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 high-priority 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.
