Dwyer UXF3 - Manual (Page 12)

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5. MODBUS COMMUNICATION PROTOCOL
5.1 General
The communication system by the MODBUS protocol is that the communication is always started from the master
station and a slave station responds to the received message.
Transmission procedures is as shown below.
1) The master station sends a command message to a slave station.
2) The slave station checks that the station No. in the received message matches with the own station No. or
not.
3) If matched, the slave station executes the command and sends back the response message.
4) If mismatched, the slave station leaves the command message and waits for the next command message.
a) In case when the station No. in the received command message matches the slave station No.
Command message
Response message Slave to master
Master to slave
Data on
the line
b) In case when the station No. in the received command message mismatches the slave station
No.
Command message
(Not respond) Slave to master
Master to slave
Data on
the line
5) To assure safety, provide a structure where the response message is checked and retry is made three (3)
times or more if no response is made or an error occurs.
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.
Bulletin F-107-UXF3-S