Using a planning scheduler to improve the flexibility of real-time fieldbus networks

Abstract A typical approach to real-time fieldbus arbitration is to use an off-line scheduler that generates a cyclic static table containing the allocation of bus-time-slots to the transaction of process-control variables. This approach, e.g. as used in the Factory Instrumentation Protocol fieldbus, is rather inflexible in the sense that any system changes, such as the addition of a sensor, requires an interruption of the fieldbus operation. In this paper the use of a planning scheduler is proposed to overcome such inflexibility. This scheduler compromises between the advantages and disadvantages of typical dynamic and static scheduling. A sufficient schedulability condition is also derived, in order to overcome the typical inability of dynamic (or even planning) schedulers to guarantee schedulability for long-term system operation. The evaluation of this condition incurs very small run time overhead, and can therefore be used with advantage in a fieldbus system that relies on the planning scheduler. An experimental test is described, to illustrate how the planning scheduler works.