System for keeping account of predetermined homogeneous units

摘要

Each card of a set of credit cards includes a memory having plural memory fields. The memory of each card in a set includes a first unalterable multi-bit field indicative of an identification of an external means with which the card can be used, as well as a second unalterable mutli-bit field indicative of operations that can be performed with the card on the external means and an alterable multi-bit field indicative of the number of credit units available to a card holder. Each card includes circuits for controlling the memory, and means for temporarily coupling the memory and circuits to the external means. Each of the first and second unalterable bit fields includes n bits. A change from one bit state to the other bit state of a predetermined number, m, of bits in the unalterable fields of each card in the set is irreversible after imprinting, where m is equal to or less than n.