each of the three connections 23 of the microcomputer 11 is connected to a locking apparatus in the motor vehicle 39 s central locking system and transmits the actuation signal output by the microcomputer 11 the coded information item is formed using a fixed code and an alternating code the alternating code is generated using the random number generated by the random number generator 19 17 and using the algorithm stored in the microprocessor the value of the alternating code is advanced in the portable transmitter unit 1 after a particular event in the receiver device 2 the values of the alternating code are split into permissible and impermissible values in this context the permissible values are always in the future and the impermissible values are always in the past or the present the dividing line between these ranges is determined by the current value of the alternating code and by the capture range the current value corresponding to the present and the capture range corresponding to a prescribed number of future information items among the coded information items which are to be transmitted operating the pushbutton switch 6 gives the microcomputer 3 in the portable transmitter unit 1 the instruction to start the algorithm for generating a coded information item with a random number being supplied by the generator 19 the coded information item generated in this way is transmitted via the antenna 9 to the receiver 2 in the motor vehicle where it is received by the antenna 21 and is passed on to the microprocessor 11 once transmission of the coded information item has ended the microprocessor 3 waits for a prescribed interval before.