GB2198014A

GB2198014A - Information transfer system

Info

Publication number: GB2198014A
Application number: GB08627268A
Authority: GB
Inventors: Derek Thomas Wright
Original assignee: British Broadcasting Corp
Current assignee: British Broadcasting Corp
Priority date: 1986-11-14
Filing date: 1986-11-14
Publication date: 1988-06-02
Also published as: GB8627268D0

Abstract

Information is transmitted between two information processing circuits through an inductive coupling (1). The first circuit absorbs power from the inductive coupling, and varies the absorbed power in response to information to be transmitted to the second circuit. The second circuit generates an a.c. signal (10) and applies this signal to the inductive coupling (1). A sensing means (14) detects the current passed by the coupling in response to the a.c. signal and provides an output (40) which is dependent on the detected a.c. signal and representative of information transmitted from the first circuit. <IMAGE>

Description

INFORMATION TRANSFER SYSTEM This invention relates to an electronic information or data transfer system for transmitting data between two information processing circuits through an inductive coupling.

Systems which use inductive coupling to transmit data are known, and it has been proposed to construct modules which carry out electronic processing of input data to produce output data and in which the input data to the module and the ouput data from the module are transferred through the same inductive coupling.

However, such systems can not permit the simultaneous transfer of data in the two directions through the coupling unless two frequencies are used or alternatively two inductive couplings are employed. Such systems then become relatively complex.

In some applications for example there is a particular desire to make the inductive coupling and one of the information or data processing circuits associated with it as small as possible. This arises in particular where one of the circuits is constituted in a 'smart card' or similar device, this being a card similar in size to a credit card but containing sophisticated processing circuitry including possibly a micro-processor. In such an application it would be undesirable to have to use two inductive couplings and certainly not possible to have two such couplings with their magnetic axes orthogonal as is preferred to avoid interference between the two directions of transmission.

The existing inductive coupling arrangements do not meet the requirements of such applications in an economical and effective manner.

The present invention is defined in the appended claims to which reference should now be made.

The invention thus provides an information transmission system which is particularly adapted to implementation for bidirectional data transfer to a 'smart card' or the like. However its size advantages and the relatively simple circuitry required of the card circuits make it applicable to information transfer systems of other types.

In this specification the term "card" is used to mean a plastics card of substantially the dimensions of an I.S.O. standard credit card subject possibly to being slightly thicker than normal credit cards.

The invention will be described in more detail by way of example with reference to the accompanying drawings, in which: Figure 1 is a block schematic diagram of a first data processing circuit for use on a 'smart card' or the like; and Figure 2 is a block schematic diagram of a second data processing circuit for use in a 'reader' or host unit for the card of Figure 1.

The data transmission system illustrated includes a magnetic inductive coupling device 1, effectively a transformer, shown in both Figures. The card circuit of Figure 1, which may be termed a sub-system, then has three main functions. To the coils 20 of the coupling 1 are connected rectifying diodes 2 and a smoothing capacitor 22 to provide a rectifying and smoothing circuit 24 for a power supply 26 for the card circuitry. A constant current regulator 3 is connected to the rectifying and smoothing circuit 24 and comprises a series-connected current sensor 28 and a parallelconnected variable current load device 30, the control input of which is connected to the current sensor 28. A further smoothing capacitor 32 is provided followed by a voltage regulator 4 which ensures that a constant voltage, in this case 5 volts, is fed to the rest of the card circuitry.The current regulator operates so that the current drawn from the rectifying and smoothing circuit 24 is always constant, change in the current drawn by the voltage regulator 4 being accomodated by an inverse change in the current drawn by the variable current load 30.

Also connected to the output of the coils 20 is a phase or frequency demodulator 5 (as appropriate) which feeds a UART (universal asynchronous receiver/transmitter) circuit 6 operative as a receiver. The output of receiver 6 is applied to a processor and memory circuit 7 which may comprise a micro-processor and constitutes the data processing circuit for the card. The output of the circuit 7 is applied to another UART 8 operative as a transmitter. The output of transmitter 8 is applied to the control input of a variable current load 9 which can in fact be a switchable current load variable between two values, one of which may be "off".

As the current drawn by the current regulator 3 is constant the current or power drawn through the inductive coupling is solely dependent upon the load 9 and hence on the data to be transmitted by transmitter 8.

The 'reader' or host unit associated with the card is shown in Figure 2 and includes a reference frequency oscillator 10 connected to a phase or frequency modulator 11 which receives on a line 36 data to be transmitted to the card sub-system of Figure 1 through the inductive coupling 1. The output of modulator 11 is applied to a power amplifier 12 which feeds the coil 38 of the coupling arrangement 1. The current through the coil also passes through a current sensing resistor 13. An amplitude modulation detector 14 is connected across this resistor and detects variations in current through the resistor which will be caused by variations in the value of the load 9 of Figure 1. The demodulated signal is applied to an output 40.

The operation of the system illustrated is thus as follows.

The system uses a single carrier frequency from reference frequency generator 10, and a single magnetic coupling 1. The carrier signal is continuously generated by the host system of Figure 2. This carrier signal has a nominally constant envelope, and input data to the sub-system is conveyed by phase or frequency modulation of this carrier, applied by modulator 11. The sub-system obtains its operating power supply by rectification of the received carrier signal using the diodes 2. The current input to the magnetic coupling is monitored at the host side of the interface by means of the sensing resistor 13. The sub-system returns data to the host by modulating the load that its power supply places on the hostgenerated carrier. Thus, in effect, the sub-system amplitude modulates the current of the carrier signal generated by the host.

Subsequent detection of such modulation is carried out by the a.m.

detector 14 fed from the sensing resistor 13, to provide the output data signal.

As indicated above, except when modulation of the input carrier is intended, the dc power drawn from the inductively coupled carrier must be kept constant regardless of the instantaneous functional condition of the electronic circuitry. This is achieved by the constant current regulator 3. Also when the carrier is modulated, by increasing and decreasing this constant load between a first and second constant value by means of switched current load 9, there must be some voltage regulating means 4 to ensure that the voltage applied to the electronic circuits within the sub-system is not altered outside their working limits. The processing system is formed from serial interfaces 6 and 8, and a microprocessor system with memory arrangements shown as the one block 7.

With a system such as the one described the input data rate can be high because phase or frequency modulation of a carrier is easily detected for modulating frequencies up to about half the carrier frequency. With a carrier frequency of 100 kHz data rates of 9600 baud should be achievable. The data output is amplitude modulated and to avoid complex detection arrangements at the host the data rate is preferably limited to about 1200 baud.

This arrangement of asymmetric input and output baud rates is particularly suited to certain applications, for example subscription television which uses over-air addressing, in which the input may contain data addressed in turn to many different groups of users but the output need contain only the information forming the keys needed to control descrambler functions; typically, two 64-bit keys might need to be output every ten seconds.

Several sub-systems of the type described may be connected in tandem to one host provided that only one is allowed to modulate the host-generated carrier at any one time. This is indicated in Figure 2. All sub-systems can retain continous access to high speed input data. With very many sub-systems connected (say 20) the dc power drain of those with quiescent output will dilute the effect of the amplitude modulation caused by varying the power drain of the active unit. It is thus likely that the higher level of power loading will need to be many times the quiesent level so that the depth of amplitude modulation is adequate for easy detection at the host.

Also, the magnetic coupling between the host and all sub-systems should Dreferably be reasonably tight.

It will be seen that the system illustrated permits continuous bidirectional data transfer without causing interruptions in the power supply derived from the input carrier signal and without complication of the circuit caused by the need for receive/transmit changeover arrangements.

Claims

1. An electronic information transfer system for transmitting information from a first circuit to a second ciruit through an inductive coupling, the system comprising first and second information processing circuits, and inductive coupling means connecting the first and second information processing circuits, the first information processing circuit comprising means for absorbing power from the inductive coupling means and means for varying the absorbed power in response to information to be conveyed to the second circuit, and the second circuit comprising means for generating an a.c. signal to be applied to the inductive coupling means, and means for sensing the current passed by the coupling means in response to the a.c. signal and to provide an output dependent thereon and representative of information transmitted from the first circuit.

2. An information transfer system according to claim 1, in which the second circuit includes means for phase or frequency modulating the a.c. signal and the first circuit includes means for demodulating the phase or frequency modulation whereby the system permits simultaneous bidirectional information transfer through the coupling.

3. An information transfer system according claim 1 or 2, in which the first circuit includes power supply means for rectifying the a.c. signal transmitted by the coupling to the first circuit, and means for maintaining the current drawn by the power supply means substantially constant regardless of the load thereon.

4. An information transfer system according to claim 3, including a voltage regulator circuit connected to the output of the power supply circuit.

5. An information transfer system according to any preceding claim, in which the first information processing circuit is constituted in a card (as herein defined).

6. An electronic information transfer system substantially as herein described with reference to and as shown in the drawings.