MemberSeptember 4, 2020 at 12:56 pm
have you tried to run the Modbus RTU communication with your slave device from PC? It is always good (and recommended) first step – just take a RS485 USB adapter, use some free Modbus RTP Master application and try to run the Modbus communication with this setup. Then you are sure that everything is OK at the Slave side (protocol, address, baudrate, supported services, register number …) and you can try it with PLC.
And from the HW point of view – make sure that the GND is also interconnected.