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`RS-485 - Wikipedia
`
`RS-485
`
`RS-485, also known as TIA-485(-A) or EIA-485, is a standard,
`originally
`introduced
`in
`1983, defining
`the
`electrical
`in serial
`characteristics of drivers and receivers for use
`communications systems. Electrical signaling is balanced, and
`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 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 systems and similar applications.
`Overview
`
`RS-485 supports inexpensive local networks 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 Mbit/s[a] or, at lower speeds,
`distances up to 1,200 m (4,000 ft).[2] As a rule of thumb, the
`speed in bit/s multiplied by the length in metres should not
`exceed 108. Thus a 50-meter cable should not signal faster than
`2 Mbit/s.[3]
`
`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 topologies using only two wires. The
`equipment
`located along a
`set of RS-485 wires are
`interchangeably called nodes, stations or devices.[4] The
`recommended arrangement of the wires is as a connected series
`of point-to-point (multidropped) nodes, i.e. a line or bus, not a
`star, 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.
`
`Available
`signals
`Connector
`types
`
`Ideally, the two ends of the cable will have a termination resistor connected across the two wires.
`Without termination resistors, signal reflections off the unterminated end of the cable can cause data
`corruption. Termination resistors also reduce electrical noise sensitivity due to the lower impedance.
`The value of each termination resistor should be equal to the cable characteristic impedance
`(typically, 120 ohms for twisted pairs). The termination also includes pull up and pull down resistors
`to establish fail-safe bias for each data wire for the case when the lines are not being driven by any
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`TIA-485-A (Revision of EIA-
`485)
`ANSI/TIA/EIA-485-
`A-1998
`Approved: March 3,
`1998
`Reaffirmed:
`December 7, 2012
`Balanced
`interconnecting
`cable
`Point-to-point, multi-
`dropped, multi-point
`At least 32 unit
`loads
`Not specified
`
`Standard
`
`Physical
`media
`
`Network
`topology
`Maximum
`devices
`Maximum
`distance
`Mode of
`operation
`
`Different receiver
`levels:
`binary 1 (OFF)
`(Voa–Vob <
`−200 mV)
`binary 0 (ON)
`(Voa–Vob >
`+200 mV)
`A, B, C
`
`Not specified
`
`1
`
`Exhibit 1058
`Samsung v. Smart Mobile
`IPR2022-01249
`
`

`

`RS-485 - Wikipedia
`8/31/23, 11:14 AM
`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.[5]
`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.[6] The initial
`edition of EIA RS-485 was dated April 1983.[7]
`
`Typical bias network together with
`termination. Biasing and termination
`values are not specified in the RS-
`485 standard.
`
`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 full-duplex by using four wires.[8] 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.[9]
`
`Converters, repeaters and star topology
`
`Converters between RS-485 and RS-232 are available to allow a personal computer to communicate
`with remote devices. By using repeaters very large RS-485 networks can be formed. TSB-89A,
`Application Guidelines for TIA/EIA-485-A does not recommend using star topology.[10]
`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 this specification 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
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`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 controllers and on factory floors. 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. 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.[11] 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 railways. The external interface to the
`DCC command station is often RS-485 used by hand-held controllers[12] or for controlling the layout
`in a networked PC environment. 8P8C modular connectors are used in this case.[13]
`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.
`Signals
`
`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 (logic 1) condition is traditionally represented (e.g. in
`RS-232) with a negative voltage and 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):[14]
`
`RS-485 3 wire connection
`
`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.[b]
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`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[15]
`Maxim, as seen in their data sheet for the MAX483 transceiver[16] and for the new generation 3.3v
`micro controller the MAX3485
`Linear Technology, as seen in their datasheet for the LTC2850, LTC2851, LTC2852[17]
`Analog Devices, as seen in their datasheet for the ADM3483, ADM3485, ADM3488, ADM3490,
`ADM3491[18]
`FTDI, as seen in their datasheet for the USB-RS485-WE-1800-BT[19]
`
`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.[c] 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.[28]
`
`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)[29] 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 V to +12 V, i.e. ±7 V on top of the 0–5 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 terminals, 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
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`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.
`
`A signal shown in blue, B in red
`
`See also
`
`List of network buses
`UART
`
`Notes
`
`a. Under some conditions it can be used up to data transmission speeds of 64 Mbit/s.[1]
`b. 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.
`c. 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.[20][21] As long as standard excludes logic function of the generator or
`receiver,[22] 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.[23] That so-called 'Pesky Polarity' problem [24] raised
`confusion which made authors think A is inverting within the TIA-485-A standard itself [25] 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".[26] It is now a common design decision to make this inversion which involves the following
`polarity chain: UART/MCU idle → TTL/CMOS = +5 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.[27]
`
`References
`1. RS-485 Reference Guide (https://web.archive.org/web/20180517101401/http://www.ti.com/lit/sg/sl
`yt484a/slyt484a.pdf) (PDF), archived from the original (http://www.ti.com/lit/sg/slyt484a/slyt484a.p
`df) (PDF) on 2018-05-17
`2. "How Far and How Fast Can You Go with RS-485? - Application Note – Maxim" (https://www.maxi
`mintegrated.com/en/app-notes/index.mvp/id/3884). www.maximintegrated.com.
`
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`RS-485 - Wikipedia
`3. Soltero, Manny; Zhang, Jing; Cockril, Chris; Zhang, Kevin; Kinnaird, Clark; Kugelstadt, Thomas
`(May 2010) [2002]. RS-422 and RS-485 Standards Overview and System Configurations,
`Application Report (http://focus.ti.com/lit/an/slla070d/slla070d.pdf) (pdf). Texas Instruments
`(Technical report). SLLA070D.
`4. Electronic Industries Association (1983). Electrical Characteristics of Generators and Receivers
`for Use in Balanced Multipoint Systems. EIA Standard RS-485. OCLC 10728525 (https://www.wor
`ldcat.org/oclc/10728525).
`5. "Application Note 847 FAILSAFE Biasing of Differential Buses" (http://www.ti.com/lit/an/snla031/sn
`la031.pdf) (PDF). Texas Instruments. 2011.
`6. "Trim-the-fat-off-RS-485-designs" (https://www.eetimes.com/trim-the-fat-off-rs-485-designs). EE
`Times. 2000.
`7. "EIA Standard RS 485 Electrical Characteristics of Generators and Receivers for Use in Balanced
`Digital Multipoint Systems", reproduced in " Data Communications Standards LibraryÈ, Telebyte
`Technology Inc., Greenlawn, New York 1985.
`8. RS-485 CONNECTIONS FAQ (http://www.bb-elec.com/Learning-Center/All-White-Papers/Serial/R
`S-485-Connections-FAQ.aspx), Advantech B+B SmartWorx, retrieved 2019-03-08
`9. What is the difference between RS422 communication and RS485 communication? (http://us.brai
`nboxes.com/faq/items/what-is-the-difference-between-rs422-communication-and-rs485-com),
`Brainboxes LLC, retrieved 2019-03-08
`10. TSB-89A, Application Guidelines for TIA/EIA-485-A (https://e2e.ti.com/cfs-file/__key/telligent-evol
`ution-components-attachments/00-138-00-00-00-33-63-91/TSB_2D00_89_2D00_A.pdf) (PDF),
`retrieved 2019-04-06
`11. "DH-485 Industrial Local Area Network Overview" (https://web.archive.org/web/20120310095800/
`http://www.ab.com/en/epub/catalogs/12762/2181376/214372/1535907/3404063/). Rockwell
`Automation. Archived from the original (http://www.ab.com/en/epub/catalogs/12762/2181376/2143
`72/1535907/3404063/) on 2012-03-10. Retrieved 10 September 2010.
`12. lenzusa.com (http://www.lenzusa.com/techinfo/xpressnetfaq.htm), XpressNET FAQ, accessed
`July 26, 2015 Archived (https://web.archive.org/web/20171117050219/http://www.lenzusa.com/tec
`hinfo/xpressnetfaq.htm) November 17, 2017, at the Wayback Machine
`13. bidib.org (http://www.bidib.org/bidibus/bidibus_e.html#T2), "BiDiBus, a Highspeed-Bus for model-
`railways", accessed July 26, 2015.
`14. "Polarity conventions" (http://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/
`13-143-00-00-00-26-49-60/RS485-_2D00_-Polarity-Conventions.pdf) (PDF). Texas Instruments.
`2003.
`15. "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" (https://web.archive.org/web/20041
`204120233/http://www.intersil.com/data/fn/fn6074.pdf) (PDF). Intersil Corporation. 28 April 2006.
`Archived from the original (http://www.intersil.com/data/fn/fn6074.pdf) (PDF) on 2004-12-04.
`16. "Data Sheet 19-0122 – MAX481/MAX483/MAX485/MAX487–MAX491/MAX1487: Low-Power,
`Slew-Rate-Limited RS-485/RS-422 Transceivers" (http://datasheets.maxim-ic.com/en/ds/MAX148
`7-MAX491.pdf) (PDF). Maxim Integrated. September 2009.
`17. "LTC2850/LTC2851/LTC2852 3.3V 20Mbps RS485/RS422 Transceivers" (https://web.archive.org/
`web/20110302044542/http://www.linear.com/docs/Datasheet/285012fd.pdf) (PDF). Linear
`Technology Corporation. 2007. Archived from the original (http://cds.linear.com/docs/Datasheet/28
`5012fd.pdf) (PDF) on 2011-03-02.
`18. "ADM3483/ADM3485/ADM3488/ADM3490/ADM3491 (Rev. E)" (http://www.analog.com/static/imp
`orted-files/data_sheets/ADM3483_3485_3488_3490_3491.pdf) (PDF). Analog Devices, Inc. 22
`November 2011.
`
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`RS-485 - Wikipedia
`19. "USB to RS485 Serial Converter Cable Datasheet" (http://www.ftdichip.com/Support/Documents/D
`ataSheets/Cables/DS_USB_RS485_CABLES.pdf) (PDF). Future Technology Devices
`International Ltd. 27 May 2010.
`20. "Profibus Interconnection Guideline (PDF)" (http://www.profibus.com/download). 1.4. P
`International. January 2007. p. 7.
`21. "SIMATIC NET Profibus Network Manual (PDF)" (https://cache.industry.siemens.com/dl/files/591/3
`5222591/att_105793/v1/mn_pbnets_76.pdf) (PDF). Siemens. April 2009. p. 157.
`22. "RS-485 Technical Manual, TIA-485 section" (https://en.wikibooks.org/wiki/Serial_Programming/R
`S-485#RS-485). Wikibooks.
`23. "Profibus Interconnection Guideline (PDF)" (http://www.profibus.com/download). 1.4. P
`International. January 2007. p. 8.
`24. "RS-485 Technical Manual, That Pesky Polarity" (https://en.wikibooks.org/wiki/Serial_Programmin
`g/RS-485#.5BThat_Pesky.5D_Polarity). Wikibooks.
`25. "RS485 Polarity Issues" (http://www.chipkin.com/rs485-polarity-issues). Chipkins Automation
`Systems.
`26. "Application Bulletin AB-19, Profibus Compliance: A Hardware Design Guide" (http://www.nve.co
`m/Downloads/ab19.pdf) (PDF). NVE Corporation. 2010.
`27. "White paper: Polarities for Differential Pair Signals" (http://www.bb-elec.com/Learning-Center/All-
`White-Papers/Serial/%E2%80%A2-Polarities-for-Differential-Pair-Signals-%28RS-422.aspx).
`Advantech B+B SmartWorx.
`28. "The RS-485 Design Guide" (https://www.ti.com/lit/an/slla272c/slla272c.pdf) (PDF). Texas
`Instruments.
`29. ANSI/TIA/EIA-485-A, page 15, A.4.1
`
`External links
`
`"TIA Standards Store: TIA-485 Revision A" (https://global.ihs.com/doc_detail.cfm?&csf=TIA&item_
`s_key=00032964&item_key_date=870024&input_doc_number=485&input_doc_title=&org_code=
`TIA). TIA. 7 December 2012. Retrieved 2 September 2020. - The Standard for sale from the
`current publisher.
`"Tutorial 763: Guidelines for Proper Wiring of an RS-485 (TIA/EIA-485-A) Network" (http://www.m
`aximintegrated.com/app-notes/index.mvp/id/763). Maxim Integrated. 19 November 2001.
`"RS232 to RS485 cable pinout" (http://pinouts.ru/Converters/rs485_cable_pinout.shtml).
`Pinouts.ru. 7 October 2012.
`"RS485 serial information" (https://www.lammertbies.nl/comm/info/RS-485.html). Lammert Bies.
`August 2012. Retrieved 12 November 2012. – Practical information about implementing RS485
`Marais, Hein (2008). "Application Note AN-960: RS-485/RS-422 Circuit Implementation Guide" (ht
`tp://www.analog.com/static/imported-files/application_notes/AN-960.pdf) (PDF). Analog Devices.
`
`Retrieved from "https://en.wikipedia.org/w/index.php?title=RS-485&oldid=1167032786"
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