RS-485 is half-duplex 4-wire serial communication standard. Though it can be made into a full-duplex 3-wire system.

Whereas the maximum transmission distance for RS-232 is officially 50ft (100-250ft is possible with good quality equipment, cabling, and some luck), the maximum transmission distance for RS-485 is just under a mile at 4,000. Rs-485 accomplishes this by removing the common ground wire and replacing the RX and TX wires with RX and TX pairs. In RS232 a single bit is transmitted down the line by pulling the TX wire up to 3v relative to the common ground. This is called absolute voltage level because in order for the receiving device to properly detect the signal it must be above 3v. With RS232's elimination of the ground wire, and usage of TX and RX pairs (TX+ and TX-, etc), it need only achieve a 3-5v difference between the + and - pairs. This combined with twisted pair cabling allows for RS485 to hit distances and transmission speeds that are unheard of with RS232. The transmission speeds for RS485 range from 10Mbs at 40ft, to 100kbs at 4,000 ft. Even at its slowest, RS485 is orders of magnitude faster than RS232.

RS-485 also takes RS-422's one-to-many topology to the next step, creating a small netowork of up to 32 devices that can communicate with each other over a single cable. A "Master-Slave" configuration is common where a single device is configured as the master, regulating the communication among all the other devices to prevent collisions and providing the bias. Common Master-Slave configuration puts the master as close to the center of the line as possible, with two "slaves" providing the termination at either end, though each application is different.

A common application for RS-485 is on aircraft, where instruments and other devices communicate with each other over a single wire, providing significant weight savings over traditional one-to-one or hub topologies.

Other applications include automation, where RS485 provides the physical link for many common automation protocols used in Industrial Control Systems, such as Modbus or Profibus. Because of its differential nature, RS-485 resists the electromagnetic interference commonly generated from automated equipment's motors or welding systems.

Additional applications include building security, where RS485 might provide the physical link for entry, lighting, and camera controls, sensors, and other devices where the simple bus design and long cable runs trumps dedicated wiring.

The RS/EIA-485 specification does not designate any connector or pin-out. Most systems utilize DB9 ports for simple integration and assembly, as well as common availability of cabling. Screw terminals are also a common configuration.