Real-time performance evaluation of urgent aperiodic messages in FF communication and its improvement

In distributed control system (DCS), computation tasks, which usually reside in different nodes and communicate with one another to accomplish a common goal, have to be executed timely. Therefore, there is a need for special-purposed real-time industrial networks. Foundation Fieldbus (FF) is characterized by explicitly distinguishing between periodic and aperiodic messages. Centralized Media Access Control (MAC) is utilized by FF to support periodic messages, and distributed MAC to support aperiodic messages. It is indicated that FF's current mechanism using Pass Token (PT) priority and PT circulation period cannot effectively guarantee the real-time requirement of important aperiodic messages. A detailed analysis shows the method of computing Actual PT Circulation Period (APTCP) should be improved. An improved method of computing APTCP is proposed in this paper, and then a method of determining Setting PT Circulation Period (SPTCP) is presented accordingly. In the end, simulation validates the effect of the improved method on the real-time performance of urgent aperiodic messages in FF.