• Modbus功能码
• 标准的modbus协议组成(RTU和ASCII)
• Modbus TCP/IP协议组成
• MBAP头消息解析
• Modbus正常解析流程
• Modbus异常解析流程
• 缩写
• 错误码
• 参考资料

Modbus功能码

• 功能码，Modbus功能码有三种，第一种是公共功能码(Public Function Codes),第二种是 用户自定义功能码(User-Defined Function Codes),第三种是保留功能码(Reserved Function Codes), 每部分的定义如下

• 其中公共功能码如下：

标准的modbus协议组成(RTU和ASCII)

Modbus TCP/IP协议组成

MBAP头消息解析

Modbus正常解析流程

• The header is 7 bytes long:
  • Transaction Identifier - It is used for transaction pairing, the MODBUS server copies in the response the transaction identifier of the request.
  • Protocol Identifier – It is used for intra-system multiplexing. The MODBUS protocol is identified by the value 0.
  • Length - The length field is a byte count of the following fields, including the Unit Identifier and data fields.
  • Unit Identifier – This field is used for intra-system routing purpose. It is typically used to communicate to a MODBUS+ or a MODBUS serial line slave through a gateway between an Ethernet TCP-IP network and a MODBUS serial line. This field is set by the MODBUS Client in the request and must be returned with the same value in the response by the server.
  • All MODBUS/TCP ADU are sent via TCP to registered port 502.

Remark : the different fields are encoded in Big-endian.

Modbus异常解析流程

缩写

错误码

参考资料

Modbus官方文档:《Modbus_Application_Protocol_V1_1b3》