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RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. The signal is a voltage change across a load as low as 1.5 volts differential from a driver, with a receiver detecting a differential voltage as minute as 200 millivolts. RS-485 standard conformant drivers provide a differential output of a minimum 1.5 V across a 54-Ω load, whereas standard conformant receivers detect a differential input down to 200 mV. The termination also includes pull up and pull down resistors to establish bias for each data wire for the case when the lines are not being driven by any device. Termination resistors also reduce electrical noise sensitivity due to the lower impedance. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices.

Interoperability of even similar devices from different manufacturers is not assured by compliance with the signal levels alone. Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. RS-485 communication involves a driver that produces signals and a receiver that detects them. Unlike Ethernet standards like TCP that repeatedly transmit packets until acknowledged (error correction), RS-485 signals transmit without requiring confirmation. RS-422 and RS-485 Standards Overview and System Configurations, Application Report (pdf). The EIA once labeled all its standards with the prefix "RS" (Recommended Standard), but the EIA-TIA officially replaced "RS" with "EIA/TIA" to help identify the origin of its standards.

The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. The EIA has officially disbanded and the standard is now maintained by the TIA as TIA-485, but engineers and applications guides continue to use the RS-485 designation. In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. RS-485, also known as TIA-485(-A) or EIA-485, is a standard, originally introduced in 1983, defining the electrical characteristics of drivers and receivers for use in serial communications systems. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. RS-485 serves as a physical layer for the AES3 digital audio interconnect. RS-485 does not specify any connector or pinout.

With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. Care must be taken when using A/B naming. This allows RS-485 to implement linear bus topologies using only two wires. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. The equipment located along a set of RS-485 wires are interchangeably called nodes, stations or devices. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. It may be used to control video surveillance systems or to interconnect security control panels and devices such as access control card readers. It is also used in Digital Command Control (DCC) for model railways. These characteristics make RS-485 useful in industrial control systems and similar applications. The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method.

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