How to physically connect a voltage monitoring meter device(ND20) with RS485 out, to a computer with Modbus RTU? Should I use a converter to RS232 and connect to serial COM port of computer? Should I use RS485 to USB converter to connect? Can I connect directly to serial COM port from A,B and Ground of RS485 using D9 connector pin? Then A,B and Ground to which pins of D9 in Modbus RTU?
If your computer doesn't support RS485 protocol, and if we're talking about regular PC's it most probably doesn't, then you cannot directly connect A and B to serial port. I would suggest using an USB-RS485 converter.
Serial port on almost all IBM compatible PCs is of type RS232. You should check your motherboard's manual. If you MB is from an industrial manufacture (e.g. Advantech or MOXA ) most likely you MB has a RS485 port. And this just the beginning! And then you need some sort of Modbus software that can talk to your device.