User`s manual
43
FX3U Series Programmable Controllers
User’s Manual - MODBUS Serial Communication Edition
7 MODBUS Standard Commands
7.2 Frame Specifications
1
Outline
2
Specifications
3
System
Configuration
4
Wiring
5
Communication
Setup
6
Related
Devices and
Comm. Status
7
MODBUS
Standard
Commands
8
Master
Specification
9
Slave
Specification
10
Creating
Programs
7.2 Frame Specifications
The following shows the frame specifications for the MODBUS protocol.
The following table details the frame specification for the MODBUS Protocol.
*1. The error check method differs depending on the frame mode. See Subsection 7.2.1.
Note
Refer to the Subsection 7.2.1 for the data size of each area.
Area name Description
Address field
[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.
[When the slave sends a response message to the master]
The host station number is stored when sending a response message.
Note: 247 is the MODBUS maximum address number. The FX
3U MODBUS Master can
address from 1 to16 stations within this range.
Function code
[When the master sends a request message to a slave]
The master specifies the number of the action to be taken by the slave.
[When the slave sends a response message to the master]
A requested function code is stored in the case of normal completion. The most
significant bit turns ON in the case of error completion.
Data
[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.
Error check *1
The node 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.
DataAddress field Function code Error check
MODBUS Protocol Data Section
Section 7.3 to section 7.18