On the effects of the 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 data exchange between master and slave devices. Nevertheless, Profibus DP explicitly foresees the presence of acyclic traffic on the network that is typically related to the transfer of alarm messages. However, an additional type of acyclic traffic may take place on Profibus DP networks, determined by access to services of the data link layer protocol (Fieldbus Data Link, FDL) by users different from the DP protocol. Both types of acyclic traffic may heavily influence the network performances as they introduce a jitter on the cycle time which affects the periodic operations. In this paper, an analytical model of the Profibus DP cycle time is proposed and the effects of both types of acyclic traffic are evaluated. Finally, some directions on how the network performances could be improved are given.