Ethernet-based real-time control data bus

Wendelstein 7-X is designed as a steady-state experiment to demonstrate the fusion reactor relevance of the advanced stellarator concept. The experiment's control and data acquisition will be performed by a distributed system of computers and programmable logic controllers (PLCs). Data of several systems have to be combined flexibly to control the machine, requiring a data exchange on a millisecond time scale between several connected units. A discharge can last up to half an hour, thus, the connections may vary during a discharge. Hence, it is desired to provide control relevant data, e.g., measurands, set points, interlock signals, cyclically via a bus system. The paper will analyze the special quality of control data streams and deduce the basic requirements for a real-time data bus. Ethernet is a candidate for the data bus since it is a widely used broad-band bus with a foreseeable potential for development. Because of its nondeterministic arbitration algorithm Ethernet is generally considered not to be suited for hard real-time applications. It is possible to circumvent this disadvantage of Ethernet either by using switching techniques or a software token to obtain a reliable base for hard real-time data transport.