Abstract: A method for recording usage data of a card-operated device, such as a public telephone set or a vending machine. In particular, usage data, such as totalized charges, is stored in a storage unit associated with the device. At the onset of a transaction, e.g., a telephone call, an initial balance, representing a existing pre-paid credit, is read from memory in the card and transferred to the device. Based upon usage of the device, during the course of the transaction and/or at its completion, the device instructs the card, at least once, to reduce its balance by, e.g., a fixed amount to yield a current balance, which is, in turn, then stored back into the card. The card then sends the current balance back to the device. Device usage for this transaction is subsequently determined, within the device, as a difference between the initial and current balances, and is added to (totalized) usage data stored therein. The security of the data transfers may be cryptographically enhanced.