A system is disclosed including a transmitter and receiver for the communication of security codes which may be validated at the receiver to operate equipment. In one embodiment, the transmitter transmits a security code at two frequencies contemporaneously to the receiver which may receive both frequencies and resolve the security code therefrom. The receiver may lock onto one frequency to the exclusion of the other frequency when parts of a security code are detected. In another embodiment, the transmitter selectively transmits security codes at a default frequency which is selected because of a recorded count of prior apparent successful transmission.