JPH05128319A - Information card system - Google Patents

Abstract

PURPOSE:To normally transmit and receive data regardless of a short distance between a contactless IC card and a card reader/writer. CONSTITUTION:If a contactless IC card 1 and a card reader/ writer 2 are too close to each other, the intensity of a radio wave is too high, and data cannot be normally transmitted neither received; and therefore, a voltage detecting circuit 20 which detects the reception voltage of a resonance circuit 22 is provided, and a variable capacity control circuit 19 and a variable capacitor 21 which reduce the reception sensitivity are provided. When the reception voltage of the voltage detecting circuit 20 exceeds a prescribed voltage level, a CPU 14 changes the electrostatic capacity of the variable capacity 21 by the variable capacity control circuit 19 and shifts the resonance frequency to reduce the voltage level of the reception voltage, and the reception sensitivity is adjusted. Thus, communication error is reduced, and the reliability of data transmission and reception in an information card system is improved, and the communication rate is raised to increase the system speed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an information card system for transmitting and receiving data by radio waves between an information card and a card reader / writer, and particularly to a transmission / reception level even when the transmission / reception distance between the information card and the card reader / writer is not constant. The present invention relates to an information card system capable of normally adjusting data transmission and reception.

[0002]

2. Description of the Related Art FIG. 6 is a block diagram showing an information card side of a conventional information card system. In FIG. 6, 3
Reference numeral 0 is an information card non-contact IC card, 31 is a card reader / writer for transmitting and receiving data to and from the non-contact IC card, 12 is an input / output control circuit for modulating / demodulating operation, 13 is a battery, and 14 is a non-contact IC card 31. CPU to control
(Central processing unit), 15 is an RO with a built-in program
M (read only memory), 16 RAM (readable and writable memory) for storing data and processing result, 17 CP
A bus that connects U and peripheral circuits and transfers data,
Reference numeral 18 is a resistor for keeping the voltage level of the transmission voltage constant. On the other hand, FIG. 7 is a block diagram showing a card reader / writer side of a conventional information card system. In FIG. 7, 32 is a CPU, 33 is a ROM, 34 is a RAM, and 35.
Is a display unit for displaying the result, 36 is a bus, 37 is an input / output control circuit, 38 is a power supply, 39 is a resistor, and 40 is a resonance circuit. The function of each unit is the same as that of the non-contact IC card 31.

FIG. 8 is a waveform diagram showing the waveform of the received voltage when the distance to the non-contact IC card reader / writer is large and when it is small in the information card system.
In FIG. 8, a is a transmission data string, b is a transmission radio wave, c is a reception radio wave received by the resonance circuit of the contactless IC card, and d is reception demodulated from the reception radio wave c by the input / output control circuit of the contactless IC card. Data string, e is the received radio wave when the distance between the contactless IC card and the card reader / writer is too short,
f is a received data string demodulated from the received radio wave e.

Next, the operation of the conventional information card system will be described. First, the card reader / writer 31 sends a command (transmission data string a in FIG. 8) to the non-contact IC card 30 by radio wave (transmission radio wave b in FIG. 8), and the non-contact IC card 30 responds to the command, The card reader / writer 31 displays the response on the display unit 35 as a result. When the non-contact IC card 30 receives a command by radio waves, the non-contact IC card 30 exchanges the electric signal with the resonance circuit 22 and sends the electric signal to the input / output control circuit 12.
The input / output control circuit 12 demodulates the electric signal into a digital signal and sends it to the CPU 14 via the bus 17. C
The PU 14 processes the data of the digital signal sent from the input / output control circuit 12 and outputs the result to the input / output control circuit 12
Then, the signal is modulated by the resonance circuit 22 and is transmitted to the card reader / writer 31 by radio waves. Also, in the card reader / writer 31,
The CPU 32 receives the radio wave from the non-contact IC card 30 at the resonance circuit 40 and displays the result on the display unit 35. By the above operation, the non-contact IC in the conventional information system
When the card 30 receives the reception radio wave c shown in FIG. 8 when the distance from the card reader / writer 31 is normal, the input / output control circuit 12 sets the reception buffer threshold level 50 to the reception data string c shown in FIG. I was demodulating to good data. Further, when the distance from the card reader / writer 31 is short, the non-contact IC card 30 receives a radio wave such as the reception radio wave e in FIG. 8 and the input / output control circuit 12 converts the data into the data shown in the reception data string f in FIG. It was demodulating.

[0005]

Since the conventional information card system is constructed as described above, the contactless I
Compared to the signal demodulated by the non-contact IC card when the distance between the C card and the card reader / writer is normal (signal t1 of the reception data string d in FIG. 8), the contactless IC card and the card reader / writer are When the distance is short, it can be seen that the signal demodulated by the non-contact IC card (t2 in the reception data string in FIG. 8) has a longer time axis and normal data transmission / reception is not performed. That is, when the distance between the non-contact IC card and the card reader / writer is short, the information card system cannot properly send and receive data.

The present invention has been made to solve the above-mentioned problems, and data can be normally transmitted and received even when the distance between an information card (contactless IC card) and a card reader / writer is short. The purpose is to get an information card system.

[0007]

As shown in FIG. 1, an information card system according to the first invention comprises an information card and a card reader / writer, and the information card (contactless IC card 1) variable capacity 2 And a resonance circuit 11 for transmitting and receiving radio waves to and from a card reader / writer, and an input / output for demodulating the radio waves received by the resonance circuit into a reception signal and modulating the transmission signal to emit the radio waves from the resonance circuit. Control circuit 12
And a voltage detection means (voltage detection circuit 20) for detecting a received voltage when the radio wave from the card reader / writer is received by the resonance circuit, and a variable capacitance control means for controlling the variable capacitance of the resonance circuit. A (variable capacity control circuit 19) and a control unit (CPU 14) for controlling the entire information card are provided. As shown in FIG. 2, the information card system according to the second invention has a resonance circuit 22 for transmitting and receiving radio waves to and from the information (non-contact IC card 1) of the card system with the card reader / writer.
And an input / output control circuit 12 that demodulates a radio wave received by the resonance circuit into a reception signal, modulates a transmission signal, and emits the radio wave from the resonance circuit. Further, the resonance circuit receives the radio wave from the card reader / writer. Voltage detection means (voltage detection circuit 20) for detecting the received voltage when received by the variable resistor 23, a variable resistor 23 provided between the input / output control circuit and the resonance circuit, and a resistance value for controlling the resistance value of the variable resistor. The control means (variable resistance control circuit 24) and the control unit (CPU 14) for controlling the entire information card are provided. As shown in FIG. 3, the information card system according to the third invention includes a resonance circuit 22 for transmitting and receiving radio waves to and from the information card (non-contact IС card 1) of the card system, with the card reader / writer,
When the resonance circuit receives the radio wave from the card reader / writer, the input / output control circuit 12 demodulates the radio wave received by the resonance circuit into a reception signal, modulates the transmission / reception signal with the carrier wave, and emits the radio wave from the resonance circuit. Voltage detecting means (voltage detecting circuit 20) for detecting the received voltage of the carrier, carrier frequency changing means (carrier frequency changing circuit 25) for changing the frequency of the carrier wave of the input / output control circuit, and a control section (CPU 14 for controlling the entire information card. ). As shown in FIG. 4, the information card system according to the fourth invention is an input card for demodulating a received radio wave into a reception signal and modulating a transmission signal to an information card (contactless IC card 1) of the carder system. An output control circuit 12 is provided and radio waves are transmitted / received to / from the card reader / writer. First resonance circuit (resonance circuit 2
2), a second resonance circuit (resonance circuit 26) having a reception sensitivity different from that of the first resonance circuit, and a changeover switch 28 for supplying the electric wave received by the first or second resonance circuit to the input / output control circuit. And a changeover switch control means (changeover switch control circuit 27) for controlling the changeover switch
A voltage detection means (voltage detection circuit 20) for detecting a reception voltage when the radio wave from the card reader / writer is received by the first or second resonance circuit, and a control unit (CPU 14) for controlling the entire information card. Equipped with. The information card system according to the fifth aspect of the present invention (as shown in FIG. 5, the information card of this card system is not provided with a voltage detecting means. Further, a method of controlling a transmission / reception level between the information card and the card reader / writer. Uses the means of the first to fourth inventions.

[0008]

In the information card system according to the first aspect of the present invention, the control section controls the variable capacitance when the received voltage detected by the voltage detecting means exceeds a predetermined voltage level when receiving the radio wave from the card reader / writer. The control unit changes the variable capacitance of the resonance circuit and shifts the number of resonance cycles, thereby controlling the voltage level of the reception voltage to be lowered. In the information card system according to the second aspect of the present invention, when the control unit receives a radio wave from the card reader / writer and the received voltage detected by the voltage detection unit becomes equal to or higher than a predetermined voltage level, the resistance value control unit performs resistance. The value is increased to control the voltage level of the transmission signal output from the input / output control circuit to the resonance circuit. In the information card system according to the third aspect of the present invention, the control unit, when receiving a radio wave from the card reader / writer,
When the voltage level of the reception voltage detected by the voltage detection means becomes equal to or higher than a predetermined voltage value, the carrier wave frequency varying means shifts the frequency of the carrier wave of the input / output control circuit from the resonance frequency, thereby changing the voltage level of the reception voltage. Control to decrease. In the information card system according to the fourth aspect of the present invention, the control unit controls the changeover switch according to the voltage level of the received voltage detected by the voltage detection unit when receiving a radio wave from the card reader / writer. By controlling the changeover switch to change the changeover switch, the first or second resonance circuit is selected to adjust the reception sensitivity and lower the reception voltage level. In the information card system according to the fifth aspect of the present invention, when the control section receives power from the card reader / writer, the control section detects the voltage level of the reception voltage of the reception voltage because the number of errors in the reception data from the input / output circuit is large and small. By doing so, the voltage detecting means is substituted. As described above, the information card system according to the present invention can adjust the transmission intensity or the reception sensitivity between the information card and the card reader / writer according to the distance between the information card and the card reader / writer.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an information card system in an embodiment of the first invention. In FIG. 1, 1
Is a contactless IC card as an information card, 2 is a card reader / writer, 11 is a resonance circuit, 12 is an input / output control circuit,
13 is a battery, 14 is a CPU (central processing unit), and 15 is R
OM (read only memory), 16 RAM (read / write memory), 17 bus, 18 resistance, 19 variable capacitance control circuit as variable capacitance control means, 20 voltage detection circuit as voltage detection means, 21 It has a variable capacity. It should be noted that each of the parts 12 to 17 in the non-contact IC card 1 has the same function as that of the conventional one (FIG. 6), and is therefore given the same reference numeral as in FIG. 6, and the following description of the configuration is simple. Therefore omitted.

The voltage detecting means 20 detects the received voltage when the radio wave is received by the resonance circuit 11 from the carder reader / writer 2 and sends it to the CPU 14 via the bus 17. CPU
When the received voltage detected by the voltage circuit 20 becomes a predetermined voltage level or higher, 14 controls the variable capacitance control circuit 19 to change the electrostatic capacitance of the variable capacitance 21 in the resonance circuit 11 to shift the resonance frequency, The voltage level of the reception voltage, that is, the reception sensitivity is adjusted. The variable capacitance control circuit 19 controls to change the electrostatic capacitance of the variable capacitance 21 in the resonance circuit 11 according to an instruction from the CPU 14.

Next, the operation of the embodiment of the first invention will be described with reference to FIGS. First, a signal such as a command (transmission data string a in FIG. 8) is transmitted from the card reader / writer 2 by radio waves (transmission radio wave b in FIG. 8). The radio wave from the card reader / writer 2 is received by the resonance circuit 11 of the non-contact IC card 1 (received radio wave c in FIG. 8), and the received voltage at that time is detected by the voltage detection circuit 20. The voltage detection circuit 20 sends the measured reception voltage to the CP via the bus 17.
Send to U14. When the voltage received by the voltage detection circuit 20 is higher than a certain voltage level, the CPU 14 uses the contactless IC.
It is determined that the distance between the card 1 and the card reader / writer 2 is short. Then, as it is, it is impossible to demodulate normally received data such as the received radio wave e and the received data string f in FIG. 8, so the variable capacitance control circuit 19 is instructed via the bus 17. Variable capacitance control circuit 1
9 changes the electrostatic capacitance of the variable capacitor 21 in the resonance circuit 11 according to an instruction from the CPU 14 and shifts the resonance frequency to reduce the voltage level of the reception voltage, and the input / output control circuit 12 normally receives the radio wave ( The reception voltage C and the reception data string d) of FIG. 8 are demodulated.

FIG. 2 is a block diagram of an information card system according to an embodiment of the second invention. In FIG. 2, 22 is a resonance circuit, 23 is a variable resistance, and 24 is a variable resistance control circuit as a resistance value control means. The second invention is characterized by these 22 to 24. The voltage detection means 20 has the same function as that of FIG. The variable resistance control circuit 24 is
The resistance value of the variable resistor 23 is changed. The CPU 14 controls the variable resistance control circuit 24 to change the resistance value of the variable resistance 23, so that the input / output control circuit 12
Control is performed to change the voltage level of the transmission voltage transmitted from to reduce the transmission intensity.

The operation of the embodiment of the second invention will be described. When the CPU 14 receives a command or the like from the card reader / writer 2 as a radio wave, the CPU 14 receives the command via the resonance circuit 22, the reception voltage / voltage detection circuit 20, and the bus 17. When the received voltage is higher than a certain voltage level, the CPU 14 sends the variable resistance control circuit 2 via the bus 17.
4 to increase the resistance value of the variable resistor 24. As a result, the voltage level of the transmission signal output from the input / output control circuit 12 via the variable resistor 23 can be lowered, and the transmission intensity can be adjusted according to the distance between the contactless card 1 and the card reader / writer 2.

FIG. 3 is a block diagram of an information card system according to an embodiment of the third invention. In FIG. 3, reference numeral 25 is a carrier frequency changing circuit as a carrier frequency changing means. The voltage detection circuit 20 has the same function as that in FIGS. The CPU 14 controls the carrier frequency changing circuit 25 to control the input / output control circuit 1
2 controls the carrier wave for modulating the signal transmitted from H.

The operation in the experimental example of the third invention will be described. When receiving a command or the like from the card reader / writer 2 by radio waves, the CPU 14 receives the received voltage via the resonance circuit 22 voltage detection circuit 20 and the bus 17. When the received voltage is equal to or higher than a certain voltage level (voltage value), the CPU 14 instructs the carrier frequency changing circuit 25 via the bus 17 to change the carrier frequency of the input / output control circuit 12. As a result, the frequency of the carrier wave of the input / output control circuit 12 deviates from the resonance frequency, and the voltage level of the received voltage decreases according to the distance between the non-contact IC card 1 and the card reader / writer 2.

FIG. 4 is a block diagram of an information card system according to an embodiment of the fourth invention. In FIG. 4, 22 is a resonance circuit as a first resonance circuit, 26 is a resonance circuit as a second resonance circuit, 27 is a changeover switch control circuit as changeover switch control means, 2
Reference numeral 8 is a changeover switch. The parts other than 22, 27 and 28 have the same functions as those of the first to third inventions. The resonance circuit 11 has the same receiving sensitivity as those of FIGS. The resonance circuit 26 is set to have lower reception sensitivity than the resonance circuit 11. The changeover switch control circuit controls the changeover switch 28 to change over the switch. The changeover switch 28 is a switch that connects the resonance circuit 11 or the resonance circuit 26 and the input / output circuit 12.
The CPU 14 controls the switch control circuit 27 to switch the changeover switch 28.

The operation of the embodiment of the fourth invention will be described. When the CPU 14 receives a command or the like from the card reader / writer by radio waves, the CPU 14 receives the reception voltage via the resonance circuit 22, the voltage detection circuit 20, and the bus 17. Then, the CPU 14 instructs the changeover switch control circuit 27 via the bus 17 when the received voltage is equal to or higher than a certain voltage level voltage value, that is, when the distance between the non-contact IC card 1 and the card reader / writer 2 is short. The change-over switch 28 is switched to connect the input / output control circuit 12 with the resonance circuit 26 having low reception sensitivity. As a result, the receiving sensitivity can be adjusted according to the distance between the non-contact IC card 1 and the card reader / writer 2.

FIG. 5 is a block diagram of the information card system in the embodiment of the fifth invention. The feature of this embodiment is that the voltage detection circuit 20 for detecting the reception voltage when receiving the radio wave from the card reader / writer 2 is not provided. CPU 14 is a non-contact IC card 1
From the resonance circuit 22 and the input / output control circuit 12, the non-contact IC
Know that the distance between the card 1 and the card reader / writer 2 is short. Utilizing this result, the CPU 14 adjusts the reception intensity or the reception intensity by the method of the first to fourth embodiments of the invention.

In the embodiment described above, the case where the non-contact IC card 1 is provided with the function of adjusting the reception intensity or the transmission output according to the distance between the non-contact IC card 1 and the card reader / writer 2 has been described. The function may be provided in the card reader / writer 2. In addition, its function is non-contact I
It may be provided in both the C card 1 and the card reader / writer 2. In addition, by combining some of the functions for adjusting the implementation strength or the transmission output in the embodiments of the first to fourth inventions,
It may be provided on one or both of the non-contact IC card 1 or the card reader / writer 2.

[0020]

As described above, according to the first aspect of the present invention, when the distance between the information card and the card reader / writer is short, the variable capacitance of the resonance circuit is changed by the variable capacitance control means to adjust the medical examination sensitivity. With such a configuration, even if the information card and the card reader / writer are too close to each other, data such as commands can be normally transmitted and received. As a result, communication errors are reduced, and reliability in data transmission / reception of the information card system is improved. Further, since free vibration during transmission / reception of data is suppressed quickly, the reliability of the communication rate can be increased and the speed of the information card system can be increased. According to the second aspect of the invention, since the reception sensitivity can be adjusted by changing the resistance value by the resistance value control means, there is an effect that the reception sensitivity can be adjusted more accurately than when the variable capacitance is adjusted. According to the third aspect of the invention, since the receiving sensitivity can be adjusted by changing the frequency of the carrier wave of the input / output control circuit by the carrier wave frequency changing means, it can be configured only by providing the carrier wave frequency changing circuit. If it is shared with the circuit of, there is an effect that the number of parts can be reduced. According to the fourth aspect of the invention, since the receiving sensitivity is configured to switch between the two resonance circuits by selecting the receiving sensitivity, there is an effect suitable when fine receiving sensitivity adjustment is not necessary. According to the fifth aspect of the invention, the voltage level of the received voltage is detected because the number of errors due to the received data is large and small.
The voltage detection circuit can be omitted.

[Brief description of drawings]

FIG. 1 is a block diagram of an information card system showing an embodiment of the first invention.

FIG. 2 is a block diagram of an information card system showing an embodiment of the second invention.

FIG. 3 is a block diagram of an information card system showing an embodiment of the third invention.

FIG. 4 is a block diagram of an information card system showing an embodiment of the fourth invention.

FIG. 5 is a block diagram of an information card system showing an embodiment of the fifth invention.

FIG. 6 is a block diagram of a card reader / writer side of a conventional information card system.

FIG. 7 is a block diagram of an information card side of a conventional information card system.

FIG. 8 is a waveform chart in each part of the conventional information card system.

[Explanation of symbols]

 1 Non-contact IC card 2 Card reader / writer 12 Input / output control circuit 14 CPU 19 Variable capacitance control circuit 20 Voltage detection circuit 21 Variable capacitance 22, 26 Resonance circuit 23 Variable resistance 24 Variable resistance control circuit 25 Carrier frequency variable circuit 27 Changeover switch control Circuit 28 Changeover switch

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[Procedure amendment]

[Submission date] September 1, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0002

[Correction method] Change

[Correction content]

[0002]

2. Description of the Related Art FIG. 6 is a block diagram showing an information card side of a conventional information card system. In FIG. 6, 3
Reference numeral 0 is an information card non-contact IC card, 31 is a card reader / writer for transmitting and receiving data to and from the non-contact IC card, 12 is an input / output control circuit for modulating / demodulating operation, 13 is a battery, and 14 is a non-contact IC card 31. CPU to control
(Central processing unit), 15 is an RO with a built-in program
M (read only memory), 16 RAM (readable and writable memory) for storing data and processing result, 17 CP
Connect communicating the U and a peripheral circuit, a bus for transferring data and the like, 18 is a resistor for a constant field intensity of transmission radio waves. On the other hand, FIG. 7 is a block diagram showing a card reader / writer side of a conventional information card system. In FIG. 7, 32 is a CPU, 33 is a ROM, 34 is a RAM, 3
5 is a display unit for displaying the result, 36 is a bus, 37 is an input / output control circuit, 38 is a power supply, 39 is a resistor, and 40 is a resonance circuit. The function of each unit is the same as that of the non-contact IC card 31.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0005

[Correction method] Change

[Correction content]

[0005]

Since the conventional information card system is constructed as described above, the contactless I
Compared to the signal demodulated by the non-contact IC card when the distance between the C card and the card reader / writer is normal (signal t1 of the reception data string d in FIG. 8), the contactless IC card and the card reader / writer are It can be seen that when the distance is short, the signal demodulated by the non-contact IC card (t2 of the reception data string in FIG. 8) has a longer bit width , and normal data transmission / reception is not performed. That is, when the distance between the non-contact IC card and the card reader / writer is short, the information card system cannot properly send and receive data.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0012

[Correction method] Change

[Correction content]

FIG. 2 is a block diagram of an information card system according to an embodiment of the second invention. In FIG. 2, 22 is a resonance circuit, 23 is a variable resistance, and 24 is a variable resistance control circuit as a resistance value control means. The second invention is characterized by these 22 to 24. The voltage detection means 20 has the same function as that of FIG. The variable resistance control circuit 24 is
The resistance value of the variable resistor 23 is changed. The CPU 14 controls the variable resistance control circuit 24 to change the resistance value of the variable resistance 23, so that the input / output control circuit 12
The control is performed to change the output level of the transmission signal transmitted from to reduce the transmission intensity.

Claims (5)

[Claims] 1. An information card for storing data, and a card reader / writer for writing / reading data to / from the information card via radio waves, wherein the information card has a variable capacity and is connected to the card reader / writer. An information card system including a resonance circuit for transmitting and receiving radio waves between the resonance circuit and an input / output control circuit for demodulating the radio waves received by the resonance circuit into a reception signal and modulating the transmission signal to emit the radio waves from the resonance circuit. In the information card, voltage detection means for detecting a reception voltage when the radio wave from the card reader / writer is received by the resonance circuit, variable capacitance control means for controlling the variable capacitance of the resonance circuit, and the information card A control unit for controlling the whole is provided, and this control unit receives the radio wave from the card reader / writer and receives the electric power detected by the voltage detecting means. Is higher than a predetermined voltage level, the variable capacitance control means changes the capacitance of the variable capacitance of the resonance circuit to shift the resonance frequency, thereby controlling the voltage level of the received voltage to be lowered. And information card system. 2. A resonance circuit comprising an information card for storing data and a card reader / writer for writing and reading data to and from the information card, wherein the information card transmits and receives radio waves to and from the card reader / writer. And an input / output control circuit that demodulates the radio wave received by the resonance circuit into a reception signal and modulates the transmission signal and emits the radio wave from the resonance circuit. Voltage detection means for detecting the received voltage when received by the resonance circuit,
A variable resistor provided between the input / output control circuit and the resonance circuit, a resistance value control unit for controlling the resistance value of the variable resistor, and a control unit for controlling the entire information card are provided. When receiving a radio wave from a card reader / writer,
When the reception voltage detected by the voltage detection means becomes equal to or higher than a predetermined voltage level, the resistance value is increased by the control means to lower the voltage level of the transmission signal output from the input / output control circuit to the resonance circuit. An information card system characterized in that it is controlled to. 3. An information card for storing data, and a card reader / writer for writing / reading data to / from this information card via radio waves, wherein the information card is a resonance for transmitting / receiving radio waves to / from the card reader / writer. In an information card system that has a circuit and an input / output control circuit that demodulates the radio wave received by the resonance circuit into a reception signal and modulates the transmission signal with a carrier wave and emits the radio wave from the resonance circuit, the radio wave from the card reader / writer The resonance circuit includes a voltage detection unit that detects a reception voltage, a carrier frequency changing unit that changes the frequency of the carrier wave of the input / output circuit, and a control unit that controls the entire information card. When the voltage level of the received voltage detected by the voltage detecting means becomes a predetermined voltage value or more when receiving a radio wave from the card reader / writer, By shifting the frequency of the carrier of the input-output control circuit from the resonance frequency transmitting frequency varying means, the information card system and controls to 抵下 the voltage level of the received voltage. 4. An information card for storing data, and a card reader / writer for writing and reading data on the information card via radio waves. The information card demodulates the received radio waves into a signal, and a transmission signal is transmitted. In an information card system including an input / output control circuit for modulating, a first resonance circuit for transmitting and receiving radio waves to and from a card reader / writer, and a second resonance circuit having a reception sensitivity different from that of the first resonance circuit. And a changeover switch for supplying an electric wave received by the first or second resonance circuit to the input / output control circuit, a changeover switch control means for controlling the changeover switch, and a first or second electric wave from the card reader / writer. The control circuit includes a voltage detection unit that detects a received voltage when the resonance circuit receives the voltage and a control unit that controls the entire information card. The control unit is a card reader / writer. When a radio wave is received from the voltage detection means, the first or second resonance circuit is selected by controlling the changeover switch control means and changing over the changeover switch according to the voltage level of the received voltage detected by the voltage detection means. The information card system is characterized in that the receiving sensitivity is adjusted to control the receiving voltage so as to lower the voltage level. 5. The receiving unit receives the received voltage because the number of errors in the received data from the input / output circuit is small when the control unit receives a radio wave from the card reader / writer without providing the voltage detecting unit on the information card. The information card system according to claim 1, 2, 3, or 4, wherein a voltage level is detected.