YOKOGAWA IR202, User Manual

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<3. MODBUS PROTOCOL>
3-1
IM 11G02Q02-01EN 2nd Edition : Sep. 04, 2020-00
3. MODBUS PROTOCOL
According to MODBUS protocol, devices communicate using a master-slave technique, in whch
only one device (the master) can initiate transactions. The other devices ( the slaves) respond by
supplying the requested data to the master, or by taking the action requested.
<Message transaction between Master and Slave>
(1) The master station sends out a command message to a slave station.
(2) The slave station tries to identify the station No. in the received message with its own station
No..
(3) If the slave station finds those numbers matched, it executes the action requested in the
received command and sends back a response message.
If those numbers are mismatched, the slave station leaves the command message and
waits for next command message

When the station No. in the received command message matches with the own slave station
Command message
Response message
Slave to master
Master to slave
Data on
the line

When the station No. in the received command message mismatches with the own slave station No.
Command message
(Not respond) Slave to master
Master to slave
Data on
the line
The master station can individually communicate with any one of slave stations connected on the
same line upon setting the station No. in the command message.
3.1 Composition of message
Command message and response message consist of 4 fields ; Station No., Function code, Data
and Error check code, which are sent in the following order:
Station No. (1 byte)
Function code (1 byte)
Data (2 to 133 bytes)
Error check code (CRC-16)(2 bytes)
Figure.3-1 Composition of message
(1) Station No.
Station No. is slave address or a number specifying a slave station. Station No. are set in the
maintenance mode display. Only a slave with the same number as this Station No. set in the
maintenance mode can respond to the message and take action requested in the received
message.
(2) Function code
This is a code to designate a function to request slaves to execute. For details, refer to section
“3.3 Function code”.