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EIA-485 (formerly RS-485 or RS485) is an OSI Model physical layer electrical specification of a two-wire, half-duplex, multipoint serial connection. The standard specifies a differential form of signalling. The difference between the wires’ voltages is what conveys the data. One polarity of voltage indicates a logic 1 level, the reverse polarity indicates logic 0. The difference of potential must be at least 0.2 volts for valid operation, but any applied voltages between +12 V and −7 volts will allow correct operation of the receiver.

EIA-485 only specifies electrical characteristics of the driver and the receiver. It does not specify or recommend any data protocol. EIA-485 enables the configuration of inexpensive local networks and multidrop communications links. It offers high data transmission speeds (35 Mbit/s up to 10 m and 100 kbit/s at 1200 m). Since it uses a differential balanced line over twisted pair (like EIA-422), it can span relatively large distances (up to 4000 feet or just over 1200 metres).

In contrast to EIA-422, which has a single driver circuit which cannot be switched off, EIA-485 drives need to be put in transmit mode explicitly by asserting a signal to the driver. This allows EIA-485 to implement linear topologies using only two lines.

The recommended arrangement of the wires is as a connected series of point-to-point networks, a line or bus, not a star, ring, or multiply-connected network. Ideally, the end farthest from the master controller will have two powered resistors to bias the lines apart when the lines are not being driven, as well as two low-impedance in-line resistors to damp reflections. Without biasing resistors, digital noise can be created when all stations are silent and unpowered, and the lines drift near each other’s values. Without antireflection resistors, reflections of fast driver edges can cause multiple data edges, some of which can lie. Star and ring topologies are not recommended because they have unnecessary reflections, excessively low or high termination impedance (all those resistors! or none!) and can concentrate electromagnetic interference.

EIA-485, like EIA-422 can be made full-duplex by using four wires, however, since EIA-485 is a multi-point specification, this is not necessary in many cases. EIA-485 and EIA-422 can interoperate with certain restrictions.

Uses of EIA-485

• SCSI-2 and SCSI-3 (for instance) use this specification to implement the physical layer.

• EIA-485 is often used with common UARTs to implement low-speed data communications in commercial aircraft cabins. For example, some passenger control units use it. It requires minimal wiring, and can share the wiring among several seats. It therefore reduces the system weight.

• EIA-485 also sees some use in programmable logic controllers and on factory floors in order to implement proprietary data communications. Since it is differential, it resists electromagnetic interference from motors and welding equipment.

• EIA-485 is used in large sound systems, as found at music events and theatre productions, for remotely controlling high-end sound-processing equipment from a standard computer running special software. The EIA-485 link is typically implemented over standard XLR cables more usually used for microphones, and so can be run between stage and control desk without laying special cables.

This standard is now administered by the TIA and is titled TIA-485-A, Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems (ANSI/TIA/EIA-485-A-98) (R2003), indicating that the standard was re-affirmed without technical changes in 2003.

Connectors

The following table lists the commonly used RS-485 signals and common pin assignments (RS-232, another serial standard, listed here for comparison):

External link

• What is RS485?

• Serial buses