Stochastic model of the profibus DP cycle time

Profibus; DP is a fieldbus protocol included in most important international standards relevant to industrial communication systems. The protocol specifies the presence of two types of device on the network: masters and slaves. The operation of Profibus DP is based on the repetition of a cycle during which every master polls its slaves and performs some possible acyclic activities. The duration of such a cycle, known as Profibus DP cycle time, may not be constant for two different reasons: retransmissions of the frames used to exchange data which are either corrupted or lost, and the acyclic activities. An analytical description of the Profibus DP cycle time is given, taking into consideration the random variables that make up it. A detailed stochastic description is obtained for each of such random variables. The results allow one to calculate, under some simplifying hypothesis, the stochastic description of the jitter that may affect the Profibus DP cycle time in many practical applications. A numerical example of application of the obtained results is included.