In automated manufacturing plants, and especially in distributed process control systems, Ethernet generally supports communication related to activities which require bandwidth exploitation and throughput maximisation, but no real-time guarantees. This is due to its contention-based medium-access control protocol, which cannot provide individual nodes with a predictable medium access time. However, in the last few years, increasing interest has arisen in the use of Ethernet networks even at the field level, where time-critical communication between sensors, controllers and actuators is currently supported by suitable bus-based networks called Fieldbuses.

In this paper we discuss the feasibility of using Ethernet technology to support real-time communication at the field level. We will therefore examine Ethernet behaviour in different working conditions and evaluate its limits to ascertain whether these limits really prevent the use of Ethernet in process control applications. We will also investigate the parameters which most affect the real-time performance of Ethernet.

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