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

A method for operating a safety module for a programmable logic control system, wherein the safety module comprises an assembly consisting of a plurality of modules with a plurality of function blocks connected through at least one internal bus, and at least one Central Processing Unit (CPU), the safety module being configured to be connected through at least one interface to external sensors and switches/buttons, actuators, optical/acoustic indicators, and displays/monitors for detecting at least one measurement signal, wherein a test signal tuned to an input filter of a first signal detection module, the test signal being generated by a second signal detection module of identical configuration as the first signal detection module, is added to each of the at least one measurement signal, and after scanning and filtering by the first signal detection module, the test signal is removed again by a next CPU of the at least one CPU, and a test is performed by comparing the test signal thus recovered to that of a second, redundant test signal such that if both signals are within a configurable tolerance band, the test is deemed to have passed, and in a next test cycle, the test signal is added to another of the at least one measurement signal and thereby the first signal detection module is tested.