Formal Translation of IEC 61131-3 Function Block Diagrams to PVS with Nuclear Application

The trip computers for the two reactor shutdown systems of the Ontario Power Generation (OPG) Darlington Nuclear Power Generating Station (DNGS) are being refurbished due to hardware obsolescence. For one of the systems, the general purpose computer originally used is being replaced by a programmable logic controller (PLC). The trip computer application software has been rewritten using function block diagrams (FBDs), a commonly used PLC programming language defined in the IEC 61131-3 standard. The replacement project’s quality assurance program requires that formal verification be performed to compare the FBDs against a formal software requirements specification (SRS) written using tabular expressions (TEs). The PVS theorem proving tool is used in the formal verification. Custom tools developed for OPG are used to translate TEs and FBDs into PVS code. In this paper, we present a method to rigorously translate the graphical FBD language to a mathematical model in PVS using an abstract syntax to represent the FBD constructs. We use an example from the replacement project to demonstrate the use of the model to translate a FBD module into a PVS specification.