Dependability Assessment of an Industrial Programmable Logic Controller via Parametric Fault-Tree and High level Petri Net

The case-study presented in this paper is aimed at assessing the dependability of a Programmable Logic Controller (PLC) devoted to safety functions. This case study has been brought to our attention by a national environmental agency and has been partially abstracted and anonymized to protect proprietary information. The PLC consists of a triplicated channel with a (2 : 3) majority voting logic and is modeled by means of a recently proposed extension of the classical Fault Tree (FT) formalism called Parametric Fault Tree (PFT). In the PFT replicated units are folded and parameterized so that only one representative of the various similar replicas is explicitly included in the model. The quantitative analysis of the PFT assumes s-independence among components and is based on combinatorial formulas. In order to include dependencies both in the failure and repair process, the PFT is directly converted into a particular class of High Level Petri Nets, called SWN. The paper illustrates the PFT formalism and the automatic conversion algorithm from a PFT into a SWN. Moreover, it is shown how various kind of dependencies can be accommodated in the obtained SWN model.