A supply system for monitoring the usage of services (electricity) to enable a customer to key-in a code in the form of discernable alpha-numeric format

摘要

A system for supplying pre-paid goods or services for example electricity to a user comprises: a monitoring device locate at the user's premises for monitoring usage of goods or services and a message generating device located remote from the user's premises for generating an encoded message in the form of a data encryption standard code in a discernable alpha-numeric format. The monitoring device includes: a storage means for storing a signal indicative of the current amount of credit, a keyboard for entering an encoded message in the form of a data encryption standard code which includes information indicative of an amount of credit, decryption means for decrypting the encoded message and processing means for processing the decoded message in order to determine the amount of credit and for updating the storage means. The encoded message generating device and the decryption means utilizing the same key, which key is changed for every transaction.