RS-485

RS-485 or EIA-485( ISO 8482) is an industrial bus interface. It is a differential interface, which transmits the data serially via the symmetrical lines. Due to the symmetrical line structure, the data cable is less sensitive to electromagnetic radiation.

The RS485 interface is available as 2- wire and 4-wire interface. The 2-wire interface operates in half- duplex. It has a twisted pair of wires for the data and for the common ground. In the 2-wire version, the signals are transmitted ternary with three levels. The 4-wire operates in duplex with two twisted pairs of wires, one each for the transmitted signal (transmit) and the received signal (receive).

Up to 32 transmit and receive devices can be connected to the RS485 bus system in push-pull mode. In order to be able to achieve this number, the input impedance of the connectable devices must be at least 12 kilo-ohms. The impedance of this 2-wire interface is 120 ohms. RS485 operates in half-duplex, where any connected device can communicate with any other.

The RS485 interface is defined for a length of 500 m, but it can be increased up to 1.2 km with symmetrical line drivers. The transmission speed is strongly dependent on the line length of the balanced cable, which must be terminated at both ends with the line impedance. For short cable lengths of 10 m, data rates of 10 Mbit/s are achieved; for cable lengths of more than 1 km, these rates are 100 kbit/s.

With large distances between RS-485 devices, potential differences between devices connected far away from each other cannot be excluded because of the different ground potential. Such problems can be eliminated by optocouplers.