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The Modbus slave from the FAQ Modbus Communikation Master/Slave via Ethernet is used here.

Create a "Standard project" and select CODESYS Control Win V3 as the device.
Define the target system by means of the Network scan.
Insert an Ethernet adapter in the device tree and specify the interface to be used.

If a target system is not defined yet, then the error message "Gateway not configured" is displayed.
Insert a Modbus TCP Master below the Ethernet adapter in the device tree.
Insert a Modbus TCP Slave below the Modbus TCP Master in the device tree.
Insert it in the Modbus Slave Channel tab and set the properties as follows:

Adapt the POU PLC_PRG as follows:

Declaration

VAR
    xExec                :    BOOL;
    wMasterSend          :    WORD := 100;
    mbChannel            :    ModbusChannel;
END_VAR

Implementation

mbChannel(slave := Modbus_TCP_Slave, xExecute := xExec);

Assign the variable wMasterSend to the output Channel 0[0] in the tab ModbusTCPSlave I/O Mapping.

Please note, that under the ModbusTCPSlave I/O Mapping tab, the 'Always update variables' is set to 'Enabled 1' :

CODESYS FAQ > Modbus TCP Slave: Using the "Application" Channel Trigger > image-2022-11-7_9-32-44.png

See also our OLH for Basic settings with Fieldbus Devices and I/O Drivers.

Start the project and set the variable xExecute to TRUE so that the new value is passed to the slave.