Configurable safety module for detecting digital or analog input or output signals

A method for operating a safety module (1) for a programmable logic control system (PLC), wherein the safety module (1) is designed as an assembly consisting of a plurality of modules (2, 3, 4, 14, 53, 54, 55, 62) with a plurality of function blocks (2-4, 12, 13, 17, 25, 26, 35, 36) connected through at least one internal bus (7, 7a, 8, 8a) and comprising at least one CPU (9, 10) and which can be connected through at least one interface (5, 6) to external sensors (and switches/buttons) and actuators (and optical/acoustic indicators and displays/monitors) for detecting at least one measurement signal, wherein a test signal tuned to the input filter of a signal detection module (ASIC) (3, 4), the test signal being generated by a separate signal detection module (ASIC) (3, 4) of identical design is added to each measurement signal and after scanning and filtering by the first signal detection module (ASIC) (3, 4) the test signal is removed again by the next CPU (9, 10) and the signal thus recovered is compared to that of the second, redundant signal such that if both signals are within a configurable tolerance band and if the test is passed in a next test cycle the test signal is added to the other measurement signal and thereby the other ASIC (3, 4) is tested.