The MODBUS standard defines an application layer messaging protocol, positioned at level 7 of the OSI model that provides "client/server" communications between devices connected on different types of buses or networks. It standardizes also a spec

高级数据链路控制规程HDLC,数据链路层是OSI参考模型的第二层，它在物理层提供的通信接口与电路连接服务的基础上，将易出错的数据电路构筑成相对无差错的数据链路，以确保DTE与DTE之间、DTE与网络之间有效、可靠地传送数据信息。-High-Level Data Link Control protocols HDLC, Data link layer is a second layer of the OSI reference model, which provides the physical