Introduction to RS 422 & RS 485

Chapter 1: What is RS 422, RS 485, comparison with RS 232.

RS 232 is well-known due to popularity of today's PC's, unlike the RS422 and RS 485. These are used in
industry for control systems and data transfers (small volumes, NO hundreds of Mb/s).
So, what is the main difference between RS 232 and RS 422 & 485? The RS 232 signals are represented by
voltage levels with respect to ground. There is a wire for each signal, together with the ground signal
(reference for voltage levels). This interface is useful for point-to-point communication at slow speeds. For
example, port COM1 in a PC can be used for a mouse, port COM2 for a modem, etc. This is an example of
point-to-point communication: one port, one device. Due to the way the signals are connected, a common
ground is required. This implies limited cable length - about 30 to 60 meters maximum. (Main problems are
interference and resistance of the cable.) Shortly, RS 232 was designed for communication of local devices,
and supports one transmitter and one receiver.
RS 422 & 485 uses a different principle: Each signal uses one twistedpair (TP) line - two wires twisted
around themselves. We're talking 'Balanced data transmission', or 'Differential voltage transmission'. Simply,
let's label one of the TP wires 'A' and the other one 'B'. Then, the signal is inactive when the voltage at A is
negative and the voltage at B is positive. Otherwise, the signal is active, A is positive and B is negative. Of
course, the difference between the wires A and B matters. For RS 422 & 485 the cable can be up to 1200
meters (4000 feet) long, and commonly available circuits work at 2.5 MB/s transfer rate.
What is the difference between RS 422 and RS 485? Electrical principle is the same: both use differential
transmitters with alternating voltages 0 and 5V. However, RS 422 is intended for point-to-point
communications, like RS 232. RS 422 uses two separate TP wires, data can be transferred in both directions
simultaneously. RS 422 is often used to extend a RS 232 line, or in industrial environments.
RS 485 is used for multipoint communications: more devices may be connected to a single signal cable -
similar to e.g. ETHERNET networks, which use coaxial cable. Most RS 485 systems use Master/Slave
architecture, where each slave unit has its unique address and responds only to packets addressed to this unit.
These packets are generated by Master (e.g. PC), which periodically polls all connected slave units.
This article will mainly cover the Master/Slave architecture because it is sufficient for 95% of applications.
In special cases (security systems, ...), an improved version of multiprocessor communication is used. This
system uses only a single line for bidirectional communication; however, there is no Master. All units
announce a packet transmission of a specified length, and at the same time listen whether the data has been
successfully transmitted. If it's not the case, they stop communicating and listen for what has happened. At
this time, urgent packets can be transmitted over the line. This system is ideal for devices, that need to
immediately transfer some very important and up-to-date data, without waiting for Master to give them a
chance to do so. On the other side, useful data transfer is less effective (about 30% less effective than the first
system).
In Master/Slave architecture, slave never starts the communication. It is critical for Master to send correct
addresses.
RS 485 exists in two versions: 1 TwistedPair or 2 TwistedPairs.
 Single TwistedPair RS 485
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 In this version, all devices are connected to a single TwistedPair. Thus, all of them must
have drivers with tri-state outputs (including the Master). Communication goes over the
single line in both directions. It is important to prevent more devices from transmitting at
once (software problem).
 Double TwistedPair RS 485
Here, Master does not have to have tri-state output, since Slave devices transmit over the
second twistedpair, which is intended for sending data from Slave to Master. This solution
often allows to implement multipoint communication in systems, which were originally
designed (HW as well as SW) for RS232. Of course, Master software needs to be modified,
so that Master periodically sends query packets to all Slave devices. Increased data
throughput is evident in large volumes.
Sometimes you can see a RS 485 system in a point-to-point system. It is virtually identical
to RS 422; the high impedance state of the RS 485 output driver is not used. The only
difference in hardware of the RS 485 and RS 422 circuits is the ability to set the output to
high impedance state.
Chapter 2: Balanced differential signals
First, let's talk about advantages and disadvanteges of RS 422/485. For a basic RS 422/485 system, we need
an I/O driver with differential outputs and an I/O receiver with differential inputs. Noise and interference is
introduced into the line; however, since the signal is transferred via a twisted pair of wires, the voltage
difference (between A and B) of this interference is almost zero. Due to the differential function of the RS
422/485 input amplifier of the receiver, this interference is eliminated. The same is true for crosstalk from
neighboring lines, as well as for any other source of interference, as long as the absolute maximum voltage
ratings of the receiver circuits are not exceeded. Differential inputs ignore different earth potentials of the
transmitter and the receiver. This is very important for communications of diverse systems, where great
problems would otherwise arise - e.g. different power sources, etc. TwistedPair cables, together with correct
terminations (to eliminate reflections), allow data transfer rate of over 10Mbit/s with cables up to 1 km long.

However, all of these advantages come at a cost. RS 422/485 circuits are more complex, and thus more
expensive. Higher data transfer speeds require correctly connected and matched terminations, which can be a
problem in systems where the number of
connected devices changes. And, of course,
TwistedPair cables are required.
In a RS 232 unbalanced data transmission
system, each signal is represented by a voltage
level with respect to ground. For example, the
TxD signal of a PC COMx port is negative
when idle, and switches between positive and
negative level when transmitting data.
Amplitude ranges between -15 to -5V in
negative state, and between +5 to +15V in
positive state.

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In a balanced differential system, the transmitter generates a voltage between 2 to 7V (approx.) between the
A and B outputs. Although the transmitter and the receiver are connected with a ground wire (GND) as well,
it is never used to determine logic levels at the AB wires. This implies already mentioned tolerance of
different ground potentials of the transmitter and the receiver. RS 485 transmitters have an Enable input,
allowing to set the outputs to high impedance state, allowing several devices to share a single TP. RS 422
transmitters usually don't provide such input. Voltage level of most commonly sold transmitters is 0 and 5V.
When idle, there is +5V on B and 0V on A.
RS422/485 receivers react to voltage difference betweeen the A and B inputs. If Vab is greater than 200mV,
a logic level is defined on the receiver output. For Vab less than 200mV, the logic level is opposite.

Picture shows a simplified schematics

of receiver input connections for a
twisted pair line, and a graph of level
determination.

EIA STANDARD RS 422 & RS 485

There are two standards describing
Balanced interface circuits: EIA-RS
422 (international standard ITU-T
V.11) defines point-to-point interfaces
with up to 10 receivers for a single
transmitter. The limiting parameter is
the receiver input impedance Ri=4kOhm. 10 receivers + termination resistor 100 Ohm give the maximum
transmitter load.
EIA-RS 485 (ISO 8482) defines the input impedance of RS 485 circuits Ri = 12kOhm. Then, up to 32
transmitters, receivers, or combination, can be connected to a single line. Since the data transfer is
bidirectional, the line needs to have a terminating resistor at both ends.

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 This picture compares the standards RS 485 and RS 422, summarizes individual values and shows
connections of termination resistors.

Ground potential difference between different devices is +- 7V max, according to EIA-RS 422. EIA-RS 485
defines maximum voltage range at the receiver input (ground potential difference + alternating signal
voltage) from -7V to +12V. RS 422/482 circuits have a short-circuit protection. Point-to-point (EIA-RS 422)
defines short circuit as a current greater than 150mA, between A and B or against the ground. Multipoint
(EIA-RS 485) defines short circuit as current greater than 150mA against the ground, or more than 250 mA
between A and B.
Ground connection for RS 422/485
For correct operation of the transmitter and the
receiver, a return signal path between the
grounding of individual devices is required. It
may be realised either by a third wire, or by
grounding each device (third pole in the mains
socket). If a third (ground) wire is used,
resistors (approx. 1kOhm) should be connected
in series to eliminate unwanted currents
resulting from ground potential differences.

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Chapter 3: Terminations, capacities, cable lengths, data transfer speed

RS 422/485 line termination is essential, especially for faster data transfer rates and long cables. Main
reasons for correct termination are reflections at the ends of the line, and the minimum transmitter load
requirement. For RS 422, the termination is fairly simple (see picture comparing RS 422 and RS 485). A
terminating resistor of 100Ohm is connected to the end of the line. If there are more RS 422 receivers
connected to the line, the resistor can be a little bigger. The value can be calculated since the input
impedance of the receivers is known.
For RS 485, the termination is somewhat more complex (again, see the comparison picture). Since each
device communicates bidirectionally (single TP version), we are unable to determine where is the transmitter
and where is the receiver - this changes constantly, according to which device transmits at the moment. So,
both ends of the line have to be terminated with a 100Ohm terminator. However, it is not that easy. Since all
device have tri-state outputs, situations occur (very often - every time the transmitting device or data
direction changes), when all transmitters are in high impedance state, and the line, due to termination
resistors, is in undefined state (Vab%lt;200mV). It is, however, desirable to define idle state in this situation
(Vab<-200mV).
The comparison picture shows a circuit solving this problem with termination resistor values. The values are
calculated to allow maximum number of devices (32) with input impedances 12kOhm to be connected to a
single line. It is important that there are only two Rt resistors at the end of the cable (they may be inside the
last device connected).
Maximum data transfer speeds

Graph showing the dependence of transfer

speed on several basic conditions.
1) Maximum data transfer rate over short
distances, where the line influence can be
neglected, is determined by the output
parameters of the transmitter. The duration of
the rising and falling edges matters. Standard
assumes speed of 10Mbit/s; today's fastest
chips, e.g. SN76ALS176, can achieve up to 25
MBit/s.
2) When the line length exceeds 10m, we have
to take into consideration losses caused by
capacities and the so-called skin effect, when
the current begins to flow only on the surface
of the conductors. The rule for standart TP cables says, that data transfer speed (Mbit/s) multiplied by cable
length (m) is less than 10^8. So, for example, if a cable is 100m long, we get maximum data transfer speed
of 1Mbit/s.
3) Last limitation applies to very long cables. Speed is limited by the ohmic resistance of the line, and
following the signal loss. Maximum cable length is determined by its resistance, which should be less than
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the line impedance - 100Ohm. Standard TP cable, diameter 2x0.6mm has a resistance of arround 100W/Km.
Capacity of the cable needs to be considered as well.
This table is for orientation only, but still very useful:
Data
transfer 1200bd 2400bd 4800bd 9600bd 19200bd 38400bd 57600bd 115200bd
speed

Max.
cable 250nF 120nF 60nF 30nF 15nF 750pF 500pF 250pF
capacity
Chapter 4: Protocols, software
This chapter is short because software is custom-designed for each individual application, including the
transfer protocols. RS 422, as a point-to-point connection, operates similarly to the RS 232 serial port.
However, there are certain things software needs to take into account. First, the communication media (TP
line) needs to be assigned to individual stations, so that there are no collisions and that responses are fast
enough.
We have to be aware of the fact, that only a single channel is available for communication. It has to transfer
both data and the bit and byte synchronisation. For transferring of individual bits, one of the data network
modulations can be used, e.g. coding NRZ, NRZI, phase modulation NRZ, or differential phase modulation.
These modulations are not covered in this article, they are a topic of network and protocol theory.
For byte synchronisation, several options are available. Maybe the simplest is to reserve one byte to be a sync
character. Software then needs to convert data bytes equal to the sync byte to a sequence of different bytes.
Or, protocols SLDC/HLDC, suitable for high-speed transfers, may be used. When an application requires
only slower data transfer rates (115200bd, or up to 2Mb/s with special UART circuits), we can take
advantage of the RS 232 format for both bit and byte synchronisation. This solution saves a lot of effort,
especially when a PC is used as Master, and Slave is equipped with a microcontroller containing its own
UART, e.g. 8051 compatibles.
From a network point of view, the RS 485 incorporates a bus topology. Since Slave stations have no means
of starting the communication without a risk of collision, they need to be assigned a 'right to transmit' by the
Master station. Assignment is done centrally via pooling, where the central (Master) station periodically asks
all Slaves whether they have data to transmit. If so, the questioned station sends the data immediately;
otherwise, it replies with a confirmation packet only, or does not reply at all. This method is good for
Multipoint systems with smaller number of Slave stations (approx. up to 100). For more stations, the reply
would become too slow. Of course, individual system requirements need to be considered. Mentioned 100
stations is for an "on-line" system, where stations have to interactively react to user requests, thus the reply
delay needs to be less than 0.5 sec (considered for 115200bd data transfer rate, which is seldom available in
industrial environments).
Of course, in systems where the Master has no priority function, or due to other factors (e.g. large number of
stations with low frequency of data transfers), different access methods may be used. For example, the
random access method ALOHA. Here, any station sends its data regardless of the transfer channel status. If a
collision occurs, the station does not receive a confirmation, and repeats transmission. However, this method
utilises on average only about 18% of available bandwidth, and with larger volumes of data the throughput
decreases rapidly due to larger number of collisions. With RS 485, where transmitters can at the same time
"listen" for the channel status, the ALOHA method can be improved by a "carrier" (data activity) detection.
In this case, stations begin transmission only if the channel is idle. Both methods essentially require a transfer
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protocol with error detection.
In any data transfer, including the RS 485, there is no way to 100% guarantee that a transmitted packet has
been successfully received. Especially with the ALOHA methods, collisions may occur; interference, cable
length, etc. causes errors too. It is advisable for the communication software to take such situations into
consideration. In the central assignment method it is useful for slave stations to send a reply packet with
information of last packet received. Several options are available, choice depends on individual use.

Several hints:
 Don't know which wire is A and B? When idle, B is more positive than A.
 You don't always have to use TP cables. For small distances and low speeds, common
telephone cables are good enough.
 Termination is not critical for small distances and low speeds - works fine with MAX circuits.

TP TwistedPair - Two identical wires wrapped around each other.

QUICK REFERENCE
FOR
RS485, RS422, RS232 AND RS423

INTRODUCTION

Line drivers and receivers are commonly used to exchange data between two or more
points (nodes) on a network. Reliable data communications can be difficult in the presence
of induced noise, ground level differences, impedance mismatches, failure to effectively bias
for idle line conditions, and other hazards associated with installation of a network.

The connection between two or more elements (drivers and receivers) should be
considered a transmission line if the rise and/or fall time is greater than half the time for the
signal to travel from the transmitter to the receiver.

Standards have been developed to insure compatibility between units provided by different
manufacturers, and to allow for reasonable success in transferring data over specified
distances and/or data rates. The Electronics Industry Association (EIA) has produced
standards for RS485, RS422, RS232, and RS423 that deal with data communications.
Suggestions are often made to deal with practical problems that might be encountered in a
typical network. EIA standards where previously marked with the prefix "RS" to indicate
recommended standard; however, the standards are now generally indicated as "EIA"
standards to identify the standards organization. While the standards bring uniformity to
data communications, many areas are not specifically covered and remain as "gray areas"

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for the used to discover (usually during installation) on his own.

SINGLE-ENDED DATA TRANSMISSION

Electronic data communications between elements will generally fall into two broad
categories: single-ended and differential. RS232 (single-ended) was introduced in 1962,
and despite rumors for its early demise, has remained widely used through the industry.
The specification allows for data transmission from one transmitter to one receiver at
relatively slow data rates (up to 20K bits/second) and short distances (up to 50Ft. @ the
maximum data rate).

Independent channels are established for two-way (full-duplex) communications. The

RS232 signals are represented by voltage levels with respect to a system common (power /
logic ground). The "idle" state (MARK) has the signal level negative with respect to
common, and the "active" state (SPACE) has the signal level positive with respect to
common. RS232 has numerous handshaking lines (primarily used with modems), and also
specifies a communications protocol. In general if you are not connected to a modem the
handshaking lines can present a lot of problems if not disabled in software or accounted for
in the hardware (loop-back or pulled-up). RTS (Request to send) does have some utility in
certain applications. RS423 is another single ended specification with enhanced operation
over RS232; however, it has not been widely used in the industry.

DIFFERENTIAL DATA TRANSMISSION

When communicating at high data rates, or over long distances in real world environments,
single-ended methods are often inadequate. Differential data transmission (balanced
differential signal) offers superior performance in most applications. Differential signals can
help nullify the effects of ground shifts and induced noise signals that can appear as
common mode voltages on a network.

Quando comunicando em altas velocidades, ou sobre longas distâncias em ambientes

ruidosos, métodos de comunicação unipolares são frequentemente inadequados.
Transmissão de dados diferencial (sinal balanceado diferencial) oferece performance
superior na maioria das aplicações. Sinais diferenciais podem anular os efeitos de
deslocamentos de terra e ruídos induzidos que podem aparecer como tensões de modo
comum em uma rede de comunicações.

RS422 (differential) was designed for greater distances and higher Baud rates than RS232.
In its simplest form, a pair of converters from RS232 to RS422 (and back again) can be
used to form an "RS232 extension cord." Data rates of up to 100K bits / second and
distances up to 4000 Ft. can be accommodated with RS422. RS422 is also specified for
multi-drop (party-line) applications where only one driver is connected to, and transmits on,
a "bus" of up to 10 receivers.

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While a multi-drop "type" application has many desirable advantages, RS422 devices
cannot be used to construct a truly multi-point network. A true multi-point network consists
of multiple drivers and receivers connected on a single bus, where any node can transmit or
receive data.

"Quasi" multi-drop networks (4-wire) are often constructed using RS422 devices. These
networks are often used in a half-duplex mode, where a single master in a system sends a
command to one of several "slave" devices on a network. Typically one device (node) is
addressed by the host computer and a response is received from that device. Systems of
this type (4-wire, half-duplex) are often constructed to avoid "data collision" (bus contention)
problems on a multi-drop network (more about solving this problem on a two-wire network in
a moment).

RS485 meets the requirements for a truly multi-point communications network, and the
standard specifies up to 32 drivers and 32 receivers on a single (2-wire) bus. With the
introduction of "automatic" repeaters and high-impedance drivers / receivers this "limitation"
can be extended to hundreds (or even thousands) of nodes on a network. RS485 extends
the common mode range for both drivers and receivers in the "tri-state" mode and with
power off. Also, RS485 drivers are able to withstand "data collisions" (bus contention)
problems and bus fault conditions.

To solve the "data collision" problem often present in multi-drop networks hardware units
(converters, repeaters, micro-processor controls) can be constructed to remain in a receive
mode until they are ready to transmit data. Single master systems (many other
communications schemes are available) offer a straight forward and simple means of
avoiding "data collisions" in a typical 2-wire, half-duplex, multi-drop system. The master
initiates a communications request to a "slave node" by addressing that unit. The hardware
detects the start-bit of the transmission and automatically enables (on the fly) the RS485
transmitter. Once a character is sent the hardware reverts back into a receive mode in
about 1-2 microseconds (at least with R.E. Smith converters, repeaters, and remote I/O
boards).

Any number of characters can be sent, and the transmitter will automatically re-trigger with
each new character (or in many cases a "bit-oriented" timing scheme is used in conjunction
with network biasing for fully automatic operation, including any Baud rate and/or any
communications specification, eg. 9600,N,8,1). Once a "slave" unit is addressed it is able to
respond immediately because of the fast transmitter turn-off time of the automatic device. It
is NOT necessary to introduce long delays in a network to avoid "data collisions." Because
delays are NOT required, networks can be constructed, that will utilize the data
communications bandwidth with up to 100% through put.

SPECIFICATIONS RS232 RS423 RS422 RS485

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 SINGLE SINGLE
Mode of Operation DIFFERENTIAL DIFFERENTIAL
-ENDED -ENDED
Total Number of Drivers and
1
Receivers on One Line (One 1 DRIVER 1 DRIVER 32 DRIVER
DRIVER
driver active at a time for RS485 10 RECVR 10 RECVR 32 RECVR
1 RECVR
networks)
Maximum Cable Length 50 FT. 4000 FT. 4000 FT. 4000 FT.
Maximum Data Rate (40ft. -
20kb/s 100kb/s 10Mb/s-100Kb/s 10Mb/s-100Kb/s
4000ft. for RS422/RS485)
Maximum Driver Output Voltage +/-25V +/-6V -0.25V to +6V -7V to +12V
Driver Output Signal +/-5V to
Loaded +/-3.6V +/-2.0V +/-1.5V
Level (Loaded Min.) +/-15V
Driver Output Signal
Unloaded +/-25V +/-6V +/-6V +/-6V
Level (Unloaded Max)
Driver Load Impedance (Ohms) 3k to 7k >=450 100 54
Max. Driver Current in
Power On N/A N/A N/A +/-100uA
High Z State
Max. Driver Current in +/-6mA
Power Off +/-100uA +/-100uA +/-100uA
High Z State @ +/-2v
Slew Rate (Max.) 30V/uS Adjustable N/A N/A
Receiver Input Voltage Range +/-15V +/-12V -10V to +10V -7V to +12V
Receiver Input Sensitivity +/-3V +/-200mV +/-200mV +/-200mV
Receiver Input Resistance
(Ohms), (1 Standard Load for 3k to 7k 4k min. 4k min. >=12k
RS485)

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