在基金会现场总线网络中，设备之间通过虚拟通信关系VCR来传送信息。
Devices in the FF network can communicate with each other through VCR.
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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