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Real-Time Fieldbus Communications Using Profibus Networks
Ref: HURRAY-TR-9823       Publication Date: Dec 1999

Abstract:
This paper provides a comprehensive study on how to use Profibus fieldbus networks to support real-time industrial communications, that is, on how to ensure the transmission of real-time messages within a maximum bound time. Profibus is based on a simplified Timed Token (TT) protocol, which is a well-proved solution for real-time communication systems. However, Profibus differs with respect to the TT protocol, thus preventing the application of the usual TT protocol real-time analysis. In fact, real-time solutions for networks based on the TT protocol rely on the possibility of allocating specific bandwidth for the real-time traffic. This means that a minimum amount of time is always available, at each token visit, to transmit real-time messages. Conversely, with the Profibus protocol, in the worst-case, only one real-time message is processed per token visit. We propose two approaches to guarantee the real-time behaviour of the Profibus protocol: an Unconstrained Low Priority Traffic profile and a Constrained Low Priority Traffic profile. The proposed analysis will show that the first profile is a suitable approach for more responsive systems (tighter deadlines), whilst the second allows for increased non-real-time traffic throughput.

Eduardo Tovar

,

Francisco Vasques

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