The design and the growing complexity of mechanical equipment prompts need for supervision, to help the operator. It is argued that rule based reasoning using events allows efficient man machine collaboration, both when using and when designing a supervision system. The way events can be defined is discussed, as well as the need for qualitative ordering of events defined on an «asynchronous» basis. Links between causal modeling in physic dynamics and ordering of events is discussed. One generic example with mechanical coupling in 2D allow to check influence of system's causality and structure on efficient ways to define events for computing qualitative system state.