In the information on Victron Energy ET430 online manual fails to mention that the ModBus should be correctly terminated by connecting the EM340 pin 7 to Pin 9. When this is done you should be able to measure 120 ohms resistance between the ModBus pair (without the cable connected). The Master end (RS485 USB adaptor) should also be terminated in 120 ohms, again it's easy to check with a multimeter. By making sure each end of the RS485 cable is correctly terminated the Modbus transmission line now will reduce any errors which may be caused by pulse reflections and also reduces the impedance of the line which lowers the susceptibility to any interference.