The effects of acyclic traffic on Profibus DP networks

Profibus DP is a very popular fieldbus included in the IEC 61158 International Standard. The features of such a fieldbus make it particularly suitable for cyclic operations, but it also foresees the possibility of performing acyclic activities between masters and slaves. A second (undesired) type of acyclic traffic is that generated by the access to services of the Profibus data link layer protocol (Fieldbus Data Link, FDL), by users different from the DP protocol. The acyclic traffic can heavily influence the network performances as it introduces a jitter on the cycle time which affects the cyclic operations. In this paper an analytical expression of the Profibus DP cycle time is calculated and the effects of both types of acyclic traffic are evaluated. Besides, a technique to eliminate the jitter and to synchronise the network operation is proposed. This solution, which requires only slight changes to the Profibus DP protocol, is compatible with the existing applications.