EIA-485, earlier known as RS-485 or RS485 is an OSI Model physical layer electrical specification of a two-wire, half-duplex, multipoint serial connection. EIA-485 standard specifies a differential form of signaling. The data is conveyed by the difference between voltages. One polarity of voltage indicates 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 is better described as un-balanced interface, as the balanced usually implies that the voltages on the differential wires are balanced with respect to ground or earth potential (e.g. +5V and -5V), but EIA-485 is usually +5V and 0V. 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.
- RS-485 communication is asynchronous in nature.
- Since it uses a differential balanced line over twisted pair similar to EIA-422, it can span moderately large distances.
- While working with RS-485 one encounters various grounding, shielding, and termination issues.
- In comparison to EIA-422 (having a single driver circuit which cannot be switched off), EIA-485 drivers need to be put in transmit mode explicitly by asserting a signal to the driver. This enables EIA-485 to implement linear topologies using only two wires.
- The equipments located along a set of EIA-485 wires are interchangeably called nodes, stations and devices.
- Although many applications use EIA-485 signal levels, the speed, format, and protocol of the data transmission is not specified by EIA-485.
- EIA-485, like EIA-422 can be made full-duplex by using four wires.
- EIA-485 does not specify any connector or pinout. Circuits may be terminated on screw terminals, D-subminiature connectors, or other types of connectors.
EIA-485 is used:
- In an extensive range of computer and automation 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.
- For low-speed data communications in commercial aircraft cabins vehicle bus where it requires minimal wiring, and can share the wiring among several seats, resulting in reduced weight.
- 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.
- In programmable logic controllers and on factory floors. Since it is differential, it resists electromagnetic interference from motors and welding equipment.
- In theatre and performance venues to control sound systems and lighting. The EIA-485 link is typically implemented over standard XLR cables more commonly used for microphones, and so can be run between stage and control desk without laying special cables. Theatrical and disco lighting is controlled with the DMX protocol.
- EIA-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining 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.