Architecture of a Safe Node for a Fieldbus System

Fieldbus systems are more and more responsible for functions affecting people's health. Therefore they are supposed to work safely, i.e. the fieldbus system must be free from unacceptable risk. Thus different measures specified in the international standard IEC 61508 have to be taken to reduce the risk, for example, a redundant hardware structure, add-ons to the protocol, integration of online self tests. A fieldbus system that is going to be enhanced with safety measures is the Local Operating Network (LON), standardized in EN 14908. Within the SafetyLon project hardware and software of a safe node for LON is developed. A safe node consists of an EN 14908 controller and two microcontrollers. The software comprises three layers. One of them is the safety layer including the safe protocol stack and the online self tests.