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5. MODBUS COMMUNICATION PROTOCOL
The master controls communication between master and slave. A typical message consists of a request (query message) sent from the master followed by an answer (response message) from the slave. When master begins data transmission, a set of data is sent to the slave in a fixed sequence.
When it is received, the slave decodes it, takes the necessary action, and returns data to the master.
5.1 Message Format
The message consists of four parts: slave address, function code, data, and error check code which are always transmitted in the same sequence.
Slave address
Function code
Data
Error check CRC-16
Message format
The slave address is a number from 1 to 99 manually set at the front key panel of the controller.
For details, see 3.3 Device Address (Slave Address) Setting (P. 7).
Although all connected slaves receive the query message sent from the master, only the slave with the slave address coinciding with the query message will accept the message.
The function codes are the instructions set at the master and sent to the slave describing the action to be executed. The function codes are included when the slave responds to the master.
For details, see 5.2 Function Code (P. 31).
The data to execute the function specified by the function code is sent to the slave and corresponding data returned to the master from the slave.
For details, see 5.6 Message Format (P. 35), 5.7 Data Configuration (P. 38) and
5.8 Communication Data List (P. 40).
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An error checking code (CRC-16: Cyclic Redundancy Check) is used to detect an error in the signal transmission.
For details, see 5.5 Calculating CRC-16 (P.33).
IMR01D02-E3
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