1. An access control system for transmitting data from a central station to a plurality of receiving stations each of said receiving stations each of said receiving stations comprising: first memory means for permanently storing a unique serial number; second temporary memory means for storing a manually inputted access code including in scrambled form said serial number and a service cutoff and also including a cypher pointer which is not scrambled; map means responsive to said cypher pointer for unscrambling said scrambled serial number and cutoff date; and means for comparing said unscrambled serial number with said permanently stored serial number for validating said receiving station to allow such station to receive said transmitted data.