Frame Specifications - Mitsubishi Electric QJ71MB91 User Manual

4
MODBUS STANDARD FUNCTIONS

4.2 Frame Specifications

The following shows the frame specifications for the MODBUS protocol.
Address field
Area name
Address field
Function code
Data
*1
Error check
Remark
Function code
MODBUS Protocol Data Section
Section 4.3 to Section 4.20
Figure 4.1 Frame specifications
Table4.4 Frame specifications
[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.
[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.
[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.
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 message is correct or not.
The message is discarded if it has an error.
* 1 The error check method differs depending on the frame mode.(
Refer to the following for the data size of each area.
Section 4.2.1
Data Error check
Description
Section 4.2.1)
4.2 Frame Specifications
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