Diagnostics of an output interface of a safety-related system with safety PLC

Safety programmable logic controllers (S-PLCs) provide broad application options and are being used to perform control but primarily safety functions (SFs) on the process control level. An essential part of control systems based on S-PLC is the sensors and actuators. In order to ensure the SF with high safety integrity level (SIL), the control system performing this SF has to dispose of a suitable failure detection mechanism and a mechanism providing its safe reaction in case of a failure detection. The diagnostics provided by S-PLC is mainly focused on its own components, and the diagnostics of sensors and actuators has to be resolved on the application program level while respecting some specific properties of S-PLC. This paper addresses the analysis of the influence of S-PLC parameters and diagnostic parameters on the safety function SIL, which is performed by a safety relevant electronic system. In the practical part is described the implementation of application diagnostics in S-PLC and the results of diagnostic parameters influence on safety integrity of SF for a selected example connected to a practical application.