View source for RS-485

{{Short description|Multidrop serial communication standard}}

{{Infobox fieldbus protocol
|name              = TIA-485-A<br/>(Revision of EIA-485)
|standard          = ANSI/TIA/EIA-485-A-1998 <br /> Approved: March 3, 1998 <br /> Reaffirmed: December 7, 2012
|governing_body    =
|type_of_network   =
|physical_media    =[[Balanced line|Balanced interconnecting cable]]
|network_topology  =[[Point-to-point_(telecommunications)|Point-to-point]], [[Multidrop bus|multi-dropped]], [[Bus network|multi-point]]
|maximum_devices   =At least 32 unit loads
|maximum_distance  = Not specified
|maximum_speed     =
|device_addressing =
|mode_of_operation =Different receiver levels: <br /> binary 1 (OFF)<br />(Voa–Vob < −200&nbsp;mV)<br />binary 0 (ON)<br />(Voa–Vob > +200&nbsp;mV)
|maximum_baud_rate =
|maximum_binary_rate  = 
|voltage           =
|mark1             =
|space0            =
|available_signals =A, B, C
|connector_types   =Not specified
}}

'''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 communication]]s systems. Electrical signaling is [[balanced]], and [[Telecommunications link#Multipoint|multipoint]] systems are supported. The standard is jointly published by the [[Telecommunications Industry Association]] and [[Electronic Industries Alliance]] (TIA/EIA). Digital communications networks implementing the standard can be used effectively over long distances and in [[Electromagnetic compatibility|electrically noisy environments]]. Multiple receivers may be connected to such a network in a linear, [[multidrop bus]]. These characteristics make RS-485 useful in [[industrial control system]]s and similar applications.

== Overview ==
RS-485 supports inexpensive [[local network]]s and [[multidrop communications]] links, using the same [[differential signaling]] over [[twisted pair]] as [[RS-422]]. It is generally accepted that RS-485 can be used with data rates up to 10&nbsp;[[bitrate|Mbit/s]]{{efn|Under some conditions it can be used up to [[data transmission]] speeds of 64&nbsp;Mbit/s.<ref>{{citation |title=RS-485 Reference Guide |url=http://www.ti.com/lit/sg/slyt484a/slyt484a.pdf |archive-url=https://web.archive.org/web/20180517101401/http://www.ti.com/lit/sg/slyt484a/slyt484a.pdf |archive-date=2018-05-17}}</ref>}} or, at lower speeds, distances up to {{convert|1200|m|abbr=on|-3}}.<ref>{{cite web|url=https://www.maximintegrated.com/en/app-notes/index.mvp/id/3884|title=How Far and How Fast Can You Go with RS-485? - Application Note – Maxim|website=www.maximintegrated.com}}</ref> As a [[rule of thumb]], the speed in bit/s multiplied by the length in meters should not exceed 10<sup>8</sup>. Thus a {{nowrap|50-meter}} cable should not signal faster than {{nowrap|2 Mbit/s}}.<ref name=slla070d>{{cite tech report |url=http://focus.ti.com/lit/an/slla070d/slla070d.pdf |format=pdf |website=[[Texas Instruments]] |first=Manny |last=Soltero |first2=Jing |last2=Zhang |first3=Chris |last3=Cockril |first4=Kevin |last4=Zhang |first5=Clark |last5=Kinnaird |first6=Thomas |last6=Kugelstadt |title=RS-422 and RS-485 Standards Overview and System Configurations, Application Report |id=SLLA070D |date=May 2010 |orig-year=2002}}</ref>

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. This allows RS-485 to implement [[linear bus topology|linear bus topologies]] using only two wires. The equipment located along a set of RS-485 wires are interchangeably called nodes, stations or devices.<ref>{{cite book |author=Electronic Industries Association |series=EIA Standard RS-485 |title=Electrical Characteristics of Generators and Receivers for Use in Balanced Multipoint Systems |year=1983 |oclc=10728525}}{{page needed|date=October 2011}}</ref> The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, i.e. a line or [[Bus network|bus]], not a [[Star network|star]], [[Ring network|ring]], or multiply connected network. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans.

[[File:Rs485-bias-termination.svg|thumb|Typical bias network together with termination. Biasing and termination values are not specified in the RS-485 standard. However, bias resistors are commonly not recommended any more by component suppliers.]]
Ideally, the two ends of the cable will have a [[termination resistor]] connected across the two wires. Without termination resistors, [[signal reflection]]s off the unterminated end of the cable can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to the [[Brownian noise|lower impedance]].{{elucidate|discuss=[[Talk:Electrical termination#Noise sensitivity]]|date=July 2018}} The value of each termination resistor should be equal to the cable [[characteristic impedance]] (typically, 120&nbsp;ohms for twisted pairs). 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. 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.<ref>{{cite web | title = Application Note 847 FAILSAFE Biasing of Differential Buses | url = http://www.ti.com/lit/an/snla031/snla031.pdf | publisher = [[Texas Instruments]] |year = 2011}}</ref>

== Standard ==
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 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.<ref>{{cite web | url = https://www.eetimes.com/trim-the-fat-off-rs-485-designs | title = Trim-the-fat-off-RS-485-designs | publisher = [[EE Times]] | year = 2000}}</ref> The initial edition of EIA RS-485 was dated April 1983.<ref>"EIA Standard RS 485 Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems", reproduced in "Data Communications Standards Library", Telebyte Technology Inc., Greenlawn, New York 1985.</ref>

RS-485 only specifies the electrical characteristics of the generator and the receiver: the [[physical layer]]. It does not specify or recommend any [[communications protocol]]; Other standards define the protocols for communication over an RS-485 link. The foreword to the standard references ''The Telecommunications Systems Bulletin TSB-89'' which contains application guidelines, including data signaling rate vs. cable length, stub length, and configurations.

Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. These characteristics include: definition of a unit load, voltage ranges, open-circuit voltages, thresholds, and transient tolerance. It also defines three generator interface points (signal lines); A, B and C. The data is transmitted on A and B. C is a ground reference. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals. If A is negative with respect to B, the state is binary 1. The reversed polarity (A positive with respect to B) is binary 0. The standard does not assign any logic function to the two states.

== Full duplex operation ==
RS-485, like RS-422, can be made [[duplex (telecommunications)|full-duplex]] by using four wires.<ref>{{citation |title=RS-485 Connections FAQ |url=https://www.advantech.com/en-eu/resources/white-papers/02cb2f4e-4fb2-4a87-be3b-508325bd61d6 |publisher=Advantech B+B SmartWorx |access-date=2023-09-15}}</ref> Since RS-485 is a multi-point specification, however, this is not necessary or desirable in many cases. RS-485 and RS-422 can interoperate with certain restrictions.<ref>{{citation |title=What is the difference between RS422 communication and RS485 communication? |url=https://www.brainboxes.com/faq/what-is-the-difference-between-rs422-communication-and-rs485-com |publisher=Brainboxes LLC |access-date=2024-10-27}}</ref>{{fv|reason=Although the opportunity is apparent to the skilled reader, there is no mention of interoperability in the source.|date=October 2024}}

==Converters and repeaters==
Converters between RS-485 and [[RS-232]] are available to allow a [[personal computer]] to communicate with remote devices. By using [[repeater]]s very large RS-485 networks can be formed.

==Network topology==
TSB-89A, Application Guidelines for TIA/EIA-485-A does not recommend using star topology, as doing so may lead to long stubs (branches of the star), which can cause signal reflections that make data transmission unreliable.<ref>{{citation |url=https://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/00-138-00-00-00-33-63-91/TSB_2D00_89_2D00_A.pdf |title=TSB-89A, Application Guidelines for TIA/EIA-485-A |access-date=2019-04-06}}</ref>

==Protocols==
RS-485 does not define a [[communication 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.

== Applications ==
RS-485 signals are used in a wide range of computer and automation systems. 

In a computer system, [[SCSI]]-2 and SCSI-3 may use RS-485 to implement the [[physical layer]] for data transmission between a controller and a disk drive.

RS-485 is used for low-speed data communications in commercial aircraft cabins' [[vehicle bus]]. It requires minimal wiring and can share the wiring among several seats, reducing weight.

These are used in [[programmable logic controller]]s and on factory floors. RS-485 is used as the physical layer underlying [[List of automation protocols|many standard and proprietary automation protocols]] used to implement [[industrial control systems]], including the most common versions of [[Modbus]] and [[Profibus]]. '''{{visible anchor|DH 485}}''' is a proprietary communications protocol used by [[Allen-Bradley]] in their line of industrial control units. Utilizing a series of dedicated interface devices, it allows PCs and industrial controllers to communicate.<ref>{{cite web
|url=http://www.ab.com/en/epub/catalogs/12762/2181376/214372/1535907/3404063/ |archive-url=https://web.archive.org/web/20120310095800/http://www.ab.com/en/epub/catalogs/12762/2181376/214372/1535907/3404063/ |archive-date=2012-03-10 |title=DH-485 Industrial Local Area Network Overview |publisher=[[Rockwell Automation]] |access-date=10 September 2010}}</ref> Since it is differential, it resists electromagnetic interference from motors and welding equipment.

In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the [[DMX512]] protocol. RS-485 serves as a physical layer for the [[AES3]] digital audio interconnect.

RS-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.

It is also used in [[Digital Command Control]] (DCC) for [[model railway]]s. The external interface to the DCC command station is often RS-485 used by hand-held controllers<ref>[http://www.lenzusa.com/techinfo/xpressnetfaq.htm lenzusa.com], XpressNET FAQ, accessed July 26, 2015 {{Webarchive|url=https://web.archive.org/web/20171117050219/http://www.lenzusa.com/techinfo/xpressnetfaq.htm |date=November 17, 2017 }}</ref> or for controlling the layout in a networked PC environment. [[8P8C modular connector]]s are used in this case.<ref>[http://www.bidib.org/bidibus/bidibus_e.html#T2 bidib.org], "BiDiBus, a Highspeed-Bus for model-railways", accessed July 26, 2015.</ref>

== Signals ==
[[File:RS-485 transceiver.svg|thumb|RS-485 interface consisting of a line driver and line receiver. Single-ended signals are shown on the left. The RS-485 bus, shown on the right, has three signals consisting of a differential pair and signal common.]]
{| class="wikitable"
|+RS-485 signal states
!Signal
!Mark (logic 1)
!Space (logic 0)
|-
|A
|Low
|High
|-
|B
|High
|Low
|}

The RS-485 differential line consists of two signals:
* '''A''', which is low for logic 1 and high for logic 0 and,
* '''B''', which is high for logic 1 and low for logic 0.

Because a [[Mark and space|mark]] (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage; and [[Mark and space|space]] (logic 0) represented with a positive one, A may be considered the ''non-inverting'' signal and B as inverting. The RS-485 standard states (paraphrased):<ref>{{cite web | url = http://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/13-143-00-00-00-26-49-60/RS485-_2D00_-Polarity-Conventions.pdf | title = Polarity conventions | publisher = [[Texas Instruments]] | year = 2003}}</ref>
* For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal.
* For an on, space or logic 0 state, the driver's A terminal is positive relative to the B terminal.{{efn|There is an apparent typo in this statement as both states in the standard are designated ''binary 1''. It is clear in the figure that follows that the off state corresponds to binary 1 and on corresponds to binary 0.}}

The truth tables of most popular devices, starting with the SN75176, show the output signals inverted. This is in accordance with the A/B naming used by most differential transceiver manufacturers, including:
* [[Intersil]], as seen in their data sheet for the ISL4489 transceiver<ref>{{cite web | url = http://www.intersil.com/data/fn/fn6074.pdf |url-status=dead |archive-url=https://web.archive.org/web/20041204120233/http://www.intersil.com/data/fn/fn6074.pdf | archive-date=2004-12-04 | title = Data Sheet FN6074.3: ±15kV ESD Protected, 1/8 Unit Load, 5V, Low Power, High Speed and Slew Rate Limited, Full Duplex, RS-485/RS-422 Transceivers | publisher = [[Intersil Corporation]] | date = 28 April 2006}}</ref>
* [[Maxim Integrated|Maxim]], as seen in their data sheet for the MAX483 transceiver<ref>{{cite web | url = http://datasheets.maxim-ic.com/en/ds/MAX1487-MAX491.pdf | title = Data Sheet 19-0122 – MAX481/MAX483/MAX485/MAX487–MAX491/MAX1487: Low-Power, Slew-Rate-Limited RS-485/RS-422 Transceivers | date = September 2009 | publisher = [[Maxim Integrated]] | access-date = 2009-06-17 | archive-date = 2009-09-27 | archive-url = https://web.archive.org/web/20090927022738/http://datasheets.maxim-ic.com/en/ds/MAX1487-MAX491.pdf | url-status = dead }}</ref> and for the new generation 3.3v micro controller the MAX3485
* [[Linear Technology]], as seen in their datasheet for the LTC2850, LTC2851, LTC2852<ref>{{cite web | url = http://cds.linear.com/docs/Datasheet/285012fd.pdf |archive-url=https://web.archive.org/web/20110302044542/http://www.linear.com/docs/Datasheet/285012fd.pdf |archive-date=2011-03-02 | title = LTC2850/LTC2851/LTC2852 3.3V 20Mbps RS485/RS422 Transceivers | publisher = [[Linear Technology Corporation]]| year = 2007}}</ref>
* [[Analog Devices]], as seen in their datasheet for the ADM3483, ADM3485, ADM3488, ADM3490, ADM3491<ref>{{cite web | url = http://www.analog.com/static/imported-files/data_sheets/ADM3483_3485_3488_3490_3491.pdf | title = ADM3483/ADM3485/ADM3488/ADM3490/ADM3491 (Rev. E) | publisher = [[Analog Devices, Inc.]] | date = 22 November 2011}}</ref>
* [[FTDI]], as seen in their datasheet for the USB-RS485-WE-1800-BT<ref>{{cite web | url = http://www.ftdichip.com/Support/Documents/DataSheets/Cables/DS_USB_RS485_CABLES.pdf | title = USB to RS485 Serial Converter Cable Datasheet | publisher = [[Future Technology Devices International Ltd]] | date = 27 May 2010}}</ref>
These manufacturers all agree on the meaning of the standard, and their practice is in widespread use. The issue also exists in programmable logic controller applications.{{efn|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.<ref>{{cite web | url = http://www.profibus.com/download | title = Profibus Interconnection Guideline (PDF) |version=1.4 |date=January 2007 |publisher = P International |page=7 |url-access=registration }}</ref><ref>{{cite web | url = https://cache.industry.siemens.com/dl/files/591/35222591/att_105793/v1/mn_pbnets_76.pdf | title = SIMATIC NET Profibus Network Manual (PDF) |date=April 2009 |publisher = Siemens|page=157}}</ref> As long as standard excludes logic function of the generator or receiver,<ref>{{cite web | url = https://en.wikibooks.org/wiki/Serial_Programming/RS-485#RS-485 | title = RS-485 Technical Manual, TIA-485 section | publisher = Wikibooks}}</ref> it would make sense '''A''' (green, negative) is higher than '''B''' (red, positive). However this contradicts the facts that an idle '''mark''' state is a logical '''one''' ''and'' the termination polarization puts '''B''' at a higher voltage in Profibus guidelines.<ref>{{cite web | url = http://www.profibus.com/download | title = Profibus Interconnection Guideline (PDF) |version=1.4 |date=January 2007 |publisher = P International |page=8 |url-access=registration }}</ref> That so-called 'Pesky Polarity' problem <ref>{{cite web | url = https://en.wikibooks.org/wiki/Serial_Programming/RS-485#.5BThat_Pesky.5D_Polarity | title = RS-485 Technical Manual, That Pesky Polarity | publisher = Wikibooks}}</ref> raised confusion which made authors think '''A''' is inverting within the TIA-485-A standard itself <ref>{{cite web | url = http://www.chipkin.com/rs485-polarity-issues | title = RS485 Polarity Issues | publisher = Chipkins Automation Systems}}</ref> and advise to swap what is '''A''' and '''B''' in drivers and line labeling as can be read in a section of an application bulletin: "Design Consideration #3: Sometimes Bus Node '''A''' Isn’t Really Bus Node '''A'''".<ref>{{cite web | url = http://www.nve.com/Downloads/ab19.pdf | title = Application Bulletin AB-19, Profibus Compliance: A Hardware Design Guide  | publisher = NVE Corporation | year = 2010}}</ref> It is now a common design decision to make this inversion which involves the following polarity chain: [[UART]]/[[Microcontroller unit|MCU]] idle → TTL/CMOS {{=}} +5&nbsp;V → Line '''B''' voltage > Line '''A''' voltage, implying '''A''', the green wire, is indeed connected to the driver ''inverting'' signal, as seen in a whitepaper.<ref>{{cite web | url = https://www.advantech.com/th-th/resources/white-papers/2fde048f-f42c-439b-b0a9-485cd548f172 | title = White paper: Polarities for Differential Pair Signals  | publisher = Advantech B+B SmartWorx}}</ref>}} Care must be taken when using A/B naming. Alternate nomenclature is often used to avoid confusion surrounding the A/B naming:
* TX+/RX+ or D+ as alternative for B (high for mark i.e. idle)
* TX−/RX− or D− as alternative for A (low for mark i.e. idle)

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 two values provide
a sufficient margin for a reliable data transmission even under severe signal degradation across the cable
and connectors. This robustness is the main reason why RS-485 is well suited for long-distance
networking in noisy environment.<ref>{{cite web | url = https://www.ti.com/lit/an/slla272c/slla272c.pdf | title = The RS-485 Design Guide | publisher = Texas Instruments}}</ref>

In addition to the '''A''' and '''B''' connections, an optional, third connection may be present (the TIA standard requires the presence of a common return path between all circuit grounds along the balanced line for proper operation)<ref>ANSI/TIA/EIA-485-A, page 15, A.4.1</ref> called '''SC''', '''G''' or '''reference''', the common signal reference ground used by the receiver to measure the A and B voltages. This connection may be used to limit the [[common-mode signal]] that can be impressed on the receiver inputs. The allowable common-mode voltage is in the range −7&nbsp;V to +12&nbsp;V, i.e. ±7&nbsp;V on top of the 0–5&nbsp;V signal range. Failure to stay within this range will result in, at best, signal corruption, and, at worst, damage to connected devices. 

Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together – it is wise to add some [[current limiting]] to the SC connection. 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.

RS-485 does not specify any connector or pinout. Circuits may be terminated on [[screw terminal]]s, [[D-subminiature]] connectors, or other types of connectors.

The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds.

== Waveform example ==
The diagram below shows [[Electric potential|potentials]] of the A (blue) and B (red) pins of an RS-485 line before, during, and after transmission of one byte (0xD3, least significant bit first) of data using an [[asynchronous start-stop]] method.

[[File:RS-485 waveform.svg|thumb|upright=1.35|left|B (U+, inverting) signal shown in red,<br> A (U−, non-inverting) signal shown in blue]]
{{clear}}

== See also ==
* [[List of network buses]]

== Notes ==
{{Notelist}}

== References ==
{{Reflist}}

== External links ==
{{Wikibooks|Serial Programming:RS-485 Technical Manual}}
* [https://store.accuristech.com/standards/tia-tia-485-a?product_id=2591400 "TIA-485-A"] - purchase official standard
* [https://www.analog.com/en/resources/technical-articles/rs485-cable-specification-guide--maxim-integrated.html "Guidelines for Proper Wiring of an RS-485 Network"] - Maxim
* [https://www.analog.com/en/resources/app-notes/an-960.html "RS-485 Circuit Implementation Guide"] - Analog
* [https://www.ti.com/lit/ta/sszt500/sszt500.pdf "RS-485 Frequently Asked Questions"] - TI
* [https://www.renesas.com/en/document/apn/an1986-external-fail-safe-biasing-rs-485-networks "External Fail-Safe Biasing of RS-485 Networks"] - Renesas
* [http://www.ti.com/lit/an/slyt324/slyt324.pdf "RS-485 Passive Failsafe for an Idle Bus"] - TI, and [https://dcs-bios.a10c.de/rs485-resistors.html RS-485 Resistor Calculator]

{{Computer-bus}}

[[Category:Serial buses]]
[[Category:Telecommunications-related introductions in 1998]]
[[Category:EIA standards]]
[[Category:Serial digital interface]]