The present invention relates to a security arrangement comprising a data processing unit for serial transmission of data for controlling outputs and for querying inputs of a process and a corresponding method; further comprising at least one parallel-to-serial converter comprising a shift register for converting data to query the inputs; at least one serial-to-parallel converter comprising a shift register for converting data to control the outputs; wherein the data to be transmitted includes diagnostic bits that are output from the serial-to-parallel converter and read back from the parallel-to-serial converter; and the read-back data is checked for errors by the data processing unit to ensure a secure data transmission.