4
MODBUS(R) STANDARD FUNCTIONS
4.2 Frame Specifications
4 - 10
1
OVERVIEW
2
SYSTEM
CONFIGURATION
3
SPECIFICATIONS
4
MODBUS(R) STANDARD
FUNCTIONS
5
FUNCTION
6
PRE-OPERATIONAL
PROCEDURES AND
SETTINGS
7
PARAMETER SETTING
8
UTILITY PACKAGE
(GX Configurator-MB)
4.2 Frame Specifications
The following shows the frame specifications for the MODBUS protocol.
* 1 The error check method differs depending on the frame mode.( Section 4.2.1)
Remark
Refer to the following for the data size of each area.
Section 4.2.1
Figure 4.1 Frame specifications
Table4.4 Frame specifications
Area name Description
Address field
[When master sends a request message to slave]
0: Sends a request message to all the slaves. (Broadcast)
1 to 247: Stores the target slave station No.
[When slave sends a response message to master]
The host station number is stored when sending a response message.
Function code
[When master sends a request message to slave]
The master specifies the number of the action to be taken by the slave.
[When slave sends a response message to 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 master sends a request message to slave]
The information needed to execute the action specified by a function code is stored.
[When slave sends a response message to 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 master adds a check code in a request message and transmits the request message.
The slave, which received the request message, recalculates the check code in the request
message and determines whether the messageis correct or not.
The message is discarded if it has an error.
Data
R
Address field Function code Error check
MODBUS Protocol Data Section
Section 4.3 to Section 4.20
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