RS485 interface gives some more benefits compared to RS232: simplicity and the robustness for long distance transfer. RS485 is simpler because it uses only two wire (or a single pair) to carry out the data in bidirectional manner, using RS232, we need 3 wires at minimum to carry the data bidirectionally. RS485 cable length can be as long as 1.2 km, while RS232 only about 25 meters.

RS232 to RS485 Converter

To connect an RS485 device or RS485 network to any PC or controller, the PC or the controller should provide RS485 interface also. If your PC or controller only has RS232 port then you need an RS232 to RS485 converter or adapter for your PC or controller.

RS485 Circuit

The RS232-RS485 adapter circuit diagram is show below, RS232 DB9 is used as the RS232 port, while only a terminal block is used as RS485 connector. A twisted pair is ideal for RS485 cable, since the twisting make both wires (+ and -) exposed to an identical noise figure, thus canceling the noise itself when received at the interface port.

rs232 rs485 adapter circuit schematic

Electrical Insulation Using Optics

Especially when the transmission is carried over a long wire outdoor, the wires is potential to carry the dangerous signal like lightning or other high voltage induction. This can be fatal to your PC or controller. The circuit uses optical isolation (opto-coupler) to isolate your RS232 port from the RS485 cables.