Using the planning scheduler in real-time fieldbuses: theoretical model for run-time overhead

A typical approach to real-time fieldbus arbitration is the use of an off-line scheduler that generates a cyclic static table containing the allocation of bus-time-slots to the broadcasting of process control variables. The use of a planning scheduler is proposed instead. This scheduler compromises between the advantages and disadvantages of typical dynamic and static scheduling. Particularly, it allows for higher operational flexibility than the traditional cyclic static approaches and presents a lower run-time overhead than the traditional dynamic approaches. A possible implementation of the planning scheduler is described and a theoretical model is presented for the runtime overhead caused by the scheduler on an average-case assumption. The validity of the model is checked against the results of several experiments conducted on an experimental low-cost real-time fieldbus.