Deterministic and Real-Time Communication over Ethernet in EPA Control System

Due to the low price and robustness resulting from its wide acceptance and deployment, ISO/IEC 8802-3 based Ethernet is becoming the de-facto network in DCS and PLC, and is being attempted to connect field devices. It is difficult to build a real-time field control network based on Ethernet because it has unpredictable delay characteristics due to packet collisions. To resolve this problem, some papers and reports presented the so-called hardware methods to improve real-time performance but they changed the circuits of Ethernet. Some other authors designed the so-called software methods (e.g. static or dynamic traffic smoothers by giving Ethernet packets priority) which only reduced the packet collision without avoiding collision. In addition, these methods are designed based on some presuppositions or preconditions and are difficult to be implemented and used in real control systems. Considering that some real-time packets can be divided in two types: (1) cyclic packets (e.g. measurement and control data) which shall be transferred at each communication cycle, and (2) acyclic packets (e.g. configuring data and alert/alarm data) which are transferred randomly, we define a EPA-based Communication Scheduling Management Entity (EPA_CSME) beyond Ethernet MAC protocol referring to ISO/OSI model to avoid collisions in Ethernet system. With the precise time synchronization based on IEC61588 protocol with the same time server, each node has the same absolute time in the whole system.