Functions of Units and Restrictions on Combination
1-6
1.2.3 MODBUS RTU Master/Slave Communication
Overview of function
This is used for communicating with other devices that support the MODBUS RTU protocol.
In master communication, communication is performed when the master unit sends
instructions (command messages) to slave units and the slave unit returns responses
(response messages) according to the instructions. Messages in accordance with the
protocol are automatically generated by PLC. In the user program, reading and writing can
be done simply by specifying the station no. and memory address and executing
SEND/RECV instructions.
Slave communication is performed when the higher device connected to PLC has the
sending right, and sends commands, and PLC returns responses. In slave communication,
PLC responds automatically, so no program concerning communication is necessary on the
PLC side.
The data size that can be sent or received in a single communication is up to 127 words for
register transmission and 2,040 bit for bit transmission.
Master function
Slave function
Examples of applications of MODBUS-RTU master communication
This is used for connection with a device that supports the MODBUS-RTU protocol.
Thermoregulator KT series
Devices from other manufacturers that support MODBUS-RTU
Examples of applications of MODBUS-RTU slave communication
This is used when access is made from the higher device using MODBUS-RTU commands.
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