EIA-485 (originally RS-485) is a serial communication standard defined in 1983 by EIA association. It is used mainly in industrial environments. RS-485 standard is designed to establish a two-wire half-duplex multi-point serial link. It has the same basis as RS-232, from which it differs in signal levels, the absence of modem signals and the possibility of networking of up to 32 devices. The serial link can reach up to 1200 meters. The advantage of RS-485 is also the possibility to create the RS-485 link from widespread standard RS-232 using simple level converters.
Sample of transmission of a character No. 211 (D3 in hexadecimal, 11010011 in binary code) is shown in Fig. 9.2. Firstly, the start bit is transmitted, then 8 bits starting with LSB, no parity, and the sequence is finished by the stop bit.
RS-485(same as RS-422) provides connection of the communicating units over a pair of conductors. These wires are labeled by letters A and B, sometimes also as "-" and "+". In the idle state, voltage should be lower on the wire labeled as A (or "-") in comparison to the wire labeled as B (or "+"). Even when working with the differential voltage, in the case of connections over longer distances it is necessary to connect also the ground (GND, G), not only the signal lines (+ RxTx and RxTx-) of the communicating devices (see e.g. Modbus specification). The reason is that in remote locations, there can be significant difference in the potential of the ground.
When communicating over long distances, terminal resistors should be applied on both sides of the line. The purpose of terminal resistors is to avoid reflections from the ends of the signal line, and also to increase the endurance of the line against interfering signals. Terminal resistors should ideally have the value of 110 Ω (i.e. image impedance), so that the resulting impedance of the line would be 55 Ω (110 Ω || 110 Ω).