Synchronised execution of discrete event models using sequential function charts

The discrepancy between the event based supervisory control theory and its signal based implementation in programmable logic controllers using sequential function charts is explored. Different execution models and the related problems are discussed. Specifically, synchronisation of sequential function charts is treated in the context of modular implementation of the control functions. The supervisory control theory requires that conflicting events are never generated. This is guaranteed by the synchronous execution of events present in all modules. However, in a signal based environment the synchronous "execution of events" is not well defined. We propose an event monitor to resolve this problem.