RS-485 (aka TIA-485-A, ANSI/TIA/EIA-485, TIA/EIA-485, EIA-485) is a standard defining the electrical characteristics of drivers and receivers for use in digital serial communications systems. Electrical signaling is balanced, and multipoint systems are supported.
RS-485 does not define a protocol; merely an electrical interface. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. Interoperability of even similar devices from different manufacturers is not assured by compliance with the signal levels alone.
RS-485 standard similar to rs232 but with an important addition, RS-485 uses a balanced line (an simple electrical method for rejecting inducted noise) which enables it to run either at higher speed or longer distance.
The RS-485 differential line consists of two pins:
- A, which is low for logic 1 and high for logic 0.
- B, which is high for logic 1 and low for logic 0.
In addition to the A and B connections, the EIA standard also specifies a third interconnection point called SC, which is the common signal reference ground. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. This third signal is the reference potential used by the transceiver to measure the A and B voltages.
RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. This allows RS-485 to implement linear bus topologies using only two wires. The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, i.e. a line or bus, not a star, ring, or multiply connected network.
More Information:
https://en.wikipedia.org/wiki/RS-485
Polarities for Differential Pair Signals RS-422 and RS-485
Guidelines for Proper Wiring of an RS-485 (TIA/EIA-485-A) Network
