RS-485 (EIA-485) - the standard for asynchronous physical layer interface. Regulates the electrical parameters of the half-duplex differential multipoint link such as "common bus". 
Implementing ─ Half way communication device at a time can either transmit or receive information. Typically, such a device is based on the transceiver circuit. 
Network built by RS-485 interface is a transceiver connected via twisted pair cable - two twisted wires. At the heart of the RS-485 based on the principle of differential (balanced) data transmission. Its essence lies in the transfer of a signal on two wires. And one wire (conventionally A) is the original signal, and on another (conditionally B) - its inverted copy. In other words, if one wire "1", the other "0" and vice versa. Thus, between the two wires of the twisted pair is always a difference in potentials at the "1" is positive, while "0" - negative. 
It is this potential difference and the signal is transmitted. Such a method of transmission ensures high resistance to common mode noise. Common mode interference is called acting on both wire line equally. For example, the electromagnetic wave passing through the portion of the communication line, leads to the potential for both wires. If the signal is transmitted on one wire potential relative to the total, as in RS-232, then laying on the wire can distort the signal is relatively well absorbed total crosstalk ("land"). In addition, the resistance of the long common wire will decrease the potential difference of land - an additional source of distortion. A differential gear when distortion occurs. In fact, if two wires lie close to each other, but still intertwined, the subject is in the same two wires. Potential weighted equally in both conductors equally changed while the potential difference informative remains unchanged.