The Profibus is a token-based communication protocol widely used in distributed process control systems to support time-critical communication between field devices (sensors, actuators, controllers). At the field level the transmission dynamics of the stations may differ quite significantly according to the dynamics of the processes supported, but the Profibus protocol, even if it distinguishes between high-priority and low-priority traffic, cannot take the transmission dynamics of the stations into account. Thus, if the stations featuring highpriority traffic have greatly differing transmission dynamics, the token rotation speed is set according to the station with the fastest dynamic, and all the stations receive the token with the maximum frequency. Thus, at each rotation the token will be passed to a station even if it does not have high-priority traffic to transmit, and this causes drawbacks. To avoid them, the token should be passed to a station when it really has data to transmit, and more frequently to stations with faster dynamics. To this aim, this paper presents a strategy to modify the token passing policy, called the Rotating Ring, and compares its performance with that of the Profibus standard.
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