分析了两个目标层：数据链路层和现场总线访问子层。

Secondly two layers are described in the thesis: the data link layer(DLL) and the fieldbus access sublayer(FAS).

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研究现场总线访问子层为通信提供的虚拟通信关系的工作情况和内部协议状态机。

These descriptions include operations of virtual communication relationships which are provided by the FAS for communication, and FAS protocol state machines.

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