Chapter 4 Modbus Communication Specifications; Modbus Protocol - Mitsubishi Electric MELSEC iQ-F FX5 User Manual

4
MODBUS COMMUNICATION SPECIFICATIONS
This chapter explains the details of MODBUS Protocol for MODBUS serial communication and the MODBUS standard
functions supported by the FX5.
• For the list of supported MODBUS standard functions, refer to Page 21 List of supported MODBUS standard functions.
• For usage of the supported MODBUS standard functions, refer to Page 32 FUNCTION.
4.1

MODBUS Protocol

The following shows the frame specifications for the MODBUS protocol.
Address field
*1 For details of the MODBUS protocol data section, refer to Page 65.
The following table details the frame specifications for the MODBUS Protocol.
Area name
Address field
Function code
Data
Error check
*1 The address number that can be used in FX5 master station differs depending on the version. (Page 77 Added and Enhanced
Functions)
Refer to Page 21 Frame mode for the data size of each area.
4 MODBUS COMMUNICATION SPECIFICATIONS
20
4.1 MODBUS Protocol
Function code
MODBUS Protocol Data Section
Description
[When the master sends a request message to a slave]
0: Sends a request message to all the slaves. (Broadcast)
1 to 247: Sends a request to a specific Slave number.
Note: 247 is the MODBUS maximum address number.
[When the slave sends a response message to the master]
The host station number is stored when sending a response message.
[When the master sends a request message to a slave]
The master specifies the function code to the slave.
[When the slave sends a response message to the master]
The requested function code is stored in the case of normal completion. The most significant bit turns ON in the case of abnormal
end.
[When the master sends a request message to a slave]
The information needed to execute the action specified by a function code is stored.
[When the slave sends a response message to the master]
The execution result of the action specified by a function code is stored. An exception code is stored when failed.
The node (master or slave) adds the check code automatically to all transmitted messages and recalculates the check code for any
received message. The received message is discarded if it has an error.
Data Error check
*1
*1