The Matlab Simulink tool is widely used to construct and analyse control law diagrams. Many have worked on techniques to enhance analysis facilities, and previously, we have considered the complementary problem of proving correctness of implementations of diagrams. We use Circus, a refinement language that combines Z and CSP, and can capture both functional and behavioural aspects of diagrams and programs. We defined a Circus semantics for an extensive subset of discrete-time diagrams, and now extend it to cover Stateflow blocks, which are themselves defined by diagrams written in (a variant of) the statechart notation. We highlight the challenging features of the semantics of a diagram, describe how Circus models can be constructed, and discuss the formalisation of the Circus semantics as algebraic translation rules.
