Semantics of reactive systems in abstract time

We explain that real-time reactive systems pose specific problems in defining languages to specify and program them. Three criteria are formulated, responsiveness, modularity, and causality, that are important to have for a high-level specification language for these systems. We prove that these properties can not be combined in one semantics. Since these properties are mandatory for a structured development of real-time reactive systems, we introduce a two-levelled semantics in which the three properties hold on different levels of the semantics: global events are treated more abstractly with respect to time than local events.