in Claim 1 below. Advantageous develop- course S, (broken line). In the
first case, the ments of the invention form the subject of symbol corresponds a signal activity main other claims dependent therefrom. tained solely for the last third of T. In the The invention will now be described, purely 120 second case, the signal is held in a state of by way of non-limiting example, with reference deactivation for the first third of the period to the appended drawings, in which: and is reactivated in the other two thirds of Figures 1 and 2 illustrate schematically the the period T. The example illustrated thus cor waveforms of signals used for transmitting responds to a pulse-width modulation (PWM) signals in the system of the invention, 125 effected by the variation of the so-called duty Figure 3 illustrates, in block schematic form, cycle of the signal. Each symbol S, S12 thus the structure of one of the transmitters used has a character of a binary logic signal and in the system of the invention, carries an associated bit of information. The Figures 4 and 5 illustrate, also in block symbol SO or, more precisely, the descending schematic form, the structure of the receiver 130 front thereof is used as the synchronisation 2 GB2188459A 2 signal between the transmitter and the retime, there is also a need to avoid the re ceiver. ceiver 20 being disturbed by messages sent The radio frequency signal which carries the by transmitters of other systems. For this pur- string of symbols S1 S12 is transmitted by pose, it suffices to consider the risks of inter- the oscillator 11. The latter is constituted 70ference which could arise in motor vehicles essentially by a matched oscillator, for provided with the system according to the in example, at a frequency of the order of 430vention parked alongside each other, for 440 MHz, which is supplied with a voltage + example, in a garage or depot. According to V generated by a battery or like source 13 the present invention these requirements are when an enabling line 1 '1 a is connected to the 75 satisfied by the subdivision of the symbols earth M of the transmitter 10. The oscillator S1 S12 of the coded signal into at least two 11 is controlled in an---allor nothing- modula- groups.
tion scheme by a string of binary characters The first group, comprising, for example, (modulated according to the scheme substan- two or four symbols, may be coded so as to tially like that of the symbols S1 S12) gener- 80 carry an indication of the transmitter 10 which ated by a coding circuit 14 the structure of has output the coded signals. In the case of a which will be explained in greater detail be- motor vehicle, the first two symbols S, and S, low. may be coded, for example, as---00--- ---01-, The radio frequency stage 22 of the re- ---10---and---11---according to whether the ceiver 20 is constituted essentially by an su- 85 message is output by the transmitter associ per-reaction unit 23 of the selective type, ated with the front left-hand wheel, the front tuned about a frequency corresponding to the right-hand wheel, the rear left-hand wheel and output frequency of the transmitters 11. Pre- the rear right-hand wheel, respectively. In the ferably, a voltage control oscillator (VCO) 24 case of vehicles having a larger number of enables the frequency of maximum sensitivity 90 wheels (for example, commercial vehicles with of the unit 23 to be spread periodically about 16 wheels), the first group of decoding sig a central frequency F0. Thus, it is possible to nals could include four symbols which can ensure the reliable reception of the messages carry associated binary values corresponding generated by the transmitters associated with to the decimal numbers from 1 to 16. The the various sensors even in the presence of 95 same criterion may naturally be extended to small variations in frequency. In cascade with an even greater number of wheel transmitters.
the radio frequency unit 23 are an amplifier The second group of symbols, comprising 25a and a squaring circuit 25b supplied, like the remaining symbols of the sequence, may the unit 23, by a stabilizer 22'. The squaring be used to convey the information for identify- circuit thus outputs a string of binary charac- 100 ing the receiver 20, and hence the central ters reproducing the course of the symbols unit, for which the message is intended.
received. This string of binary characters is The number of symbols of the first group intended for transfer to a main decoding cir- (identifing the transmitter which outputs the cuit 26 the structure of which will be exmessage) and the number of symbols of the plained more fully below. 105 second group (identifing the receiver for which From what has been seen, the system ac- the message is intended) may be freely se- cording to the invention includes a receiver 20 lected in dependence on the requirements of and a plurality of transmitters 10 each of use. In particular, the overall number of sym which can transmit a coded radio frequency bols in the code sequence remaining the signal corresponding to the message output 110 same, an increase in the number of symbols by a corresponding sensor P to the receiver in the first group (transmitter identification) 20. To this end, it should be mentioned that causes a corresponding reduction in the num the present description relates, by way of ber of corresponding symbols in the group example, to a configuration of use in which a (receiver identification) and vice versa. This single sensor P is connected to each transmit- 115 fact does not conflict with the applicational ter 10. Naturally, however, it is possible to requirement described above. In the case of connect several sensors P to each transmitter vehicles with a smaller number of wheels (for 10: for example, in the case of a commercial example, motor cars) for which two symbols vehicle provided with paired wheels, it is pos- are sufficient for the identification of the sen sible to connect the sensors associated with 120 sor, a relatively greater number of symbols the tyres of the two paired wheels to a single must be provided in the group which identifies transmitter. The transmission is effected with the receiver. This is so as to reduce the pro the use of structurally similar coded signals bability of interference arising between the output at a frequency which is the operating messages transmitted in two motor vehicles frequency common to the oscillators 11 of all 125 whose receivers are identified by the same the transmitters 10 and of the receiving unit group of symbols. On the other hand, in the 23 of the receiver 20. There is thus a require- case of commercial vehicles with a high num ment for enabling the receiver 20 to recognise ber of wheels (for example, 16 wheels pro and identify which of the transmitters 10 con- vided with relative transmitters each of which nected to it has sent a message. At the same 130 is identified by a group of four symbols), a 3 GB 2 188 459A 3 lesser number of symbols of the second cates univocally a corresponding binary logic group suffices to minimize risks of interfer- value selectively predetermined to the corre ence, if account is taken of the relatively small sponding symbol of the sequence which is number of vehicles of this type in circulation. transmitted. In particular, the coding terminals If we examine the diagram of the transmitter 70 16, 1612 and the corresponding lines in greater detail (with reference to Figure 171 1712 of the code selection network as- 3), it may be seen that its core is constituted sociated therewith may be considered as be by the decoding circuit 14 which, in a cur- ing divided into at least two respective rently preferred embodiment, is constituted by groups.
a microprocessor such as, for example, the 75 The first group includes the coding terminals microprocessor MM5320ON. The coding cir- and the lines of the coding network which cuit is supplied with the voltage + V through enable the -signature- of the transmitter 10 a RC network the time constant of which to be inserted in the coding sequence. For identifies the clock frequency of the circuit 14. example, in the case of a commercial vehicle More particularly, the circuit 14 has a number 80having 16 wheels, the first four coding termi- of terminals or coding stems 161 16,, equal nals 161 16, of the transmitter 14 may re- to the number of symbols S, S, of the ceive a binary code with all the values from code sequence. The terminals 161 1612 have ---0000---to---1111---in which, for example, the an associated coding network 17 which is value -0- may correspond to the absence of constituted essentially by a plurality of electri- 85 the conductive bridge and the binary value cally conductive lines all connected to the -1- may correspond to the presence of the earth M of the device and each connectible respective conductive bridge for connection through a conductive bridge 171 1712 to a the earth M. For example, in the case in respective coding terminal 161 1612. Conse- which there are 16 transmitters 10, each of quently, each coding terminal 161 1612 of 90 these will receive a different binary code on the coder 14 is connected to the earth M of the first four code terminals 161 16, the transmitter 10 only when a corresponding All the other coded terminals (with the ex- conductive bridge 17, 1712 has been applied ception of the terminal 16, which is intended between the terminal 161 16,2 itself and the to permit an auxiliary signal, as will be more coding network 17.
95 fully explained below) receive (again in this If we ignore for the moment the function of case with the selective disposition of the re- the parts of the transmitter enclosed in the spective conductive bridges) a binary code rectangle in broken outline, generally indicated which is identical for all the transmitters 10 19, the transmission function of the coding intended to terminate at a single receiver 20.
circuit 14 may be described essentially in the 100 In other words, the second group of coding following terms. terminals and the lines of the code selection As long as the switch 1 controlled by the network 17 are used for inserting into the pressostatic sensor P remains in a floating po- code sequence transmitted the -address- of sition, the enabling line 11 a of the oscillator the receiver 20 for which the messages are 11 which is connected to the earth M keeps 105 intended.
the oscillator 11 in a de-energised condition. In the region of the auxiliary circuit ele There is thus no radio frequency signal ments, generally indicated 18, (the presence transmission to the aerial 12. Immediately, the of which is optional in the system of the in switch 1 connects the negative terminal of the vention) a threshold circuit which is sensitive battery 11 a to the earth M, as a result of the 110 to the charge voltage of the battery 13.
movement of the pressostatic sensor P, the This circuit, which contains a fixed reference enabling line 1 la (which is connected to the supply such as, for example, a Zener diode, is earth M) brings the oscillator 11 into the oper- intended to control an electronic switch con ating condition. The oscillator 11 then outputs stituted by a transmitter 19a the base of a radio frequency signal modulated in depen- 115 which is connected to the output of the circuit dence on the sequence of binary values out- 19 itself and the emitter of which is con put by the coder 14, which is also activated nected to the negative terminal of the battery at the same time as a result of the connection 13. More particularly, the circuit 19 is ar of the coding network 17 to the negative ter- ranged to output a low logic level (keeping the minal of the battery 13. More particularly, 120 transistor 19a cut off) as long as the tension each character of the string output by the co- across the terminals of the battery 13 remains der 14 will assume either one or the other of above a threshold level, for example of the two possible logic values (-0- or---1-) ac- order of 7 volts, and to generate a signal of cording to whether or not the corresponding high logic level at its output (saturating the coding terminal 161 161, is connected to the 125 transistor 19a) immediately the voltage across earth M through a corresponding conductive the terminals of the battery 13 fails below the bridge 171 17,,. The selective arrangement threshold as a result of gradual exhaustion. As of these bridges 17, 171, thus enables the long as the transistor 19a is kept in the cut application to each coding terminal 16, 16,2 off condition, the combination of circuit ele- of a corresponding state signal which indi- 130ments 18 has practically no influence on the 4 GB2188459A 4 operation of the transmitter. In practice, the the first four symbols of the sequence. In code terminal 16. which is connected to the other words, one is dealing with the four collector of the transistor 19a remains floating symbols which contain the -signature- of the and, with regard to the coding, is arranged in transmitter (and hence of the sensor P) which an entirely similar manner to the other code 70 outputs the message received. The decoding terminals 16, 1612 for which there is no pro- terminals of the first group are connected to vision for its connection to the earth M an identification circuit 29 which, according to through a respective conductive bridge a sequence timed by a clock circuit 30, scans 171 1712. A diode 19b, having its anode the decoding terminals connected thereto and connected to the earth M (that is, to the en- 75 can produce, upon the reception on an input abling line 1 '1 a and to the coding network 17) line 29a of an enabling signal generated by and its cathode connected to the collector of the decoding circuit 26, the emission of a the transistor 19, remains inversely polarised warning signal on a visual display unit 31. In in these conditions and ensures the isolation accordance with two possible embodiments, of the terminal 16, from the earth M when it 80 illustrated in full outline and in broken outline is connected to the negative terminal of the respectively in Figure 4, the visual display unit battery 13 as a result of the movement of the 31 may be constituted, for example, by a digi switch 1. On the other hand, when the output tal visual display which presents an Arabic of the circuit 19 which detects the exhaustion numeral corresponding to the binary configura- of the battery 13 causes the saturation of the 85 tion output by the identification circuit 29, or transistor 19a, the cathode of the diode 19b again by an ideographic visual display such as, is connected to the negative terminal of the for example, a drawing representing the battery 13 and is polarised directly. As a re- vehicle on which the system is mounted sche sult of the direct polarisation, the coding net- matically in plan. The ideographic visual dis work 17 and the enabling line 11 a of the 90 play is then provided with separate visual dis transmitter 15 are connected to the negative play elements, such as, for example, a LED, terminal of the battery 11. Thus, whatever the each of which identifies a co ' rresponding wheel position adopted by the switch 1, conditions of the vehicle. In other words, when an en are established which cause the transmission abling signal is received on the line 29a, the of the code sequence by the transmitter 10. 95 identification circuit 29 transmits a signal to More particularly, for each transmitter, the the visual display unit 31 which causes the code of the symbol S, corresponding to the presentation of the Arabic numeral or the terminal 16, is opposite that present in the switching on of the LED corresponding to code sequences emitted as a result of the wheel identified by the binary code input on connection of the earth M to the negative ter- 100 the decoding terminals 281 to 28, of the first minal of the battery caused by the movement group. A memory 31a enables the warning of the switch 1. In normal operating conditions signal to be presented continuously until the (the battery 13 charged) the code terminal 16, memory itself is cancelled by means of a re is in fact floating. When the transistor 19a setting head 31b.
passes into conduction, however, the same 105 The output of the enabling signal on the line terminal 16, is connected to negative terminal 29a by the decoding circuit 26 is subordi of the battery 13 and is then placed in a nated to the recognition by the decoding cir condition of connection which is practically cuit itself of the presence in the coded se identical to that of the coding terminals 16, quence received of the symbols corresponding connected to the earth M through a respective 110 to the second group of symbols (the---ad- conductive bridge 17, 17,,. dress- of the receiver) firmly memorised on The heart of the receiver 20 is constituted the decoding terminals 28, 2812 through a by the decoding circuit 26 which, according to decoding selection network 32 structurally the embodiment currently preferred, may also similar to the coding selection network 17 of be made-like the coder 14-with the micro- 115 the transmitter 10. Again in this case, the processor MM5320ON. The decoding circuit selection network comprises a plurality of 26 is connected to the supply voltage + V of lines 32, 3212 constituted by conductive the receiver through an RC network 27 the bridges which enable the selective connection time constant of which determines the clock to earth of the receiver of each of the termi frequency of the decoding circuit 26. The cir- 120 nals of the second group of the decoding se- cuit 26 is provided with decoding terminals lection terminals 28, 2812. In other words 28, 2812 each of which corresponds to one (omitting the function of the decoding terminal of the symbols of the coding sequence con- 28, for the moment, for simplicity of explana tained in the signal received from the various tion) the output of the enabling signal on the transmitters 10. As in the case of the coding 125 line 29a occurs only when the decoding circuit terminals 14, the terminals of the decoder 26 26 recognises the correspondence between are also divided into two groups. the values of the symbols of the coded se A first group (comprising the four terminals quence received and the values stored on the 281 to 284 in the embodiment illustrated) in- decoding terminals of the second group cludes the decoding terminals associated with 130 through the decoding selection network 32. In GB2188459A 5 this case, the decoder 26 recognises that the parison test carried out within it has a nega coding sequence received is effectively in- tive result. Immediately one of the transmitters tended for the receiver of which it forms a 10 generates the sequence corresponding to part and not for another receiver. The output the detection of the exhaustion of the respec of the enabling signal on the line 29a, as seen 70 tive supply, the roles of the decoders 26 and previously, causes the presentation of a warn- 33 are reversed. In this case, the main deco ing message which identifies the transmitter der 26 remains inactive and there is thus no 10, and hence the sensor P, which has output emission of the enabling signal on the line a message indicative of the fact that the pres- 29a. The emission of the enabling signal at sure of the corresponding tyre has fallen be- 75 the output of the auxiliary decoder 33, how low a lower limit or is excessive. ever, causes the activation of the identification However, as seen previously, each of the circuit 29 (the line 29b) and the corresponding transmitters 10 of the system according to transmission to the signalling unit 31 of a sig the invention is also able to transmit, with the nal which produces the presentation of the use of the symbol S, of the sequence, the 80 Arabic numeral or the switching on of the LED information relating to the eventual exhaustion corresponding to the transmitter and sensor of the corresponding supply 13. In the re- which have produced the message. In this ceiver according to the embodiment illustrated case, however, the signal output by the aux in Figure 4, the recognition and consequent iliary decoder 33 also causes the activation of interpretation of this additional signalling infor- 85 the intermittence circuit 35. The signal pre mation is carried out with the use, in addition sented on the unit 31 in this case is intermit to the main decoder 26, of an auxiliary decotent. The driver of the vehicle may then real der 33 which is structurally similar, both with ise immediately that the signal transmitted regard to the supply through a RC network 34 corresponds not to the detection by one of which controls its clock frequency, and with 90 the sensors P of a pressure level which is too regard to the connection of a first group of low or too high but to the exhaustion of the decoding terminals 33, 33, to the identifica- depletion of the supply (battery) 13 which tion circuit 29 and the connection of a second supplies the corresponding transmitter 10.
group of decoding terminals 33. 33,2 to the Thus, the driver is warned of the need to decoding selection network 32. The difference 95 replace or recharge this battery.
between the main decoder 26 and the aux- Figures 6 and 7 illustrate two receivers 20' iliary decoder 33 relates to the different con- and 2W corresponding to two simplified em nection of the two decoding terminals 28. and bodiments of the receiver illustrated and de 33, corresponding to the fifth symbol S, of scribed in detail with reference to Figure 4.
the code sequence, that is, to the symbol 100 The receiver 20' of Figure 6 is a receiver in which conveys the information relating to the tended to be mounted in a motor car in which eventual exhaustion of the supply 13 of the each sensor P carries an associated transmit respective transmitter 10. In the case of the ter, such as the transmitter 10 of Figure 3, main decoder 26, the decoding terminal 28, is without the circuit elements 18 intended to in fact connected to a---high-logic level, for 105 enable the detection of the exhaustion of the example, the supply voltage + V. In the aux- battery 13. In this case, in the code sequence iliary decoder 33, the corresponding terminal used for communication between the transmit 33, is instead kept at a---low-logic level, for ters 10 and the receiver 20', the first group example, by means of a connection to the of symbols which identify the transmitter (and earth of the receiver. The output of the aux- 110 the wheel) is constituted by only two sym iliary decoder 33 is connected to a further bols, while the second group of symbols enabling input 29b of the identification circuit which identify the receiver comprises the re 29,as well as to an intermittence circuit 35 maining ten symbols of the sequence. With which controls the operation of the switch 36 this number of symbols in the second group interposed between the identification circuit 29 115 of the sequence, it is possible considerably to and the signalling unit 31. If it is assumed that reduce and virtually to nullify the probability of when normal transmission occurs (that is, interference occurring between the transmis when none of the signalling circuits for the sion systems mounted in two different drop in the supply 18 of one of the transmit- vehicles which are temporarily close to each ters 10 intervenes by signalling the exhaustion 120 other. In this case, the identification circuits of the corresponding supply) the symbol S, of 29 to which only the decoding terminals 281 the coding sequence is transmitted with a and 282 are connected may be enabled to value corresponding to the value set on the operate, causing the presentation of a corre coding terminal 28, of the main decoder 26, sponding warning signal on the unit 31, only one may understand that the reception of a 125 as a result of the output of the enabling signal corresponding signal by the receiver 20 by the decoder 26. In the example illustrated, causes only the output of the enabling signal the visual display unit has an associated 29a according to the criteria described above. sound signalling device such as a bell, a The auxiliary decoder 33 does not in fact probuzzer or a voice synthesising circuit 37, as duce any signal at its output since the corn- 130well as the memory 31a which enables the 6 GB2188459A 6 permanent display for the desired time and a receiver associated with the central unit, the eventual cancellation by means of the re- which can detect the coded signals generated setting key 31b. by the transmitters associated with the sen In the further variant illustrated in Figure 7, sors and has stored within it a respective pre the receiver 20", although including a single 70 determined second group of symbols indica decoding circuit 26, is also able to recongise tive of the central unit, the receiver including and interpret the information conveyed by the decoding means which can select from among symbol S, that is, the information relating to the coded signals including the first and the the exhaustion of the supply 13 of one of the second groups of symbols all and only the transmitters. In this case, the corresponding 75 coded signals comprising a second group of decoding terminal 28. of the decoder 26 is symbols identical to the respective predeter connected to the selection circuit 29 to which mined second group of symbols, and which there are also connected the decoding termi- can identify from among the signals selected nals 28, and 282 corresponding to the sym- and in dependence on the first group of sym bols of the sequence identifying the transmit- 80 bols contained in each signal selected the ter which has output the message. According transmitter which has generated the signal it to a periodic sequence subservient to the self.
rhythm of the clock circuit 30, the selection 2. A system according to Claim 1, wherein circuit 29 maintains the terminal 28. at a high least one transmitter associated with one of logic level during a first interval and at a low 85 the sensors includes means for detecting the logic level during a second interval of equal exhaustion of the supply for the transmitter duration. In other words, in the circuit of Fig- itself and is able to generate a coded signal ure 7, the identification circuit 29 is able to including at least one auxiliary symbol indica make the decoding circuit 26 function as a tive of the exhaustion of the supply.
main decoder during the first interval and as 90 3. A system according to Claim 1 or Claim an auxiliary decoder (with a function similar to 2, wherein the coded signals are transmitted that carried out by the decoder 33 of the in the form of radio frequency signals.
receiver 20 of Figure 4) during the second 4. A system according to Claim 1, Claim 2 interval. Consequently, the emission of the en- or Claim 3, wherein the coded signals include abling signal 29a during the first interval corre- 95 symbols of binary- type.
sponds to the emission of the enabling signal 5. A system according to Claim 3 and Claim by the main decoder 26 of Figure 4, with the 4, wherein the radio frequency signals have a consequent identification on the visual display binary type amplitude modulation.
unit 31 of the sensor P which has detected an 6. A system according to Claim 4 or Claim insufficient or excessive pressure. On the 100 5, wherein the binary type symbols are consti other hand, the emission of the enabling signal tuted by substantially identical waveforms hav 29a during the second interval will correspond ing duty cycles differentiated by different logic to the reception of an alarm message indica- values.
tive of the fact that the supply battery 13 of 7. A system according to Claim 3, wherein one of the transmitters is exhausted. Again in 105 the receiver includes means for varying the this case, as in the case of the transmitter 20 reception frequency which produce periodic of Figure 4, the presentation of the message spreading of the reception frequency itself.
on the unit 31 could be effected in an inter- 8. A system according to Claim 2, wherein mittent manner through an intermittence circuit the receiver can recognise, in the range of all 28 controlled by the identification circuit 29. 110 and only the coded signals including asecond Naturally, the principle of the invention re- group of symbols identical to the respective maining the same, the constructional details predetermined second group of signals, the and forms of embodiment may be varied auxiliary symbol indicative of the exhaustion of widely with respect to that described and il- the supply for supplying the transmitter and lustrated, without thereby departing from the 115 can output a corresponding alarm signal.
scope of the present invention. 9. A system according to Claim 1 or Claim 2 and Claim 4, wherein each transmitter in CLAIMS cludes:
1. A system for transmitting messages from a supply source which generates a supply a plurality of sensors to a central unit, com- 120 voltage prising: a coding circuit having a plurality of coding at least one transmitter associated with at terminals each of which corresponds to one of least one respective sensor and able to gener- the symbols of the sequence, ate from the message output by the respec- a transmission circuit piloted by the respec tive sensor a coded signal constituted by a 125 tive sensor and able to cause the transmission sequence of symbols divided into at least a of the coded signal as a result of the connec first group of symbols identifying the transmit- tion of at least one of the coding terminals to ter which generates a signal and a second the supply, and group of symbols, which identifying a central a coding selection network associated with unit for which the message is intended, and 130 the coding terminals and including a separate 7 GB2188459A 7 line for each of the coding terminals in an presentation of each warning signal.
arrangement such that the symbol of the se- 14. A system according to Claim 2 and any quence corresponding to each coding terminal one of Claims 11 to 13, wherein:
assumes at least two separate values in de- two decoding circuits are provided, each pendence on a state signal applied to the re- 70 having a plurality of decoding terminals divided spective separate line of the coding selection into a first group of decoding terminals con network. nected to the identification circuit and a sec 10. A system according to Claim 2 and ond group of decoding terminals having the Claim 9, wherein the coding circuit includes an associated decoding selection network, auxiliary coding terminal corresponding to the 75 each decoding circuit includes an auxiliary auxiliary symbol indicative of the exhaustion of decoding terminal corresponding to the aux the supply, and in that the means for detect- iliary symbol indicative of the exhaustion of ing the exhaustion of the supply comprise: the supply of the transmitter, a threshold circuit sensitive to the supply means are provided for applying respective voltage and able to generate a signal of the 80 different state signals to the auxiliary decoding exhaustion of the supply when the supply vol- terminals of the two decoding circuits, tage falls below a predetermined threshold, the identification circuit can be controlled by and the enabling signal generated by one of the an electronic switch subservient to the two decoding circuits, and threshold circuit and able, upon the output of 85 an alarm signal generating circuit is provided the supply exhaustion signal, to connect the for signalling the exhaustion of the supply, auxiliary coding terminal to the supply and to which can be controlled by the enabling signal activate the transmission circuit, causing the generated by the other of the two decoding transmission of the coded signal. circuits.
11. A system according to Claim 9, wherein 90 15. A system according to Claim 2 and any the receiver comprises: one of Claims 11 to 13, wherein:
a decoding circuit having a plurality of de- the decoding circuit includes an auxiliary de coding terminals each of which corresponds to coding circuit corresponding to the auxiliary one of the symbols of the sequence, whereby symbol indicative of the exhaustion of the the decoding terminals are divided into a first 95 supply, group and a second group of decoding termi- means are provided for applying to the aux nals corresponding respectively to the first iliary decoding terminal of the decoding circuit group and to the second group of symbols of different state signals in a first and a second the sequence, the decoding terminals of the interval, alternating with each other cyclically, second group having an associated decoding 100 the identification circuit is controllable by the selection network including, for each of the enabling signal generated by the decoding cir decodingi terminals of the second group a se- cuit during the first interval, and parate line which can receive a stored state an alarm signal generating circuit is provided signal which identifies a value of the respec- for signalling the exhaustion of the supply, tive symbol of the sequence, and a sequence 105 which can be supplied by the enabling signal recognition network which can generate an generated by the decoding circuit during the enabling signal only when the symbols of the second interval.
second group of the sequence received have 16. A system according to Claim 14 or respective values corresponding to the value Claim 15, wherein the alarm signal generating supplied to the decoding terminals of the seccircuit for signalling the exhaustion of the sup ond group through the respective separate ply has an associated auxiliary signalling unit lines of the decoding selection network, and for presenting the alarm signal of the exhaus an identification circuit which can be acti- tion of the supply.
vated by the enabling signal, and is connected 17. A system for transmitting messages to the decoding terminals of the first group in 115 from a plurality of sensors to a central unit, order to identify, in dependence on the values substantially as herein described with refer of the symbols of the first group of the se- ence to, and as shown in, the accompanying quence, the transmitter which has transmitted drawings.
the signal coded according to the sequence Printed for Her Maiesty's Stationery Office itself and to produce a corresponding warning by Burgess & Son (Abingdon) Ltd, Dd 8991685, 1987.
signal for each transmitter. Published at The Patent Office, 25 Southampton Buildings, 12. A system according to Claim 11, London, WC2A 1 AY, from which copies may be obtained.
wherein the receiver is associated with a signalling unit for the presentation of the warning signal, comprising at least partly separate warning members for the presentation of each warning signal.
13. A system according to Claim 12, including a visual display unit of ideographic type, with separate visual display elements for the