This paper proposes a novel variant of Statecharts, called Safecharts, especially for use in the specification and the design of safety critical systems. The objective is to provide a sharper focus on safety issues and a systematic approach to deal with them. This is achieved by making a clear separation between functional and safety requirements. A novel feature of Safecharts is the safety annotation, which proposes an explicit ordering of states according to risk level. Transitions are classified according to their risk nature and given a new priority scheme for their execution in the event of any non-determinism. A railway signalling system, a well-known case study, is used as an example to demonstrate some features and semantics of Safecharts.
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