EIA-485

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EIA-485 (formerly RS-485 or RS485) is an OSI model physical layer electrical specification of a two-wire,^[1] 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 drivers 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 wires. The equipment located along a set of EIA-485 wires are interchangeably called nodes, stations and devices.

The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, a line or bus, not a star, ring, or multiply-connected network. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to the lower impedance, and bias resistors (see below) are required. The value of each termination resistor should be equal to the cable impedance (typically, 120 ohms for twisted pairs). Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance.

Somewhere along the set of wires, powered resistors are established to bias each data line/wire when the lines are not being driven by any device. 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.

Often in a master-slave arrangement when one device dubbed "the master" initiates all communication activity, the master device itself provides the bias and not the slave devices. In this configuration, the master device is typically centrally located along the set of EIA-485 wires, so it would be two slave devices located at the physical end of the wires that would provide the termination. The master device would provide termination if it itself was located at a physical end of the wires, but that is often a bad design as the master would be better located at a halfway point between the slave devices. Note that it is not a good idea to apply the bias at multiple node locations, because, by doing so, the effective bias resistance is lowered, which could possibly cause a violation of the EIA-485 specification and cause communications to malfunction. By keeping the biasing with the master, slave device design is simplified and this situation is avoided.

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.

Converters between EIA-485 and other formats are available to allow a personal computer to communicate with remote devices. By using "Repeaters" and "Multi-Repeaters" very large RS485 networks can be formed. The Application Guidelines for TIA/EIA-485-A has one diagram called "Star Configuration. Not recommended." Using an RS485 "Multi-Repeater" can allow for "Star Configurations" with "Home Runs" (or multi-drop) connections similar to Ethernet Hub/Star implementations (with greater distances). Hub/Star systems (with "Multi-Repeaters") allow for very maintainable systems, without violating any of the RS485 specifications. Repeaters can also be used to extend the distance and/or number of nodes on a network.

1 Uses of EIA-485
2 Connectors
- 2.1 Pin labelling
3 Waveform example
4 References
5 See also
6 External links

[edit] 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.

EIA-485 also is used in Building automation as the simple bus wiring and long cable length is ideal for joining remote devices.

EIA-485 also is used to control theatrical and disco lighting where it is used as the physical layer of the DMX protocol.

EIA-485 is used to control video surveillance cameras. Typically wiring runs from a central controller to a number of cameras which have stepper motors for pan, tilt and zoom. One or more joysticks are connected to the controller and each camera is assigned an address. There appear to be a number of vendor defined protocols for communication of the actual movement requests.

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.

[edit] Connectors

EIA-485 does not specify any connector.

[edit] Pin labelling

The EIA-485 differential line consists of two pins:

A aka '−' aka TxD-/RxD- aka inverting pin which is negative (compared to B) when the line is idle (ie data is 1).
B aka '+' aka TxD+/RxD+ aka non-inverting pin which is positive (compared to A) when the line is idle (ie data is 1).

These names are all in use on various equipment, but the actual standard released by EIA only uses the names A and B. However, despite the unambiguous standard there is much confusion about which is which:

The EIA-485 signalling specification states that signal A is the inverting or '-' pin and signal B is the non-inverting or '+' pin. [1] The same naming is specified in the NMEA standards.

This is in conflict with the A/B naming used by a number of differential transceivers manufacturers, including the Texas Instruments application handbook on EIA-422/485 communications (A=non-inverting, B=inverting). These manufacturers are incorrect, but their practice is in a widespread use.

Therefore, care must be taken when using A/B naming.

In addition to the A and B connections, the EIA standard also specifies a third interconnection point called C, which is the common ground.