4. Modbus RTU
4 - 5
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)
Response Message
Query Message
Slave
(servo amplifier)
Address
Function
Data
Error Check
Address
Function
Data
Error Check
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
Address
Function
Data
Error Check
END
L
H
3.5 bytes
8 bits
8 bits
n × 8 bits
8 bits
8 bits
3.5 bytes
Message field
Size
Communication
path
Description
Address
8 bits
Master → Slave
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.
Slave → Master
The station number of a slave (servo amplifier) is returned.
Function
8 bits
Master → Slave
Set a function code.
Set a function code to request to the slave.
Slave → Master
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.
Data
n × 8 bits
Master → Slave
The format changes depending on the function code selected.
Refer to section 4.8 for details.
Slave → Master
Error Check
16 bits
Master → Slave
Send data to perform the CRC check of a received message frame.
Slave → Master
Summary of Contents for MR-JE-C Series
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