Analysis of polling protocols for fieldbus networks

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Abstract
Roll-call polling mechanisms are considered for fieldbus networks. Based on a producer/consumer communication model, two different protocols are presented, namely the standardized FIP (Factory Instrumentation Protocol) and a modified FIP protocol that we propose here. The modified protocol uses redundancy to eliminate local timers used by the FIP protocol. The two protocols are analyzed for efficiency, jitter and robustness. We compare the two protocols based on implementation and analytical results. Practical aspects concerning: clock precision; computing time; and hard real-time constraints are also discussed. The results show that: (1) the original FIP protocol is fractionally more efficient with respect to its execution time than the modified one; (2) the modifications proposed, however, reduce the jitter and improve the robustness considerably; (3) the slot time of the original FIP protocol has theoretical upper bound but the modified protocol has no such limitations.