Method for safe evaluation of information at input circuit of a safety device and a safe input circuit for carrying out this method

The proposed method of a safe evaluation of information at input circuit of a safety device comprises short-circuiting of a voltage comparator circuit (NK) input by using an electronic switch (ES) and determining voltage level between input terminal connectors (VSNK1, VSNK2) of said comparator circuit (NK). The method itself is then carried out in such a manner that a microprocessor system (MS) evaluates information at the input circuit in time sequence so that first discharge of a capacitor (C) by short-circuiting thereof using the electronic switch is carried out, subsequently said short-circuiting is cancelled and then it is determined through the mediation of the voltage comparator circuit (NK) whether the capacitor (C) remained discharged below said voltage comparator circuit (NK) voltage level for at least specified minimum time of discharge with the proviso that in case when the microprocessor system (MS) determines that the capacitor (C) has been charged during the specified minimum time of discharge above said voltage comparator circuit (NK) voltage level or that no discharge of the capacitor (C) took place at all, the microprocessor system (MS) evaluates information at the input circuit as invalid. A safe input circuit for executing the method is provided with input terminal connectors (VS1, VS2) that are connected to input terminal connectors (VSNK1, VSNK2) of a voltage comparator circuit (NK) being followed up with input terminal connectors (VSMS1,VSMS2) of a microprocessor system (MS) serving for excitation of an electronic switch (ES). Said voltage comparator circuit (NK) input is parallel-connected to both said capacitor (C) and said electronic switch (ES) outlet. One side of the capacitor (C) is connected through an RC filter resistor (R) to one of the input terminal connectors (VS1, VS2).