The effect of pass token period on realtime communication of urgent aperiodic message and its improvement

The key to success in using distributed control system (DCS) is the timely execution of computational tasks which usually reside on different nodes and communicate with one another to accomplish a common goal. Therefore, the temporal property of the underlying network is important, and special-purpose realtime industrial network is needed. Foundation fieldbus (FF), which is characterized by explicitly distinguishing periodic and aperiodic message, is one of the network kinds. FF applies centralized media access mechanism (MAC) to support periodic message, and distributed MAC to support aperiodic message. As for aperiodic message, this paper indicates FF´s current mechanisms can´t effectively obtain its expected object of guaranteeing important aperiodic message although PT priority and PT rotation period are applied. Furthermore, detailed analysis expresses method of counting PT rotation period is the main cause. Then, an improved method is proposed, and accordingly, a method of setting PT rotation period is deduced. A numerical case validates the effects of the improved method.