Wat is RS 485 en het verschil met
RS422, RS232 AND RS423

Inleiding voor alleen de RS-485 de verschillen met RS-232, RS-422 en RS-423 staan ook op deze pagina in het engels :

Dit is een kleine beschrijving van het RS485 protocol, voor uitgebreidere informatie heb ik op onderaan deze pagina wat links gezet naar bedrijven, data sheets en application noteís.

De beschrijving:

De RS 485 interface levert net als de RS232 interface een seriŽle verbinding. Fysiek zou deze dan ook het best omschreven kunnen worden als een verbeterde RS232 interface, of nog beter een RS422 protocol. RS485 is namelijk een multipoint communicatie netwerk net als de RS422 De RS 485 werd in 1983 voor het eerst geÔntroduceerd en is met name geschikt voor het opzetten van besturingen en distributeren van data. Een belangrijke verbetering t.o.v. RS232 is het gebruik van een symmetrische verbinding. Door deze aanpak is een tweedjasverbinding alleen te gebruiken voor half duplex verbindingen. Alle zenders en ontvangers zijn dus met elkaar verbonden via dezelfde tweekoperaders. Er kan steeds maar een zender actief zijn , alle andere aansluitingen zijn dan hoogohmig geschakeld.De communicatie kan half duplex verlopen zoals in de volgende afbeelding:In de industrie wordt de RS 485 gebruikt als een lokale bus bijvoorbeeld het profibus systeem.In de computer-branche gebruikt men hem onder andere in de SCSI- en IPI-interface.De voordelen van deze bus zijn : een maximale data-snelheid van meer dan 10 MB/s en onder goede condities en het gebruik van een symmetrische kabel is ook tot 30 MB/s haalbaar.Er kunnen afstanden van 1200 meter of meer gehaald worden.Een regel is dat de impedantie van het netwerk identiek is aan de uitgangsimpedentie van de zender.Daarmee is ook gelijk een grens gesteld aan de lengte van de verbinding. Zelfs een goede koper ader van ongeveer 0,25 mm2 heeft al een weerstand van ongeveer 80 ohm per kilometer. Dit betekent dat bij een afstand van 1200 tot 1500 meter de kabel impedentie gelijk wordt aan de karakteriseke impedantie. Van 120 ohm.Hiermee ligt de maximale kabel lengte dus al vast. Een ander punt dat grenzen stelt aan de verbinding, zijn parasitaire capaciteiten en zelf inducties die iedere kabel bezit.

Omdat data met hoge snelheden gedistribueerd worden, komen steile vlanken in het signaal voor. Naarmate de kabellengte langer wordt, wordt het signaal minder betrouwbaar.

 

Hoe werkt RS485

De data wordt differentiaal verstuurd, het voordeel hiervan is dat dit protocol bestand is tegen data-collission en bus-fout condities.

RS485 kan zowel 2 dradig als 4 dradig uitgevoerd worden, het verschil zit dan in de half of full duplex communicatie.Bij een full duplex communicatie is er sprake van 1 master en maximaal 32 slaves, communicatie is slecht mogenlijk met een slave, de anderen staan dan tristate geschakeld.

Soms wordt dit hardwarematig gedaan, maar meestal wordt dit door de software geregeld.

Bit stream : 

De datastream van RS485 is hetzelfde zoals die van de RS232, 1 startbit 8 databits en 1 of 2 stopbits.

Links :

Informatie over RS485:

National-semiconducters

Technman

RS485.com

B&B electronics

Datasheets :

National-semiconducters

 

VERSCHILLEN TUSSEN RS-232, RS-422, RS-423, en RS-485
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" 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.

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 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.

Below are the specifications for RS232, RS423, RS422, and RS485. Please give us a call at 513-874-4796 if further information is required. We have solutions to most problems that are encountered in this area.

Thanks, Ron Smith

SPECIFICATIONS RS232 RS423 RS422 RS485
Mode of Operation SINGLE
-ENDED
SINGLE
-ENDED
DIFFER- ENTIAL DIFFER- ENTIAL
Total Number of Drivers and Receivers on One Line 1 DRIVER
1 RECVR
1 DRIVER
10 RECVR
1 DRIVER
10 RECVR
1 DRIVER
32 RECVR
Maximum Cable Length 50 FT. 4000 FT. 4000 FT. 4000 FT.
Maximum Data Rate 20kb/s 100kb/s 10Mb/s 10Mb/s
Maximum Driver Output Voltage +/-25V +/-6V -0.25V to +6V -7V to +12V
Driver Output Signal Level (Loaded Min.) Loaded +/-5V to +/-15V +/-3.6V +/-2.0V +/-1.5V
Driver Output Signal Level (Unloaded Max) Unloaded +/-25V +/-6V +/-6V +/-6V
Driver Load Impedance (Ohms) 3k to 7k >=450 100 54
Max. Driver Current in High Z State Power On N/A N/A N/A +/-100uA
Max. Driver Current in High Z State Power Off +/-6mA @ +/-2v +/-100uA +/-100uA +/-100uA
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) 3k to 7k 4k min. 4k min. >=12k




 

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