PREVENCION DE DUPLICACION ELECTRONICA DE UNA SENAL DE ENTRADA SIN LLAVE,QUE CARACTERIZA UN METODO DE TRANSMISION CONSERVADOR DE ENERGIA

A remote keyless entry device providing vehicle access which utilizes a friend/foe screening technique comprising an initial sequence of two pulses which precede a transmitted code. The first pulse (24) is transmitted on a 50 kHz carrier frequency while the second (26) is transmitted on a 38 kHz carrier frequency. Once the vehicle receives the pulses, a match is acknowledged permitting the subsequently following transmitted vehicle code to be received and compared to the code stored in the receiver memory (116). The specific vehicle code in the preferred embodiment consists of five integers. These integers are transmitted by the transmitter in serial fashion. Each integer is represented as a time interval between a start and a stop pulse. Each time interval is equal to a fixed increment of time multiplied by the respective integer value. The stop pulse of the first integer is the start pulse of the second and the stop pulse of the second is the start pulse of the third, etc. Once the vehicle receiver receives the sequence of integers, the receiver microprocessor (114) compares the integers received with integers stored therein. If a suitable match is accomplished a logic signal is generated and communicated to the central locking unit (118). A central locking unit which controls individual door access mechanisms then positions the respective mechanisms according to the instruction received therefrom.