Importance

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Abstract

A credit card includes a memory having first and second zones in which are respectively stored variable and fixed data. The data stored in the first zone includes first, second and third bit fields respectively representing a card serial number, a card invalidation character and a card validation character. The first, second and third bit fields are subject to change by improper use of the card. The first, second and third bit fields are read to derive first, second and third signals. The second signal is compared with a predetermined signal indicative of a desired value for the second field. A set of bits is written into the third field in response to the second signal being equal to the predetermined signal and the first signal being equal to a stored signal indicative of an assigned serial number. The set of bits written into the third field is tested to determine if the bits were properly written into the third field. In response to the test on the third field indicating that the bits were properly written into the third field and the second signal being equal to the predetermined signal and the first signal being equal to a stored signal indicative of an assigned serial number, the variable data stored in the second part of the memory are changed.

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First Claim