10. De-coding circuits in the various out-stations 20, 22, 24 respond selectively to the coded output signals from the central station 10 and transponders (not shown) in the out-stations 10, 22, 24 etc. generate reply signals in the form of further bursts of high frequency signals typically at a different frequency from the incoming signals from the central station 10 which are then re-transmitted along the communication channels to the central station 10 which is adapted to not only transmit but also receive signals and decode them to provide indication that a transponder in a particular out-station has received a transmitted signal from the central station 10 and has responded with an appropriate reply signal.

Fig. 2 illustrates graphically a call signal in the form of a burst of high frequency signal 28 followed by a quiescent period during which a reply signal of a different frequency of a defined duration 30 should be received from the appropriate out-station. If it is received circuits within the central station 10 respond to the reply signal 30 and generate an indication that all is well with the particular out-station concerned.

A further call signal is transmitted at the end of the quiescent period following the call signal 28 and this next call signal is designated by reference numeral 32. The call signals 28 and 32 are coded either by way of frequency or duration or some other simple coding device to that only one of the outstations responds to the call signal 28 and another to call signal 32 and so on.

As shown in Fig. 2 the out-station which responds to the call signal 32 produces its own reply signal identified by reference numeral 34 which is transmitted by the transponder in the out-station which has been addressed during the quiescent period following the call signal 32.

The process is continued until call signals for calling up all of the separate out-stations have been transmitted in turn and the process may be operated on a continuous basis or may be instigated by depressing a check switch at the central control station.

Use is made of the fact that a call signal such as 28 will be received by a transponder in an out-station and a reply signal 30 will have to be transmitted by the transponder if the line and out-station are functioning correctly. To this end a signal checking facility is shown in Fig. 3 connected to the out-station 20 of Fig. 1. In Fig. 3 only the transponder section of the out-station is shown since the remainder of the out-station circuitry is irrelevant. This transponder is designated by reference numeral 36 and is shown connected to the communication channel 1 4 serving outstation 20.

Outputs from the transponder are taken to a selector switch designated 38. The first output to terminal 40 of the switch is derived from the signals in the output of the receiver section of the transponder. The second output which is supplied to terminal 42 of the switch 38 is derived from the transmitter section of the transponder and typically from the output stage of the transmitter.

Signals from either the receiver or the transmitter section of the transponder 36 can therefore be supplied via signal path 44 to rectifying diode 46 and current rectified by the diode 46 will cause an indicator lamp 48 to be illuminated. A current limiting resistor 50 is provided in series with the lamp 48 and when so provided a capacitor 52 may be connected in the output of the rectifying diode 46 so as to provide a more or less continuous direct current for operating the indicator lamp 48.

The circuit is obviously not required to function all the time since it is only a check facility and consequently a push-button switch 54 is provided in the signal path 44. On pushing the push-button switch 54 so the switch is closed and the signal path 44 completed between the switch 38 and the rectifying diode 46. A return path to the transponder is provided by signal path 56.

The relevant signals in the system of Fig. 3 are illustrated in Fig. 4. In Fig. 4(i) the call signal burst 28' is shown preceding a reply signal burst 30'. The transponder receiver will produce an output signal as shown in Fig.

4(ii) in response to the receipt by the receiver of the call signal 28'. This receiver output signal is denoted by reference numeral 58 and is the signal which is supplied to the rectifying diode 46 when switch 38 is in the position shown in Fig. 3.

Fig. 4(iii) illustrates the transmitter signal which is generated by the reply signal 30' and the signal 60 shown in Fig. 4(iii) corresponds to the signal which will be supplied to the rectifying diode 46 if switch 38 is in the opposite position to that shown in Fig. 3.

In a preferred embodiment of the invention the circuit comprising the push-button switch 54, diode 46, capacitor 52, resistor 50 and lamp 48 is located within a test probe which is adapted to be connected to either the terminal 40 or the terminal 42 which are conveniently brought out as test points at a convenient location on the out-station equipment. An earth connection to the out-station equipment is also required and to this end a third test point is conveniently provided at 62.

Alternatively the test circuit of Fig. 3 can be built into the transponder out-station equipment and the push-button provided at a convenient point in the equipment normally adjacent the indicator lamp 48 so that a quick test can be made by an engineer to ensure that the transponder equipment is functioning correctly and that there is apparently no line fault at least between the central station and that out-station.

An alternative test facility is shown in Fig.

5. This circuit is also adapted to be connected to an out-station such as 20 in Fig. 1 and to this end the transponder unit 36 of the outstation 20 is shown connected to the communication branch 14. As in Fig. 3, signals are derived from the transponder receiver and transponder transmitter sections and are brought out to terminals 40 and 42 and an earth connection from the transponder circuit is brought out to terminal 62.

A two-input NOR gate logic circuit 64 is connected to the two terminals 40 and 42 to provide an output of a logic inverting amplifier 66 the output of which provide the driving current for an indicator lamp 68 via a current limiting resistor 70 if required.

The indicator lamp 68 constitutes a system monitor indicator and the functioning of this part of the circuit can best be understood with reference to Fig. 6. Here in Fig. 6(i) the call signal 28' and appropriate reply signal from the transponder 36, signal burst 30', are shown. The signal from the receiver is shown at 72 in Fig. 6(ii) and the signal from the transmitter is shown at 74 in Fig. 6(iii). The two pulses 72 and 74 shown in Fig. 6(iv) constitute the output of inverter 66 and the system monitor lamp 68 will therefore flash repetitively whilst the system is operating correctly. In the event that the lamp 68 ceases to flash it will be necessary to know whether or not the system has developed a fault or simply whether the circuit associated with lamp 68 has failed.To this end a checking circuit is provided comprising a signal path 76 from terminal 42 to a rectifying diode 78 for providing a direct current signal to an inverting amplifier 80. A substantially continuous D.C.

signal is obtained by using a large capacitor 82 and high value resistor 84.

The effect of the inverting amplifier 80 is to convert the D.C. level to a zero and all the time that the D.C level is maintained by the circuit 78, 80, 82, 84 so a lamp 86 connected in the output of the inverting amplifier 80 if necessary through a current limiting resistor 88, will cease to light. However in the event that the D.C. level in circuit 78, 80, 82, 84 fails the effect of the inverting amplifier 80 is to produce an output signal which immediately causes the lamp 86 to be illuminated thereby indicating a system fault. Assuming that the circuit 78, 80, 82, 84 does not fail the fault must lie in a failure of the output signals from the transmitter and operation of the lamp 86 will therefore indicate a failure somewhere in the system.

As with the Fig. 3 embodiment, the embodiment of Fig. 5 can be located at a test probe adapted to be connected to out-station equipment by an engineer but more conveniently the system of Fig. 5 is connected permanently to the out-station equipment so that a check can readily be made at any time to determine whether the equipment is functioning correctly.

In both systems, the rectifying circuit and possibly also the logic function NOR gate 64 of the system of Fig. 5 will represent a drain on the signals in the transponder and although not shown appropriate buffer amplifiers will be provided to reduce interference in the operation of the transponder circuits by the checking facility circuits.

Likewise where the output from a rectifying circuit such as 46, 52 in Fig. 3 or the output from inverting amplifier 66 or 80 in Fig. 5 is insufficient to operate an indicator lamp an appropriate buffer amplifier (not shown) may be located between the output of the rectifying circuit or inverter stage as appropriate to provide the appropriate output current for operating the lamp.

Claims

1. In a data signalling system of the type described circuit means is provided at an outstation for converting high frequency bursts of pulses received by the transponder in the outstation (or transmitted by the transponder in the out-station) into an electrical current and an indicator lamp is provided operable by the current derived from the circuit means to indicate the presence of the high frequency signal to which the circuit means responds.

2. A data signalling system as claimed in claim 1 in which the circuit means is located in a test probe adapted to be connected into the out-station so as to pick up the appropriate signals and indicate whether the outstation is receiving signals from the central station and whether the transponder is responding appropriately thereto.

3. A data signalling system as claimed in claim 1 in which the circuit is mounted permanently in the out-station and switch means is provided for supplying either the output signals from the receiver or the signals from the transponder output to the said circuit means.

4. A data signalling system as claimed in any of claims 1 to 3 in which the circuit means comprises a rectifying circuit comprising a single diode and an appropriate electrical lamp which responds to direct current pulses from the rectifying circuit.

5. A data signalling system as claimed in claim 4 in which a large capacity is connected across the output of the rectifying circuit so as to produce a continuous indication.

6. A data signalling system as claimed in claim 5 in which a resistive load is connected in series with the lamp so as to allow a charge to develope across the capacitor.

7. In a data signalling system of the type described there is provided first circuit means, including the logic circuit means for generating a signal pulse corresponding to the period between the end of a received call signal and the beginning of a transmitted reply signal from the transponder of an out-station, indicator means being provided responsive to the signal or signals so as to indicate the correct functioning of the out-station, second circuit means being provided for generating an electrical signal from the transmitted output signals from the transponder, a second indicator lamp is provided which is illuminated in the event that the transponder output signals cease or become of insufficient amplitude and/or duration and the signal generated by the said second circuit means is used to inhibit an output signal to the said second indicator lamp so that the latter is prevented from lighting up all the time that the transponder is functioning correctly and producing output signals for transmission back to the central station.

8. Data signalling system as claimed in claim 7 in which both the first and second circuit means are permanently connected to the out-station transponder so as to provide a ready system check on the operation of the transponder.

9. A data signalling system as claimed in claim 7 in which the first and second circuit means are contained in a test probe adapted to be connected to appropriate junctions within the circuits of the out-station transponder.

10. A data signalling system as claimed in claim 7, 8 or 9 in which the output from the transponder receiver and the output from the transponder transmitter are supplied as inputs to a logic circuit having a NOR function and the output of this circuit is inverted and supplied as a control signal via a current limiting resistor, if required, to the system monitor indicator lamp.

11. A data signalling system as claimed in any of claims 7 to 10 in which the second circuit means comprises a rectifying circuit for producing from the transmitter output pulses a D.C. signal which is supplied as an input to an inverting amplifier the output of which is connected to a fault indicating lamp.

1 2. A data signalling system as claimed in claim 11 in which the rectifying circuit has a time constant such that a D.C. voltage is maintained at the output of the rectifying circuit from the intermittently received pulses from the transmitter output so that the inverter amplifier is continually forced to produce a zero output except in the event of a total failure of the transmitted output pulses from the transponder.

1 3. A data signalling system as claimed in claim 1 2 in which latching circuit means is provided for latching the indicator lamp into an ON condition, in the event that the inverter amplifier output rises only intermittently for short periods as a result of insufficient amplitude or duration of the transponder transmitter output pulses.

14. A data signalling system as claimed in claim 1 3 further comprising means for overriding the latching circuit.

1 5. A data signalling system constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompany drawings.