To obtain access to the functional blocks 20, 24, 28, 32 of the different levels 18, 22, 26, 30, a separate authentication process 34, that is symbolized by an openable switch in FIG. 1, has to be carried out for each level. On each authentication process 34, a random number is generated in the embodiment shown that is transmitted to the accessing browser via the Ethernet connector 12. The browser encrypts the random number using a public key that is included in a certificate of the browser. The encrypted random number is subsequently sent back to the PLC 10 and is decrypted there using a private key. If the result of the decryption coincides with the original random number, access is granted to the respective level 18, 22, 26, 30. In order, for example, to obtain access to the operating system kernel 28, three authentication processes 34 are thus necessary, namely for the fourth level 18, for the third level 22, and for the second level 26.

The schematic design of the PLC 10 is shown in more detail in FIG. 2, with the PLC 10 substantially comprising three sectors. The first sector is a hardware level 36 to which the Ethernet connector 12 belongs. The hardware level 36 additionally comprises the hardware components for a CAN bus (controller area network bus), for a USB connection 40 (universal serial bus), for an encoder 42, and for a touch-sensitive screen 44.