Instruction manual

IMR01H02-E3

6. 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.

6.1 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

Slave address

The slave address is a number from 1 to 99 manually set at the front key panel of the controller.

For details, see 4.2 Setting the Communication Parameters (P. 9).

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.

Function code

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 6.2 Function Code (P. 45).

Data

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 6.6 Message Format (P. 49), 6.7 Data Configuration (P. 53) and

6.8 Communication Data List (P. 55).

Error check

An error checking code (CRC-16: Cyclic Redundancy Check) is used to detect an error in the signal

transmission.

For details, see 6.5 Calculating CRC-16 (P. 47).