Modbus Rtu Master / Slave Communication - Panasonic FP0H User Manual

Unit Functions and Restrictions on Combination

1.2.3 MODBUS RTU Master / Slave Communication

 Overview of function
• Used for communication with other devices that support the MODBUS RTU protocol.
• Master communication is performed when the master unit sends commands to slave units,
and the slave unit returns responses (response messages) according to the instructions.
Data can be read and written between PLC and external devices by specifying unit numbers
and memory addresses and executing F145 (SEND) / F146 (RECV) instructions in a user
program, because PLC automatically creates messages according to the protocol.
• A higher device connected to PLC has the transmission right in slave communication. Slave
communication is performed when the device sends a command, and PLC returns a
response. In slave communication, no communication program is required on the PLC side,
because PLC automatically returns a response.
• The data size that can be sent or received at a time is a maximum of 127 words for register
transmission, and 2,040 bits for bit transmission.
Master function
Slave function
 Purpose of MODBUS-RTU master communication (example)
Used for connection with devices that support the MODBUS-RTU protocol.
• Temperature regulator KT series
• Devices that support MODBUS-RTU from other manufacturers
 Purpose of MODBUS-RTU slave communication (example)
Used for access from a higher device using MODBUS-RTU commands.
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