text: The present invention relates to an IC card system wherein an IC card having a built-in IC circuit is used, and the data-writing power supplied to the memory provided within the IC circuit is controlled.
  Recent times have come to be known as the "cashless era." Now, people can buy goods without paying in cash, by using cards issued by credit companies or the like.
  Among the cards used at present are: plastic cards, embossed cards, and magnetic-stripe cards. These cards are so simple in structure that they can easily be forged. And fraudulent use of forged cards has become a major problem.
  A new type of card, generally known as the "IC card", which is free of this drawback, has been invented. The IC card contains an IC circuit having a memory storing the personal identification number of the authenticated holder of the card. The personal identification number cannot be easily read from the IC card. An IC card system, which is a combination of the IC card and a terminal device, has also been invented.
  Data can be exchanged between the IC card and the terminal, when the card is inserted in the terminal. In order to write data into the data memory (i.e., an EPROM) provided within the IC card, a data-writing voltage Vpp must be supplied from the terminal to the IC card. More specifically, the terminal supplies voltage Vpp to the IC card, in response to a request made by the card.
  In the conventional IC card system, the terminal supplies data-writing voltage Vpp unconditionally, in response to the request from the IC card. The terminal continues to supply voltage Vpp to the IC card until the IC card gives the terminal an instruction for stopping the supply thereof.
  Here arises a problem. Even if the IC card fails to function properly for some reason while the terminal is supplying voltage Vpp thereto, data-writing voltage Vpp will continue to be supplied to the card until the card gives the instruction for stopping the voltage supply. Hence, data other than that desired may be written in the EPROM, in some cases. In the event of data being erroneously written in the EPROM, not only can the IC card be no longer used, but, in addition, the data previously stored in the EPROM may be destroyed. When this happens, the card holder will be greatly inconvenienced. Therefore, the security of the conventional IC card system cannot be fully guaranteed.