I. Introduction
Industrial control systems (ICS) often rely on programmable logic controllers (PLCs). PLCs are robust industrial computers performing various control tasks. The typical execution schema of a PLC is cyclic: the user-defined program is executed in an infinite loop (so-called PLC or scan cycle). At the beginning of the loop the physical inputs are read, then the program is executed using these stable input values. At the end of each loop the physical outputs are assigned which are then stable until the end of the next cycle. The duration of the scan cycle may vary in most of the PLCs.