RS-485, also known as TIA-485(-A) or EIA-485, is a standard defining the electrical characteristics of drivers and receivers for use in
serial communication
In telecommunication and data transmission, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are ...
s systems. Electrical signaling is
balanced, and
multipoint systems are supported. The standard is jointly published by the
Telecommunications Industry Association and
Electronic Industries Alliance
The Electronic Industries Alliance (EIA; until 1997 Electronic Industries Association) was an American standards and trade organization composed as an alliance of trade associations for electronics manufacturers in the United States. They deve ...
(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
Differential signalling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conduc ...
over
twisted pair
Twisted pair cabling is a type of wiring used for communications in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted ba ...
as
RS-422
RS-422, also known as TIA/EIA-422, is a technical standard originated by the Electronic Industries Alliance that specifies electrical characteristics of a digital signaling circuit. It was meant to be the foundation of a suite of standards tha ...
. It is generally accepted that RS-485 can be used with data rates up to 10
Mbit/s
In telecommunications, data-transfer rate is the average number of bits ( bitrate), characters or symbols ( baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are mul ...
or, at lower speeds, distances up to . As a
rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 10
8. Thus a cable should not signal faster than .
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. The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, i.e. a line or
bus
A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...
, not a
star
A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...
,
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.

Ideally, the two ends of the cable will have a
termination resistor connected across the two wires. Without termination resistors,
signal reflection
In telecommunications, signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber. Some of the signal power may be reflected back to its origin rather than being carried all t ...
s 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
The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction ...
(typically, 120 ohms for twisted pairs). The termination also includes pull up and pull down resistors to establish
fail-safe
In engineering, a fail-safe is a design feature or practice that in the event of a specific type of failure, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlike inherent safe ...
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.
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. The initial edition of EIA RS-485 was dated April 1983.
RS-485 only specifies the electrical characteristics of the generator and the receiver: the
physical layer
In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip.
Th ...
. It does not specify or recommend any
communications protocol
A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchro ...
; 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 +, 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. 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.
Converters, repeaters and star topology
Converters between RS-485 and
RS-232
In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' ('' data terminal equipment'') suc ...
are available to allow a
personal computer
A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tech ...
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.
Applications
RS-485 signals are used in a wide range of computer and automation systems. In a computer system,
SCSI
Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interface ...
-2 and SCSI-3 may use this specification to implement the
physical layer
In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip.
Th ...
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 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. 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. 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
Building automation (BAS), also known as building management system (BMS) or building energy management system (BEMS), is the automatic centralized control of a building's HVAC (heating, ventilation and air conditioning), electrical, lighting, ...
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 or for controlling the layout in a networked PC environment.
8P8C modular connectors are used in this case.
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

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):
* 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.
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
Intersil is an American semiconductor company headquartered in Milpitas, California. As of February 24, 2017, Intersil is a subsidiary of Renesas. The previous Intersil was formed in August 1999 through the acquisition of the semiconductor bus ...
, as seen in their data sheet for the ISL4489 transceiver
*
Maxim, as seen in their data sheet for the MAX483 transceiver and for the new generation 3.3v micro controller the MAX3485
*
Linear Technology, as seen in their datasheet for the LTC2850, LTC2851, LTC2852
*
Analog Devices, as seen in their datasheet for the ADM3483, ADM3485, ADM3488, ADM3490, ADM3491
*
FTDI, as seen in their datasheet for the USB-RS485-WE-1800-BT
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. 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.
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)
[ANSI/TIA/EIA-485-A, page 15, A.4.1] 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 Common-mode signal is the voltage common to both input terminals of an electrical device. In telecommunication, the common-mode signal on a transmission line is also known as longitudinal voltage.
In most electrical circuit
An electrical net ...
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
The diagram below shows
potentials of the A (red) and B (blue) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an
asynchronous start-stop method.
See also
*
List of network buses
List of electrical characteristics of single collision domain segment "slow speed" network buses:
{, class="wikitable sortable" style="width:100%;"
! Name !! Multidrop !! Max nodes !! Electrical type !! Cable type !! data-sort-type="number", M ...
*
UART
Notes
References
External links
* - The Standard for sale from the current publisher.
*
*
* – Practical information about implementing RS485
*
*esp8266, esp32 and Arduino implementation with MAX485 and MAX3485
How to interface Arduino, esp8266 or esp32 to RS-485
{{Computer-bus
Serial buses
Telecommunications-related introductions in 1998
EIA standards
Serial digital interface