The Systems Modeling Language (SysML) is a semi-formal, graphical modelling language used in the specification and design of systems. We describe how Communicating Sequential Processes (CSP) and its associated refinement checker, FDR3, may be used to underpin an approach that facilitates the refinement checking of the behavioural consistency of SysML diagrams. We achieve this by utilising CSP as a semantic domain for reasoning about SysML behavioural aspects: activities and state machines are given a formal, process-algebraic semantics. These behaviours execute within the context of the structural diagrams to which they relate, and this is reflected in the CSP descriptions that depict their characteristic patterns of interaction. We describe how CSP and FDR3 can be used in conjunction with SysML in a formal, top-down approach to systems engineering. Moreover, the compositionality afforded by CSP alleviates the state space explosion problem frequently encountered with complex formal models and complements the top-down approach of SysML. Typically, a system is composed from constituent systems using the concept of blocks. SysML permits two alternative interpretations with regard to the behaviour of the resulting composition. We argue that the use of a process-algebraic formalism enables us to explore the relationships between these interpretations in a more rigorous fashion than would otherwise be the case.
