Formally Veriied Building Blocks in Functional Logic Diagrams for Emergency Shutdown System Design

The task of safeguarding systems is to bring processes from dangerous into safe states. A special class of safeguarding systems are emergency shutdown systems (ESD), which, until now, are only implemented in inherently fail safe hardwired forms. Despite their high reliability, there is an urgent industrial need to replace them by more exible systems. Therefore, in an earlier work, a dedicated programmable logic controller (PLC) was designed, which directly supports functional logic diagrams (FLD), the traditional and user oriented graphical programming paradigm of ESDs, in its architecture. In this paper we give a formal correctness proof of the functional building blocks occurring in FLDs specifying ESDs. For this task Isabelle/HOL is used as a mechanical proof assistant. In a nal step, safety licensing of ESD software can easily be carried through by back translation.