Modbus RTU issues?
En_C: Set to enable connection, I would recommend a variable (coldretained)
Channel: In your case '0.3'
Partner: Identity of slave
Id: Create new variable that will be reused on the other FB:s
Id: The variable created above
Req: Oscillator or other request
StartAddr: Address of the first register you want to read
MBWrite works like MBRead, only difference is sending instead of reading
Also verify that baudrate, parity, databits and stopbits (on COM3) matches your slave.
The flow control settings depends on your physical connection and wiring.
It is wise to use the "Valid" from the Connect block in the enable expression and the "Error" from the read/write block.
Req:= Valid and Req and not Error
This stops the read/write if the Connect is down and prevent that the connect is disabled before all requests has been answered by the slave.