![]() The second byte of every Modbus message is a function code. Slaves must specify their own id in every response message: 01 03 02 00 01 25 CA The master specifies the id of the slave to which the request message is addressed. The first byte of every Modbus message is a slave id. Slave id function function-specific data CRC ![]() Let’s see what’s inside the frame: 01 03 02 00 01 25 CA The whole message in our case is seven bytes long. Those are 8-bit hexadecimal characters that are sent over Modbus RTU network. But first lets examine the basic structure of a request frame: 01 03 02 00 01 25 CA The length and the contents of the data frame vary based on the type of read/write operation being performed. A response is a message from the slave back to the master. A request is a message from the master to a slave. Modbus RTU Data FrameĪ Modbus data frame is a message transmitted over Modbus network. The drawback is that TCP/IP stack is much more difficult to support in some types of field devices where Modbus RTU would work fine.
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