A method and a device for securely switching between operating modes of an industrial controller for machine tools or production machines are described. Using a multi-element photo array provides the basis for measurement of differential refractive index values with a heretofore unattainable combination of sensitivity of measurement and concurrent range of measurement. In order to process variable-length high-speed packets, packet multiplexing instead of simple TDM is used and a larger input bandwidth than an output bandwidth is used, so that a statistical multiplex effect is accomplished. All chains in the set of chains are extended, and a first set of paths is generated, wherein the first set of paths is a minimal set of paths covering all states of the modeled system.