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RS-485 is a half-duplex communication standard defining the electrical characteristics of drivers and receivers for use in serial communications systems. It is a differential protocol designed for use in environments requiring especially robust communication. Differential signaling is the method where information is transmitted using two complementary signals. The host converts a fundamental signal into a pair, the original, and its inverse (or complement). RS-485 requires the use of twisted-pair wiring where two wires are intertwined a specific number of times over the cable’s length. When noise couples onto a twisted-pair cable, it presents itself the same on each wire. When the receiver gets the complementary pair, a subtractor is used to find the difference between the two wires (hence the name differential), and because the added noise is the same on both wires, it is not present in the result of the equation, yielding only the fundamental signal. Because the receiver cares only about the difference between the two cables, the voltage levels of the system no longer matter. Therefore, a 3.3V system can work on the same system as 12V devices, so long as they all follow RS-485 requirements.

PTM Published on: 2010-06-29
PTM Updated on: 2022-01-19