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Schedulability Analysis in the FIP Fieldbus Accounting for Inserted Idle-Time

The FIP fieldbus (Factory Instrumentation Protocol, presently known as WorldFIP) uses, normally, an off-line scheduler that generates a cyclic static table containing the allocation of bus-time slots to the transaction of process control variables. Such table is organized in time-windows, called elementary cycles, within which the transactions are placed by the scheduler. The table is scanned on-line to timely start the respective transactions. When the FIP fieldbus is used in the synchronous mode (probably its most typical operational mode), the elementary cycles have a fixed duration. The fact that this duration is not correlated to the durations of the transactions causes the appearance of inserted idle-time whenever a transaction does not fit within a given elementary cycle. The presence of such idle-time prevents the direct usage of traditional preemptive schedulability analysis. This paper presents one sufficient and another necessary and sufficient schedulability conditions based on worst-case response times. Both conditions can be applied with any fixed priorities scheduling criterion and consider different transactions’ durations. Also, both conditions are well suited to on-line schedulability analysis, an important issue when a planning scheduler is used to allow on-line changes to the set of periodic process variables.