As seen in the above illustration from National Instruments, RS-485 ports are identical in appearance to RS-232 but with different pin assignments. You will not find them on contemporary home computer hardware, and they are only rarely seen on legacy machines. They are, however, found on various types of commercial computer and telecom equipment and if you want to explore such devices you’ll need a way to communicate with RS-485 ports. (For example, we recently encountered one while exploring a Harris Communications OpenSky base station controller/transceiver.)
According to the Wikipedia entry:
EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the electrical characteristics of drivers and receivers for use in balanced digital multipoint systems. The standard is published by the ANSI Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA). Digital communications networks implementing the EIA-485 standard can be used effectively over long distances and in electrically noisy environments. Multiple receivers may be connected to such a network in a linear, multi-drop configuration.
You can read more details on RS485 design basics in this 3 page PDF from EETimes Asia.
When hacking commercial and industrial hardware you may encounter RS-485 ports and need a quick and easy way to interface with a PC. If you have access to MAX485 converter chips and an Arduino you can use the circuit and methods described in Paddy’s Blog for interfacing RS485 and Arduino.
Of course, if you don’t wish to build the interface there is always the RS485 Shield for Arduino from RobotShop.
In any event, when exploring a new hardware find, it’s important to remember that not every DB9 connector is an RS232 serial port.