Modbus Rtu Communication Message Frame - Mitsubishi Electric MR-JE-C Series Instruction Manual

4. Modbus RTU

4.7 Modbus RTU Communication Message Frame

Query Messages sent from the master (controller) and Response Messages sent from the slaves (servo
amplifiers) are both sent in the following message frame format.
Master
(controller)
A message frame is composed of four message fields.
When a slave (servo amplifier) received a Query Message sent from the master (controller) without errors,
the Function Code in the Query Message is copied into the Function in a Response Message. When a slave
(servo amplifier) received a Query Message with an error, the slave returns a value obtained by adding "80h"
to the Function Code value in the Query Message to the master. Judge the occurrence of an error by
checking the Function Code in the Response Message on the controller side.
During the dataless time of 3.5 bytes before and after the reception of a Query Message, the servo amplifier
recognizes the received Query Message.
Message frame
START
3.5 bytes
Message field
Address
Function
Data
Error Check
Address
Function
Data
Error Check
Query Message
Response Message
Address
Function
Data
Error Check
Address Function
8 bits 8 bits
Communication
Size
path
Master → Slave
8 bits
Slave → Master
Master → Slave
8 bits
Slave → Master
Master → Slave
n × 8 bits
Slave → Master
Master → Slave
16 bits
Slave → Master
Slave
(servo amplifier)
Data
L
n × 8 bits 8 bits
Set a station number.
Set a value within 0 to 247 with 1-byte length (8 bits).
When "0" is set, the broadcast communication is executed.
The station number of a slave (servo amplifier) is returned.
Set a function code.
Set a function code to request to the slave.
Send the function code requested by the master.
When a communication error has occurred, send a value obtained
by adding "80h" to the function code requested by the master.
The format changes depending on the function code selected.
Refer to section 4.8 for details.
Send data to perform the CRC check of a received message frame.
4 - 5
Error Check
END
H
8 bits 3.5 bytes
Description