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

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 appro ach, e.g, as used in the Factory Instrumentation Protocol fieldbus, is rath er inflexible in the sense that any system changes, such as the addition of a sensor, requires an interruption of the fieldbus operation. In this pape r the use of a planning scheduler is proposed to overcome such inflexibilit y. This scheduler compromises between the advantages and disadvantages of t ypical 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 over head, and can therefore be used with advantage in a fieldbus system that re lies on the planning scheduler. An experimental test is described, to illus trate how the planning scheduler works. (C) 1999 Elsevier Science Ltd. All rights reserved.