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Introduction to RS485

Differential signals with RS485

Electrical characteristics of RS485

Network topology with RS485

RS485 functionality

RS485 quick reference

RS485 afsluitweerstanden

Introduction to RS485

RS232, RS422, RS423 and RS485 are serial communication methods for computers and devices. RS232 is without doubt the best known interface, because this serial interface is implemented on almost all computers available today. But some of the other interfaces are certainly interesting because they can be used in situations where RS232 is not appropriate. We will concentrate on the RS485 interface here.

RS232 is an interface to connect one DTE, data terminal equipment to one DCE, data communication equipment at a maximum speed of 20 kbps with a maximum cable length of 50 feet. This was sufficient in the old days where almost all computer equipment were connected using modems, but soon after people started to look for interfaces capable of one or more of the following:

• Connect DTE's directly without the need of modems
• Connect several DTE's in a network structure
• Ability to communicate over longer distances
• Ability to communicate at faster communication rates

RS485 is the most versatile communication standard in the standard series defined by the EIA, as it performs well on all four points. That is why RS485 is currently a widely used communication interface in data acquisition and control applications where multiple nodes communicate with each other.

Differential signals with RS485: Longer distances and higher bit rates

One of the main problems with RS232 is the lack of immunity for noise on the signal lines. The transmitter and receiver compare the voltages of the data- and handshake lines with one common zero line. Shifts in the ground level can have disastrous effects. Therefore the trigger level of the RS232 interface is set relatively high at ±3 Volt. Noise is easily picked up and limits both the maximum distance and communication speed. With RS485 on the contrary there is no such thing as a common zero as a signal reference. Several volts difference in the ground level of the RS485 transmitter and receiver does not cause any problems. The RS485 signals are floating and each signal is transmitted over a Sig+ line and a Sig- line. The RS485 receiver compares the voltage difference between both lines, instead of the absolute voltage level on a signal line. This works well and prevents the existence of ground loops, a common source of communication problems. The best results are achieved if the Sig+ and Sig- lines are twisted. The image below explains why.

Noise in straight and twisted pair cables

In the picture above, noise is generated by magnetic fields from the environment. The picture shows the magnetic field lines and the noise current in the RS485 data lines that is the result of that magnetic field. In the straight cable, all noise current is flowing in the same direction, practically generating a looping current just like in an ordinary transformer. When the cable is twisted, we see that in some parts of the signal lines the direction of the noise current is the oposite from the current in other parts of the cable. Because of this, the resulting noise current is many factors lower than with an ordinary straight cable. Shielding—which is a common method to prevent noise in RS232 lines—tries to keep hostile magnetic fields away from the signal lines. Twisted pairs in RS485 communication however adds immunity which is a much better way to fight noise. The magnetic fields are allowed to pass, but do no harm. If high noise immunity is needed, often a combination of twisting and shielding is used as for example in STP, shielded twisted pair and FTP, foiled twisted pair networking cables. Differential signals and twisting allows RS485 to communicate over much longer communication distances than achievable with RS232. With RS485 communication distances of 1200 m are possible.

Differential signal lines also allow higher bit rates than possible with non-differential connections. Therefore RS485 can overcome the practical communication speed limit of RS232. Currently RS485 drivers are produced that can achieve a bit rate of 35 mbps.

Characteristics of RS485 compared to RS232, RS422 and RS423

What does all the information in this table tell us? First of all we see that the speed of the differential interfaces RS422 and RS485 is far superior to the single ended versions RS232 and RS423. We also see that there is a maximum slew rate defined for both RS232 and RS423. This has been done to avoid reflections of signals. The maximum slew rate also limits the maximum communication speed on the line. For both other interfaces—RS422 and RS485—the slew rate is indefinite. To avoid reflections on longer cables it is necessary to use appropriate termination resitors.

We also see that the maximum allowed voltage levels for all interfaces are in the same range, but that the signal level is lower for the faster interfaces. Because of this RS485 and the others can be used in situations with a severe ground level shift of several volts, where at the same time high bit rates are possible because the transition between logical 0 and logical 1 is only a few hundred millivolts.

Interesting is, that RS232 is the only interface capable of full duplex communication. This is, because on the other interfaces the communication channel is shared by multiple receivers and—in the case of RS485—by multiple senders. RS232 has a separate communication line for transmitting and receiving which—with a well written protocol—allows higher effective data rates at the same bit rate than the other interfaces. The request and acknowledge data needed in most protocols does not consume bandwidth on the primary data channel of RS232.

Network topology with RS485

Network topology is probably the reason why RS485 is now the favorite of the four mentioned interfaces in data acquisition and control applications. RS485 is the only of the interfaces capable of internetworking multiple transmitters and receivers in the same network. When using the default RS485 receivers with an input resistance of 12 kΩ it is possible to connect 32 devices to the network. Currently available high-resistance RS485 inputs allow this number to be expanded to 256. RS485 repeaters are also available which make it possible to increase the number of nodes to several thousands, spanning multiple kilometers. And that with an interface which does not require intelligent network hardware: the implementation on the software side is not much more difficult than with RS232. It is the reason why RS485 is so popular with computers, PLCs, micro controllers and intelligent sensors in scientific and technical applications.

RS485 network topology

In the picture above, the general network topology of RS485 is shown. N nodes are connected in a multipoint RS485 network. For higher speeds and longer lines, the termination resistances are necessary on both ends of the line to eliminate reflections. Use 100 Ω resistors on both ends. The RS485 network must be designed as one line with multiple drops, not as a star. Although total cable length maybe shorter in a star configuration, adequate termination is not possible anymore and signal quality may degrade significantly.

RS485 functionality

And now the most important question, how does RS485 function in practice? Default, all the senders on the RS485 bus are in tri-state with high impedance. In most higher level protocols, one of the nodes is defined as a master which sends queries or commands over the RS485 bus. All other nodes receive these data. Depending of the information in the sent data, zero or more nodes on the line respond to the master. In this situation, bandwidth can be used for almost 100%. There are other implementations of RS485 networks where every node can start a data session on its own. This is comparable with the way ethernet networks function. Because there is a chance of data collosion with this implementation, theory tells us that in this case only 37% of the bandwidth will be effectively used. With such an implementation of a RS485 network it is necessary that there is error detection implemented in the higher level protocol to detect the data corruption and resend the information at a later time.

There is no need for the senders to explicity turn the RS485 driver on or off. RS485 drivers automatically return to their high impedance tri-state within a few microseconds after the data has been sent. Therefore it is not needed to have delays between the data packets on the RS485 bus.

RS485 is used as the electrical layer for many well known interface standards, including Profibus and Modbus. Therefore RS485 will be in use for many years in the future.

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 less 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" for the user 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.

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.

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 a few microseconds.

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.

Below are the specifications for RS232, RS423, RS422, and RS485.

SPECIFICATIONS

RS485 en de afsluitweerstanden

Wanneer kleine hoeveelheden data met informatie moeten getransfereerd worden over langere afstanden dan is een RS-485 interface een goede keuze.
Een RS-485 interface is een elektrische specificatie voor een meerpunt schakeling die gebruik maakt van een symmetrisch netwerk.
RS-485 laat het gebruik van meerdere zenders en ontvangers op een netwerk (bus) toe.
Het specificatie document (TIA/EIA-485-A) definieert de elektrische karakteristieken van de bus en van de zender en ontvangers.
In het document staan suggesties voor het afsluiten en bedraden van het netwerk, maar hoe de pin's zijn aangesloten of welk software protocol er wordt gebruikt is vrij te kiezen. Een RS-485 netwerk kan over 256 stations beschikken indien er gebruik gemaakt wordt van ontvangers met een hoge weerstand.

De lengte van het netwerk kan tot 1200 meter bedragen bij dataoverdracht snelheden tot 10 Mbps. Voor langere afstanden kan er gebruik gemaakt worden van versterkers die het signaal regenereren en zo een nieuw RS-485 netwerk beginnen.

De RS-485 specificatie zegt niets over het te gebruiken protocol, in de praktijk wordt er veel gebruik gemaakt van een protocol dat vertrouwd is met dat van de UART die in een PC zit. In de handel zijn er verschillende soorten RS-485 omzetters te verkrijgen, voor microcontrollers kan je een RS-485 transceiver aansluiten aan de seriële poort. De meeste netwerken maken gebruik van een extra signaal voor controle van de transceiver. Aan de zijde van de PC kan men hiervoor gebruik maken van het RTS signaal.

De reden waarom RS-485 netwerken over zo een grote lengte communiceren is omdat de ontvangers het verschil meten in spanning tussen de twee geleiders van de kabel. De meeste storingen die op de geleiders voorkomt is voor beide gelijk, hierdoor verandert het spanningsverschil tussen de geleiders niet. Deze storingen oefenen daarom geen invloed op de ontvanger waardoor een goede werking van het netwerk is verzekerd. Van een common-mode spanning (zoals bij RS232) is er ook geen sprake dit omdat de retour via de tweede geleider loopt.

Voor de zender moet het spanningsverschil minstens 1,5 V zijn, zodat de interface een voldoende tolerantie heeft voor niet gemeenschappelijke ruis en verzwakking. In een knooppunt moet de bedrading zo kort mogelijk gehouden worden.
In de meeste gevallen wordt een getwiste (twisted-pair) afgeschermde kabel gebruikt omdat deze goede eigenschappen heeft om ruis te voorkomen.

De datasheets van interface chips geven de niet inverse lijn de naam Line A en de inverse lijn de naam Line B. Wanneer spanning op Line A meer dan 200mV groter is dan deze van Line B dan de is uitgang van de ontvanger hoog. In het omgekeerde geval is de uitgang van de ontvanger laag. Indien het verschil tussen Line A en Line B minder dan 200 mV is, dan is de status van de uitgang ongedefinieerd.

De figuur toont een voorbeeld van een mogelijk RS-485 netwerk.

Afsluitweerstanden RS485 netwerk

In het begin van het netwerk staan drie weerstanden twee van 470 ohm en een van 120 ohm. Deze weerstanden zorgen ervoor dat de status van de lijn behouden blijft wanneer er geen driver actief is.

De afsluitweerstanden.
Aan het begin en het einde van een RS-485 netwerk moet een afsluitweerstand geplaatst worden.
De afsluitweerstanden reduceren reflecties in de kabel die ervoor kunnen zorgen dat de ontvanger een logische toestand verkeerd ontvangt en moeten dus steeds geplaatst worden! De waarde voor de afsluitweerstand is voor een RS-485 netwerk tussen de 100 en 150 ohm. Voor de DCIBUS die op een baudrate werkt van 115200 Hz is 120 ohm als waarde voor de afsluitweerstand een goede keus. Plaats alleen aan het begin en het einde van het netwerk een afsluitweerstand, meerdere weerstanden hebben geen nut en leggen het netwerk eigenlijk in kotsluiting waardoor de componenten voor de transmissie overbelast worden.

De RS-485 specificatie raad een 100 ohm weestand (0.5 W) aan in serie met de massa bij iedere knoop in het netwerk. Indien de spanning van de massa op twee knopen verschilt dan zullen de weerstanden de stroom beperken.