EP0356334B1 - System for exchanging information between a portable object like a key and an exchange device - Google Patents

Abstract

The exchange device (not shown) is installed in a lock and emits a modulated carrier wave. The key (2) possesses circuits (27, 28, 29, 23) for demodulating the modulated carrier wave received and for reconstructing a first signal (PR) representing the information emitted by the exchange device, and processing circuits (23, 25, 30) receiving the first signal (PR) and generating a second signal (D) representing the information to be transmitted. The second signal causes an impedance (26) to vary abruptly, so that the charge detected by the exchange device varies and so that the information is thereby transmitted to it. Moreover, the impedance (26) is designed as a limiter. A clock signal (41) is obtained from the modulated carrier wave by clipping, and the supply voltage (AC) is obtained by rectification and regulation. The system makes it possible particularly to use a contactless connection between the key and the lock, for example by means of a single pair of coils. <IMAGE>

Description

The present invention firstly relates to an information exchange system between at least one portable object and at least one sedentary exchange device.

Such a system applies in particular in the field of locksmithing. In this case, the portable object is a key provided with electronic circuits capable of receiving, storing and transmitting information in the form of electrical signals, and the exchange device is arranged on a lock. The exchange device is arranged to read, when the key is inserted into the lock, a secret identification number memorized in this key, and to prohibit unlocking of the lock if this identification number is not correct. Such a system can be installed on a lock and a key of the conventional type, that is to say already comprising a purely mechanical device for identifying the key, in order to reinforce the security provided by this device. On the contrary, such a system can also be installed on a lock and a key comprising no mechanical means of identification, if the security thus obtained is considered to be sufficient.

Systems of the type defined above are already known, described for example in German applications DE-A-35 07 871 and DE-A- 28 02 472, or in European application EP-A-0 223 715. In all these systems, the exchange of information between the key and the exchange device takes place via a plurality of conductors. This necessitates the use, on the lock, of a connector provided with a plurality of contacts which must be positioned precisely with respect to the contacts of the key, which is not without causing some problems, in particular due to the small size of the key. Furthermore, this makes it impossible, in practice, to produce a system in which the key and the exchange device are coupled, without electrical contacts, by replacing each of the conductors with a pair of two antennas, in the since it is very difficult to easily accommodate, on a key, a plurality of antennas, arranging so that, moreover, none of them pick up, in whole or in part, a signal intended for the others. However, a system without electrical contacts has, in certain cases, advantages, such as insensitivity to soiling, or the possibility of mechanically protecting the key by overmolding in plastic material.

Also known, from French patent FR 2 180 349, is a system in which the exchange of information between the key and the lock takes place by means of only two conductors, one for fixing a common reference potential, the other to transmit potential variations in one direction or the other. However, this result is obtained at the cost of a relative complexity of the signals and processing circuits, complexity which results in the fact that if one seeks to remove these conductors, it turns out to be necessary to provide two pairs of antennas , one for the passage of signals from the exchange device to the portable object, and the other for the passage of signals from the portable object to the exchange device.

• des premiers moyens de traitement engendrant un premier signal représentatif d'une première suite d'éléments binaires, ou bits, à émettre vers ledit objet portatif,
• des moyens pour, en réponse audit premier signal, moduler une porteuse,
• des moyens pour émettre la porteuse modulée, et
• des premiers moyens détecteurs, pour détecter des variations brusques d'amplitude du courant dans lesdits moyens d'émission et délivrer un signal détecté auxdits premiers moyens de traitement,

et ledit objet portatif comprend :

• des moyens pour, en réponse à la porteuse modulée reçue, restituer ledit premier signal,
• des deuxièmes moyens de traitement recevant ledit premier signal restitué et engendrant un deuxième signal représentatif d'une deuxième suite de bits à émettre vers ledit dispositif d'échange.

Document GB-A-2 167 586 also discloses a system of the type defined above, in which said exchange device comprises:

• first processing means generating a first signal representative of a first series of binary elements, or bits, to be transmitted to said portable object,
• means for, in response to said first signal, modulating a carrier,
• means for transmitting the modulated carrier, and
• first detector means for detecting sudden variations in the amplitude of the current in said transmission means and deliver a detected signal to said first processing means,

and said portable object comprises:

• means for, in response to the modulated carrier received, restore said first signal,
• second processing means receiving said first restored signal and generating a second signal representative of a second series of bits to be transmitted to said exchange device.

Such a system conforms to the preamble of claim 1.

It would be possible, in a device of this type, to ensure the transmission of information using a single pair of antennas, but at the cost of a long duration of transmission. The transmission of information from the key to the lock cannot in fact be carried out simultaneously with the transmission from the lock to the key.

The present invention aims to overcome this drawback.

To this end, the subject of the invention is a system as above, characterized in that it comprises means for, in response to said second signal, suddenly varying the charge which represents, for said means d transmission, said portable object, so that, during the duration of transmission of a bit of the first sequence, a bit of the second sequence is transmitted and detected by said first detector means.

In the system of the invention, the information originating from the exchange device and intended for the portable object is transmitted, in known manner, by modulation of a carrier. To transmit this modulated carrier, one can indifferently use a set of two conductors, or a couple of antennas. However, due to the fact that the information originating from the portable object and intended for the exchange device is transmitted by variations in the load seen by the latter, they result in variations in amplitude of the modulated carrier, that the connection between the exchange device and the portable object is made using two conductors, or using a couple of antennas. In either case, these variations are detected by the exchange device. Thus, the problem of the connection between the key and the lock can be solved in a simple way, either using two contactors, or using a pair of antennas, which considerably simplifies the positioning problems. relative contacts or antennas, for example. In addition, such a mode of transmission allows simultaneity between the transmission of the bits to the key and the transmission to the lock, which allows a reduction in the overall duration of the exchanges.

In the preferred embodiment, said modulation means are arranged to modulate the amplitude of said carrier, and said restitution means comprise second detector means for detecting sudden variations in amplitude of the modulated carrier received and said variation means abrupt are arranged to limit the amplitude of the modulated carrier received.

Then, the system is practically insensitive to the inevitable fluctuations in the level of the modulated carrier received, linked for example to the quality or cleanliness of the contacts in the case of a contact connection, or to the geometry and relative positioning of the antennas. in the case of a contactless connection. The system is therefore reliable and safe to use.

In addition, said portable object comprises on the one hand means for clipping said modulated carrier, said second processing means being synchronized by said written signal, and on the other hand means for rectifying the received modulated carrier, as well as regulation means for, in response to the rectified carrier, delivering to all the electronic components of the portable object, a supply voltage of electrical energy.

Then, it is not necessary to provide, on the portable object, an oscillator delivering a clock signal synchronous with the clock signal of the exchange device. Likewise, it is not necessary to provide a battery for the supply of electrical energy to the portable object.

Advantageously, said transmission means comprise a first coil and said restitution means a second coil and the coupling between said portable object and said exchange device is a magnetic coupling, obtained by contactless approximation of said first and second coils.

• la figure 1 représente un schéma par blocs des circuits électroniques d'un dispositif d'échange disposé dans une serrure,
• la figure 2 représente un schéma par blocs des circuits électroniques d'une clé destinée à échanger des informations avec le dispositif d'échange de la figure 1,
• la figure 3 représente, de façon détaillée, le circuit de variation de charge et de limitation, prévu sur la clé de la figure 2,
• la figure 4 représente un diagramme temporel des principaux signaux en divers points des circuits électroniques des figures 1 et 2, et,
• les figures 5 représentent des exemples d'implantation sur une clé plate et sur une clé cylindrique des circuits électroniques de la figure 2.

The present invention will be better understood from the following description of the preferred embodiment of the system of the invention, made with reference to the accompanying drawings, in which:

• FIG. 1 represents a block diagram of the electronic circuits of an exchange device arranged in a lock,
• FIG. 2 represents a block diagram of the electronic circuits of a key intended for exchanging information with the exchange device of FIG. 1,
• FIG. 3 shows, in detail, the load variation and limitation circuit, provided on the key of FIG. 2,
• FIG. 4 represents a time diagram of the main signals at various points of the electronic circuits of FIGS. 1 and 2, and,
• FIGS. 5 represent examples of installation on a flat key and on a cylindrical key of the electronic circuits of FIG. 2.

An information exchange system between a portable object, in this case a key, and an exchange device installed in a lock, and therefore sedentary, which allows, for example, prior to authorization to unlock the lock, verification of an identification number stored in the key, is now described.

With reference to FIG. 1, the exchange device 1 firstly comprises a circuit 11 for supplying electrical energy, which generates the supply voltage or voltages AS necessary for the various components of the exchange device.

An oscillator 12 delivers a carrier H1, a frequency here equal to 200 kHz.

A microprocessor processing circuit 13 is provided with a synchronization input receiving the carrier H1, an output delivering a binary signal CP for controlling the carrier, an output delivering a binary signal P representative of a first series of binary elements, or bits, to send to the key, and of an input receiving a binary signal RS.

An AND gate 14 is provided with two inputs receiving the signal CP and the carrier H1 respectively, and with an output connected to the carrier input of a modulator circuit 15, here amplitude modulator.

The modulator circuit 15 is also provided with a modulation input receiving the signal P, and with an output connected to the input of a current amplifier 16, charged by a coil 17 in series with a circuit 18 for detecting sudden variations in the current flowing through it.

The circuit 18 is arranged to detect the peak value of the amplitude of the current flowing through it and permanently compare it with the instantaneous value of this amplitude. Here it delivers an RS output signal at level 1 when the instantaneous value becomes greater than the detected peak value, which translates into a sudden increase in the current which the peak detector does not immediately take into account. On the contrary, the RS output signal goes to level 0 when the current suddenly drops. It should be noted that such a device detects an abrupt variation independently of the starting point of this variation, which is useful in the application described, as will be better understood below.

Referring now to FIG. 2, the portable object, that is to say here the key 2, is now described.

The key firstly comprises a coil 27, intended to be coupled to the coil 17, when the key 2 is inserted into the lock, the coil 17 constituting the only access which connects all of the electronic circuits which will now be described with the exterior.

A capacitor 31 is mounted in parallel on the coil 27.

A full-wave rectification circuit 28 is provided with an input access, receiving the voltage across the terminals of the coil 27, and an output access, one terminal of which is grounded, and the other delivers a signal. straightened RC.

A capacitor 32 is arranged in parallel on the output of the rectifying circuit 28.

A voltage regulator circuit 21 is provided with an input receiving the rectified signal RC and with an output delivering an AC supply voltage, worth here substantially + 5 V. This voltage allows the supply of electrical energy to all the components of the key that need it.

A clipper circuit 22, supplied by the AC voltage, is provided with an input one terminal of which is connected to ground, and the other of which is connected to one end of the coil 27. The clipper circuit 22 delivers a clipped signal binary ECR.

A reset circuit 24 is provided with an input receiving the AC supply voltage, and an output delivering a binary signal Z.

A circuit 29 for detecting sudden variations, supplied by the voltage AC, is provided with an input receiving the rectified signal RC and an output delivering a binary signal PR. Circuit 29 detects sudden variations in the RC signal. It operates on the same principle as the detector circuit 18 but it also includes an inhibition input receiving a binary signal IN. When the signal IN is at level 0, the circuit 29 goes to level 0 for an abrupt increase in the signal RC, and to level 1 for an abrupt decrease in this signal. When the signal IN is at level 1, the circuit 29 is inhibited and does not detect anything.

A circuit 26 for varying the charge and limiting the voltage across its terminals, described in more detail below, is arranged in parallel on the capacitor 32. It is provided with a control input receiving a binary signal D.

A sequencer circuit 23, supplied by the voltage AC, is provided with three inputs receiving the binary signals ECR, Z and PR respectively, and four outputs delivering the binary signal IN, the binary signal PR and two signals AU and H2, respectively.

A processing circuit 25, supplied by the voltage AC, receives the binary signals PR, AU and H2, and delivers the binary signal DE.

A door AND 30 is provided with two inputs receiving the binary signals IN and DE, and with an output delivering the binary signal D.

With reference to FIG. 3, the circuit 26 for varying the load and limiting the voltage across its terminals here comprises a bipolar transistor of NPN 261 type, the emitter of which is connected to ground. A Zener diode 262 is disposed between the collector of transistor 261 and the conductor which delivers the rectified signal RC. The cathode of the Zener diode 262 is connected to this conductor. The Zener voltage of diode 262 here is 6.2 volts.

A resistor 263 is disposed between the cathode of the diode 262 and the base of the transistor 261. A resistor 264 is disposed between the base of the transistor 261 and the collector and the base combined, of a NPN type bipolar transistor 265. The emitter of transistor 265 is connected to the collector of a NPN type bipolar transistor 266, the base of which receives the signal D and the emitter of which is connected to ground.

The system which has just been described operates as follows, with reference now to FIG. 4.

This figure shows, by way of example, four successive time intervals, or clock periods, each time interval being able to be considered as the duration of transmission of a bit from the exchange device 1 of the lock to the key 2, and possibly, and as will be better understood below, of a bit from the key 2 to the device exchange 1.

The value of the bits of the sequence to be transmitted to the key 2 is represented in the upper part of FIG. 4, which shows here that the first bit to be transmitted is worth 0, the second 1, the third 0, and the fourth 0, by example.

En réponse à un signal de détection de l'introduction de la clé 2 dans la serrure, obtenu grâce à un dispositif non représenté car connu, le circuit de traitement 13 commande le passage au niveau haut du signal CP de commande de porteuse, et la porteuse H1 se trouve effectivement appliquée à l'entrée de porteuse du circuit modulateur 15.The processing circuit 13 is arranged to determine the duration of each clock period, of value T2, so that it is a multiple of the period T1 of the carrier H1. Here, the period T2 is worth:

$\text{T2 = 64 T1}$

In response to a signal for detecting the introduction of the key 2 into the lock, obtained by means of a device which is not shown as known, the processing circuit 13 controls the passage of the carrier control signal CP to the high level, and the carrier H1 is effectively applied to the carrier input of the modulator circuit 15.

Simultaneously, the processing circuit 13 generates the signal P. As it appears in FIG. 4, the signal P is a binary signal which, at the start of each clock period, is at the high level, and at the end of each period clock, low level. The duration, or width, of each of the pulses of the pulse sequence thus formed is modulated, that is to say that it can take one of the values of a couple associated with the two values 0 and 1, possible for each bit to be sent to the key 2. Thus, if a bit to be sent to the key 2 is worth 0, the width of the corresponding pulse of the signal P is worth 12 periods T1, and if this bit is worth 1, the width of the pulse is worth 52 T1 periods. Naturally, these values are only given As an example, and the width of small value, representative of the binary value 0, can be between approximately 8 and approximately 16 times the period T1, while the width of high value, representative of the binary value 1, can be between 48 and approximately 56 times the period T1.

The binary signal P, representative of the series of bits to be transmitted to the key 2, is therefore applied to the modulator circuit 15. The latter is arranged so that the amplitude of the modulated carrier is here 10 volts peak to peak approximately when the signal P is at low level, and of 8 volts peak to peak approximately when the signal P is at high level.

The modulated carrier, after current amplification in the current amplifier 16 is applied to the coil 17.

It is received by the coil 27 of the key 2, coupled to the coil 17 by magnetic coupling when the key 2 is in the lock, coupling due to the contactless approach of the two coils 17 and 27.

Upon reception, the modulated carrier is clipped in circuit 22, which generates the ECR signal, of period T1, and which, as will be better understood below, will be used as synchronization signal by the sequencer circuit 23.

The received modulated carrier is also rectified in the rectifier circuit 28, which delivers the rectified signal RC shown in FIG. 4. In this figure, for which a correct coupling between the coils 17 and 27 has been assumed, the signal RC, after a establishment phase corresponding to the introduction of the key 2 into the lock, is worth approximately 8 volts at the start of each clock period, and substantially 10 volts at the end of each period, the duration of the plateau at 8 volts being that the pulse of the corresponding clock period of the signal P.

The signal RC is applied to the voltage regulator circuit 21, which generates the AC supply voltage of + 5 volts.

The reset circuit, in response to the establishment of the AC voltage, generates a reset pulse Z of the sequencer circuit 23.

In FIG. 4, it appears, during the third clock period, a drop in the level of the signal RC, to a value lower than 8 volts, after the plateau at 8 volts already described. As will be better understood below, this drop in level corresponds to the transmission of a bit of value 1 from key 2 to the exchange device, and it corresponds to a level 1 pulse of the binary signal IN.

Consequently, the signal PR at the output of the circuit 29 for detecting sudden variations represents the restitution of the signal P representative of the data transmitted by the exchange device 1, since the signal PR goes from 0 to 1 for the sudden decreases in the signal RC , and from 1 to 0 for the sudden growths of this signal, the transitions relating to the fall linked to the emission of a bit of value 1 by the key 2 being ignored, thanks to the signal IN.

• le signal AU, au niveau 1 pour autoriser les échanges après l'établissement des différentes tensions,
• le signal H2, signal d'horloge de période $\text{T2 = 64 T1}$
  
  
  ici au niveau 0 pendant la première demi-période,
• le signal IN comprenant une impulsion au niveau 1 pour les seules périodes d'horloge au cours desquelles un bit de valeur 0 a été émis par le dispositif d'échange, impulsion qui, dans ce cas, passe au niveau 1 après que ce soit écoulée, depuis le début de la période d'horloge, une durée sensiblement égale à 16 fois T1, et y reste pour une durée sensiblement égale à 28 fois T1. Ainsi les flancs de l'impulsion du signal IN se produisent tous deux pendant que le signal P est constant, ici au niveau bas.

The sequencer circuit 23 generates, from the clipped signal ECR, the pulse Z, and the signal PR:

• the signal AU, at level 1 to authorize exchanges after the establishment of the different voltages,
• the H2 signal, period clock signal $\text{T2 = 64 T1}$
  
  
  here at level 0 during the first half period,
• the signal IN comprising a pulse at level 1 for the only clock periods during which a bit of value 0 has been transmitted by the exchange device, pulse which, in this case, passes to level 1 after it has elapsed , since the start of the clock period, a duration substantially equal to 16 times T1, and remains there for a duration substantially equal to 28 times T1. Thus the flanks of the pulse of the signal IN both occur while the signal P is constant, here at the low level.

The processing circuit 25 is therefore able, in particular from the restored signal PR and the clock signal H2, to know the value of each of the bits transmitted by the exchange device 1, by determining the level of the restored signal PR at moment of the rising transition of the clock signal H2.

In response to the information thus received, representing for example an address of a memory included in the circuit 25, the latter generates the signal DE representative of a series of bits to be transmitted to the exchange device 1, and which represents, for example, the content of the box previously addressed. Naturally in such a real situation, the circuit 25 would wait to have received all of the bits specifying the address of the memory box before transmitting. However, here, for the clarity of the description, and also because one of the advantages of the system of the invention is the possibility of transmitting, during the duration of transmission of a bit to key 2, a bit to the transfer device 1, it is assumed that the processing circuit 25 is immediately ready to transmit.

As shown in the lower part of FIG. 4, the bits to be transmitted by the processing circuit 25 are for example 0, 1, and 0. It is noted that, during the second clock period, which here corresponds to reception, coming from the exchange device 1, of a bit of value 1, the emission of a bit by the processing circuit 25 is prohibited. Such a characteristic is not compulsory, but it has the advantage, here, that the transmission of a bit by the key 2 necessarily takes place during a clock period when the signal RC is for sure at the high level for the last three quarters of this clock period. This allows a relatively long time to vary, if necessary, and thanks to the signal D, the load that represents, for the amplifier 16 and the coil 17, all the electronic circuits of the key, so to transmit to the exchange device 1, the information at the output of the processing circuit 25.

For this purpose, the signal D at the output of the circuit AND 30 is always at level 1, except when the signal IN and the signal DE are simultaneously at level 1. The circuit 26 of load variation and limitation is arranged, as will be better understood below, to consume a large current , when the signal D goes to level 0, and therefore cause an abrupt variation in the charge seen by the exchange device 2, translated by an abrupt increase in current, detected by the detector circuit 18.

Thus, when, a bit of value 0 having been transmitted by the exchange device 1, the emission of a bit by the key 2 is authorized, the abrupt variation in charge during the time interval where the signal P is at the low level is interpreted by the processing circuit 13 of the exchange device 1 as a bit of value 1 coming from the key 2, while the absence of abrupt variation in load during this same time interval is interpreted as a bit of value 0 coming from this key 2.

The load variation and limitation circuit 26 operates as will now be described. The transistor 265 is mounted, in a known manner, to compensate for the influence of the temperature. When the signal D is at level 0, the transistor 266 is blocked, the resistor 264 is in the air, and the base potential of the transistor 261 is pulled up by the resistor 263. The transistor 261 is therefore strongly conductive, and the potential of its collector is very low. As a result, the Zener diode 262 conducts and consumes a large current, which therefore corresponds at a high load for the rectifier circuit 28, and, as a result, the load equivalent to the coil 27 followed by the electronic circuits of the key 2 is increased. In practice, the RC potential is forced to a value close to the 6.2 V Zener voltage, compatible, however, with the operation of regulator 21.

When the signal D is at level 1, the transistor 266 is highly conductive, and the transistor 261 is biased by the resistance bridge 263, 264 so that its potential VCE is approximately 4 volts. Thus, the circuit 26 can play the role of limiter, because, when the potential of the RC signal tends to exceed 10 volts, the Zener diode 262 starts to drive. This has the effect of stabilizing the load represented by the key 2, and, moreover, makes it possible to use, in the key, electronic components of the HC MOS type, the supply voltage of which must in no case exceed 10 volts.

When the identification process is relatively complex, the processing circuit 25 can comprise in particular a microprocessor, which can then ensure, in whole or in part, the functions performed by the sequencer 23.

In this case, the processing circuit 25 may also include an oscillator, powered by the AC signal to deliver to the microprocessor the high frequency clock signal necessary for its operation. This clock signal, relating to the internal functioning of the microprocessor, does not have to be synchronous with the ECR signal, without this being in contradiction with the assertion which has been made according to which the ECR signal is used as synchronization signal. Indeed, the signal ECR is always used as a synchronization signal, in particular in that it allows for example to obtain the clock signal H2, whose rising transitions define the instants when the restored signal PR is, suddenly safe, representative of the bits coming from the exchange device 2 of the lock. This therefore does not concern the internal functioning of the microprocessor and the signal delivered by the oscillator.

By way of example, FIG. 5a shows a flat key provided with a recess 3 which passes through it, inside which is disposed a module 2 a integrating the electronic circuits of FIG. 2, as well as the coil 27. The module 2 a is protected by two layers 4 of epoxy resin.

Likewise, FIG. 5b shows a cylindrical key, at the end of which is fixed, by screwing or gluing, a module 2b integrating the electronic circuits of FIG. 2, as well as the coil 27, for example a cylindrical coil of same axis as the key, located at the end of the module 2 b
Naturally, the scope of the present application is not limited to the description which has just been given and it is within the reach of the skilled person, for example, to replace the transmission by coupled coils by a transmission by contacts. or else to avoid the ban on transmitting, for the key, when a bit of value 1 has been transmitted by the exchange device, by using the time available in each clock period differently. Similarly, it is within the reach of the skilled person to replace the circuits described above, and in particular comprising bipolar transistors, by corresponding circuits produced in CMOS technology, for example.

Similarly, if it is particularly simple and advantageous to use pulse width modulation, followed by amplitude modulation, to transmit the information from the exchange device to the key, this is not mandatory, and other types of carrier modulation can also be used.

Finally, if the system of the invention proves to be particularly useful for the application which has been described in the field of locksmithing, it is obviously possible to apply it to any other field such as that of memory cards, for example.

In the field of memory cards, and particularly in that of bank cards, the complexity of the identification process generally requires the use of a microprocessor associated with the clock oscillator which has already been discussed. Naturally, if the device is designed to receive several types of microprocessor, the oscillator is programmable.

Claims (8)

1. A system for exchanging information between at least one portable object (2) and at least one sedentary exchange device (1),
   the said device comprising:

   - first processing means (13) generating a first signal (P) representing a first series of binary elements, or bits, to be transmitted towards the said portable object (2),

   - means (15) for modulating a carrier wave (H1), in response to the said first signal (P),

   - means (16, 17) for transmitting the modulated carrier wave (PM), and

   - first detector means (18) for detecting sudden variations in amplitude of the current in the said transmission means (16, 17) and for supplying a detected signal (RS) to the said first processing means (13),
   and the said portable object comprising:

   - means (27, 28, 29, 23) for restoring the said first signal (PR), in response to the received modulated carrier wave, and

   - second processing means (23, 25, 30) receiving the said first restored signal (PR) and generating a second signal (D) representing a second series of bits to be transmitted towards the said exchange device (1),
   characterised in that the said portable object additionally comprises means (26) for, in a sudden manner, varying, in response to the said second signal (D), the load which the said portable object (2) represents for the said transmission means (16, 17) so that, during the period of transmission of a bit of the first series, a bit of the second series is transmitted and detected by the said first detector means (18).
2. A system according to claim 1 in which the said modulation means (15) are arranged to modulate the amplitude of the said carrier wave (H1), and the said restoration means comprise second detector means (29) for detecting sudden variations in amplitude of the received modulated carrier wave.
3. A system according to claim 2 in which the said first processing means (13) are arranged so that the said first signal (P) is a binary signal comprising a series of pulses, the size of each of these pulses being capable of assuming one of the values of a pair associated with the pair of possible values for each bit of the first series, and the said second processing means are arranged so that the said second signal (D) is a binary signal comprising a series of pulses, each of these pulses only occurring when a bit of the first series assumes a predetermined value (0), the level of each of the pulses being capable of assuming one of the values of the pair of possible values for each bit of the second series, and the edges of each of these pulses occurring while the level of the said first signal (P) is constant.
4. A system according to one of claims 1 to 3 in which the said sudden variation means (26) are arranged to limit the amplitude of the received modulated carrier wave.
5. A system according to one of claims 1 to 4 in which the said portable object comprises means (22) for clipping the peaks of the said modulated carrier wave, and the said second processing means (23, 25, 30) are synchronised by the said peak-clipped signal (ECR).
6. A system according to one of claims 1 to 5 in which the said portable object comprises means (28) for rectifying the received modulated carrier wave, and regulation means (21) for supplying a service voltage (AC) of electrical energy to all the electronic components of the portable object in response to the rectified carrier wave.
7. A system according to one of claims 1 to 6 in which the said transmission means comprise a first coil (17) and the said restoration means, a second coil (27), and the coupling between the said portable object (2) and the said exchange device is a magnetic coupling, obtained by bringing together, but without contact, the said first and second coils (27).
8. A system according to one of claims 1 to 7, in which the said portable object is a key (2) and the said sedentary exchange device (1) is installed in a lock in order to prevent the unlocking of the said lock when the information received from the key indicates that the key is not approved for unlocking the said lock.

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