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RS-422 RS-485 Application Note
Electronics Mfg. Inc.
P.O. 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Internet Addresses: Home Page: http://www.bb-elec.com Customer Service: sales@bb-elec.com Technical Support: techsupt@bb-elec.com
Electronics Revised October 1997
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Cover Page
Table Contents
CHAPTER OVERVIEW INTRODUCTION DATA TRANSMISSION SIGNALS Unbalanced Line Drivers Balanced Line Drivers Balanced Line Receivers. STANDARD RS-422 DATA TRANSMISSION STANDARD RS-485 DATA TRANSMISSION TRISTATE CONTROL RS-485 DEVICE USING RTS. SEND DATA CONTROL RS-485 DEVICE CHAPTER SYSTEM CONFIGURATION NETWORK TOPOLOGIES. WIRE FOUR WIRE SYSTEMS TERMINATION. BIASING RS-485 NETWORK. EXTENDING SPECIFICATION CHAPTER SELECTING RS-422 RS-485 CABLING NUMBER CONDUCTORS SHIELDING CABLE CHARACTERISTICS CHAPTER TRANSIENT PROTECTION RS-422 RS-485 SYSTEMS. WHAT DOES SURGE LOOK LIKE? Surge Specifications. Common Mode Differential Mode. GROUND GROUND. TRANSIENT PROTECTION USING ISOLATION Isolation Theory. Isolation Devices TRANSIENT PROTECTION USING SHUNTING Shunting Theory. Connecting Signal Grounds Shunting Devices COMBINING ISOLATION SHUNTING SPECIAL CONSIDERATION FAULT CONDITIONS CHOOSING RIGHT PROTECTION YOUR SYSTEM RS-422/485 Application Note Table Contents
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
CHAPTER SOFTWARE INTRODUCTION RS-422 SYSTEMS RS-485 DRIVER CONTROL RS-485 RECEIVER CONTROL MASTER-SLAVE SYSTEMS Four Wire Master-Slave Systems. Wire Master-Slave Systems. MULTI-MASTER RS-485 SYSTEMS SYSTEMS WITH PORT POWERED CONVERTERS CHAPTER SELECTING RS-485 DEVICES CHAPTER SOURCES FURTHER INFORMATION APPENDIX SPECIFICATION SUMMARY APPENDIX STANDARD RS-423 DATA TRANSMISSION.
Table Contents
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Chapter Overview
Introduction
purpose this application note describe main elements RS-422 RS-485 system. This application note attempts cover enough technical details that system designer will have considered important aspects data system design. Since both RS-422 RS485 data transmission systems that balanced differential signals, appropriate discuss both systems same application note. Throughout this application note generic terms RS-422 RS-485 will used represent EIA/TIA-422 EIA/TIA-485 Standards.
Data Transmission Signals
Unbalanced Line Drivers Each signal that transmits RS-232 unbalanced data transmission system appears interface connector voltage with reference signal ground. example, transmitted data (TD) from device appears with respect (signal ground) DB-25 connector. This voltage will negative line idle alternate between that negative level positive level when data sent with magnitude volts. RS-232 receiver typically operates within voltage range volts shown Figure 1.1. Balanced Line Drivers balanced differential system voltage produced driver appears across pair signal lines that transmit only signal. Figure shows schematic symbol balanced line driver voltages that exist. balanced line driver will produce voltage from volts across output terminals will have signal ground connection. Although proper connection signal ground important, isn't used balanced line receiver determining logic state data line. balanced line driver also have input signal called "Enable" signal. purpose this signal connect driver output terminals, "Enable" signal OFF, consider driver disconnected from transmission line. RS-485 driver must have "Enable" control signal. RS-422 driver have this signal, always required. disconnected "disabled" condition line driver usually referred "tristate1" condition driver.
term "tristate" comes from fact that there third output state RS-485 driver, addition output states "0." RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Figure
Figure
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Balanced Line Receivers balanced differential line receiver senses voltage state transmission line across signal input lines, will also have signal ground that necessary making proper interface connection. Figure schematic symbol balanced differential line receiver. Figure also shows voltages that important balanced line receiver. differential input voltage greater than +200 receiver will have specific logic state output terminal. input voltage reversed less than -200 receiver will create opposite logic state output terminal. input voltages that balanced line receiver must sense shown Figure 1.3. range required allow attenuation transmission line.
Standard RS-422 Data Transmission
Standard RS-422-A entitled "Electrical Characteristics Balanced Voltage Digital Interface Circuits" defines characteristics RS-422 interface circuits. Figure typical RS-422 four-wire interface. Notice that five conductors used. Each generator driver drive (10) receivers. signaling states line defined follows: When terminal driver negative with respect terminal, line binary (MARK OFF) state. When terminal driver positive with respect terminal, line binary (SPACE state. Figure shows condition voltage balanced line RS-232 RS-422 converter when line "idle" condition state. also shows relationship terminals RS422 system terminal markings used many types equipment. terminal equivalent designation, terminal equivalent designation. same relationship shown Figure also applies RS-485 systems. RS-422 withstand common mode voltage (Vcm) volts. Common mode voltage defined mean voltage terminals with respect signal ground.
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Figure
Figure
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Figure
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Standard RS-485 Data Transmission
RS-485 Standard permits balanced transmission line shared party line multidrop mode. many driver/receiver pairs share multidrop network. Many characteristics drivers receivers same RS-422. range common mode voltage that driver receiver tolerate expanded volts. Since driver disconnected tristated from line, must withstand this common mode voltage range while tristate condition. Some RS-422 drivers, even with tristate capability, will withstand full voltage range volts. Figure shows typical two-wire multidrop network. Note that transmission line terminated both ends line drop points middle line. Termination should only used with high data rates long wiring runs. detailed discussion termination found Chapter this application note. signal ground line also recommended RS-485 system keep common mode voltage that receiver must accept within volt range. Further discussion grounding found Chapter this application note.
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Figure
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Figure
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
RS-485 network also connected four-wire mode shown Figure 1.7. Note that four data wires additional signal ground wire used "four-wire" connection. four-wire network necessary that node master node others slaves. network connected that master node communicates slave nodes. slave nodes communicate only with master node. This network some advantages with equipment with mixed protocol communications. Since slave nodes never listen another slave response master, slave node cannot reply incorrectly another slave node.
Tristate Control RS-485 Device using
discussed previously, RS-485 system must have driver that disconnected from transmission line when particular node transmitting. RS-232 RS-485 converter RS-485 serial card, this implemented using control signal from asynchronous serial port enable RS-485 driver. line connected RS485 driver enable such that setting line high (logic state enables RS-485 driver. Setting line (logic puts driver into tristate condition. This effect disconnects driver from bus, allowing other nodes transmit over same wire pair. Figure shows timing diagram typical RS-232 RS-485 converter. waveforms show what happens VRTS waveform narrower than data VSD. This normal situation, shown here illustrate loss portion data waveform. When control used, important certain that high before data sent. Also, line must then after last data sent. This timing done software used control serial port converter. When RS-485 network connected two-wire multidrop party line mode, receiver each node will connected line (see Figure 1.6). receiver often configured receive echo data transmission. This desirable some systems, troublesome others. sure check data sheet your converter determine receiver "enable" function connected.
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Figure
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Send Data Control RS-485 Device
Many Electronics' RS-232 RS-485 converters RS-485 serial cards include special circuitry, which triggered from data signal enable RS-485 driver. Figure timing diagram important signals used control converter this type. important note that transmit data line "disabled" fixed interval after last bit, typically character length. this interval short, miss parts each character being sent. this time long, your system turn data line around from transmit receive before node (with Send Data converter) ready receive data. latter case, will miss portions complete characters) beginning response.
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Figure
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Chapter System Configuration Network Topologies
Network configuration isn't defined RS-422 RS-485 specification. most cases designer configuration that best fits physical requirements system.
Wire Four Wire Systems
RS-422 systems require dedicated pair wires each signal, transmit pair, receive pair additional pair each handshake/control signal used required). tristate capabilities RS485 allow single pair wires share transmit receive signals halfduplex communications. This "two wire" configuration (note that additional ground conductor should used) reduces cabling cost. RS-485 devices internally externally configured wire systems. Internally configured RS-485 devices simply provide connections (sometimes labeled "+"). Devices configured four wire communications bring connections both transmit receive pairs. user connect transmit lines receive lines create wire configuration. latter type device provides system designer with most configuration flexibility. Note that signal ground line should also connected system. This connection necessary keep common mode voltage receiver within safe range. interface circuit operate without signal ground connection, sacrifice reliability noise immunity. Figures illustrate connections four wire systems.
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Figure Typical RS-485 Four Wire Multidrop Configuration
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Figure Typical RS-485 Wire Multidrop Network
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Termination
Termination used match impedance node impedance transmission line being used. When impedance mismatched, transmitted signal completely absorbed load portion reflected back into transmission line. source, transmission line load impedance equal these reflections eliminated. There disadvantages termination well. Termination increases load drivers, increases installation complexity, changes biasing requirements makes system modification more difficult. decision whether termination should based cable length data rate used system. good rule thumb propagation delay data line much less than width, termination needed. This rule makes assumption that reflections will damp several trips down data line. Since receiving UART will sample data middle bit, important that signal level solid that point. example, system with 2000 feet data line propagation delay calculated multiplying cable length propagation velocity cable. This value, typically speed light (c), specified cable manufacture. example, round trip covers 4000 feet cable. Using propagation velocity 0.66 round trip completed approximately assume reflections will damp three "round trips" down cable length, signal will stabilize 18.6 after leading edge bit. 9600 baud wide. Since reflections damped much before center bit, termination required. There several methods terminating data lines. method recommended parallel termination. resistor added parallel with receiver's lines order match data line characteristic impedance specified cable manufacture (120 common value). This value describes intrinsic impedance transmission line function line length. terminating resistor less than should used. Termination resistors should placed only extreme ends data line, more than terminations should placed system that does repeaters. This type termination clearly adds heavy loading system overload port powered RS-232 RS-485 converters. Another type termination, coupled termination, adds small capacitor series with RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
termination resistor eliminate loading effect. Although this method eliminates loading, capacitor selection highly dependent system properties. System designers interested termination encouraged read National Semiconductors Application Note 9032 further information. Figure illustrates both parallel termination RS-485 two-wire node. four-wire systems, termination placed across receiver node.
Figure Parallel Termination
Biasing RS-485 Network
When RS-485 network idle state, nodes listen (receive) mode. Under this condition there active drivers network, drivers tristated. Without anything driving network, state line unknown. voltage level receiver's inputs less than ±200 logic level output receivers will value last received. order maintain proper idle voltage state, bias resistors must applied force data lines idle condition. Bias resistors nothing more than pullup resistor data line (typically volts) pulldown ground) data line. Figure illustrates placement bias resistors transceiver twowire configuration. Note that RS-485 four-wire configuration, bias resistors should placed receiver lines. value bias resistors dependent termination number nodes system. goal generate enough bias current network maintain minimum between data line. Consider following examples bias resistor calculation.
Refer Chapter information National Semiconductors Application Notes. RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Bias Resistor
Bias Resistor
Figure Transceiver with Bias Resistors
Example node, RS-485 network with termination resistors Each RS-485 node load impedance 12K. nodes parallel give load 1200 Additionally, termination resistors result another load, total load Clearly termination resistors responsible majority loading. order maintain least 200mV between line, need bias current flow through load. create this bias from supply total series resistance 1428 less required. Subtract that already part load, left with 1371 Placing half this value pullup half pulldown ground gives maximum bias resistor value each biasing resistors.
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Example node, RS-485 network without termination Each RS-485 node load impedance 12K. nodes parallel gives total load order maintain least across need current 0.53 generate this current from supply requires total resistance 9375 maximum. Since this total receiver load, bias resistors must less. Notice that very little bias current required systems without termination. Bias resistors placed anywhere network split among multiple nodes. parallel combination bias resistors system must equal less than calculated biasing requirements. Electronics uses 4.7K bias resistors RS-485 products. This value adequate most systems without termination. system designer should always calculate biasing requirements network. Symptoms under biasing range from decreased noise immunity complete data failure. Over biasing less effect system, primary result increased load drivers. Systems using port powered RS-232 RS-485 converters sensitive over biasing.
Extending Specification
Some systems require longer distances higher numbers nodes than supported RS-422 RS-485. Repeaters commonly used overcome these barriers. RS-485 repeater such Electronics' 485OP placed system divide load into multiple segments. Each "refreshed" signal capable driving another 4000 feet cable additional RS485 loads. Another method increasing number RS-485 nodes load type RS-485 receivers. These receivers higher input impedance reduce load RS-485 drivers increase total number nodes. There currently half quarter load integrated circuit receivers available, extending total allowable number nodes 128.
RS-422/485 Application Note
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Chapter Selecting RS-422 RS-485 Cabling
Cable selection RS-422 RS-485 systems often neglected. Attention details selection process prevent costly prospect re-pulling thousands feet cable.
Number Conductors
signal ground conductor often overlooked when ordering cable. extra twisted pair must specified have enough conductors signal ground. wire system then requires twisted pair, four wire system requires three twisted pair.
Shielding
often hard quantify shielded cable required application not. Since added cost shielded cable usually minimal worth installing first time.
Cable Characteristics
When choosing transmission line RS-422 RS-485, necessary examine required distance cable data rate system. Appendix RS-422-A Standard presents empirical curve that relates Cable Length Data Rate twisted-pair telephone cable that shunt capacitance pF/ft. terminated ohms (see Figure 3.1). This curve based signal quality requirements Signal rise fall time equal less than, one-half unit interval applicable modulation rate. maximum voltage loss between driver load
RS-422/485 Application Note
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Figure
Losses transmission line combination losses (skin effect), conductor loss, leakage, losses dielectric. high quality cable, conductor losses dielectric losses same order magnitude. Figure included this application note point significant difference performance different cables. This chart shows Attenuation versus Frequency three different Belden cables. Note that polyethylene cables offer much lower attenuation than cables.
RS-422/485 Application Note
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Figure
Another approach choosing transmission line "E-GRADE Program," which been established Anixter Bros. Inc. Anixter worldwide distributor wiring system products. Under this program, Anixter divides data interface cables into four categories follows: E-GRADE E-GRADE E-GRADE E-GRADE LIMITED DISTANCE STANDARD DISTANCE EXTENDED DISTANCE MAXIMUM DISTANCE
Simple charts used help user select proper cable without technical understanding cable parameters. This program divides usage categories into EIA-232-D, EIA-422-A, EIA-423-A. When using this literature, EIA-422-A charts choosing RS-485 cable.
RS-422/485 Application Note
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Chapter Transient Protection RS-422 RS-485 Systems
first step towards protecting RS-422 RS-485 system from transients understanding nature energy guarding against. Transient energy come from several sources, most typically environmental conditions induced switching heavy inductive loads.
What does surge look like?
Surge Specifications While transients always conform industry specifications, both Institute Electrical Electronics Engineers (IEEE) International Electrotechnical Commission (IEC) have developed transient models evaluating electrical electronic equipment immunity surges. These models offer some insight into types energy that must controlled prevent system damage. Both 1000-4-5: 1995 "Surge Immunity Test" IEEE C62.41-1991 "IEEE Recommended Practice Surge Voltages Low-Voltage Power Circuits" define "1.2/50µs 8/20µs combination wave" surge which voltage rise time with decay across open circuit. specified current waveform rise time with decay into short circuit. Open circuit voltages levels from commonly used both positive negative polarities, although under some circumstances voltages high applied. Figures illustrate combination wave characteristics. addition, IEEE C62.41 also specifies "ring wave" test. ring wave rise time decaying oscillation with source impedance shown Figure 4.3. Typical amplitudes ring wave also range from
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
1.2/50 uSecond Voltage Wave V(t) Time,
Figure Combination Wave Voltage Waveform
8/20 uSecond Current Wave
V(t)/Vp
Time,
Figure Combination Wave Current Waveform
RS-422/485 Application Note
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100kHz Ring Wave V(t)/Vp
-0.2 -0.4 -0.6 -0.8 Time,
Figure Ring Wave Common Mode Differential Mode Identifying type surges that threaten system important part selecting appropriate levels methods transient protection. Since each conductors data cable travels through same physical space, reasonable expect transients caused environmental current switching "common mode" that present data ground conductors within data cable. some installations, there another source unwanted energy consider. there high voltage cables running anywhere near data cables, potential fault condition exists result insulation failures inadvertent contact installer. This type surge could contact number conductors data cable, presenting "differential" surge data equipment. Although voltages currents associated with this type surge much lower than types surges modeled ANSI IEC, they have particularly destructive quality their own. Instead dissipating within several milliseconds, they exist steady state condition data network.
RS-422/485 Application Note
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Ground Differences
Realizing that transient energy high frequency nature leads some disturbing observations. frequencies this magnitude, difficult make impedance electrical connection between points inductance path between them. Whether that path several feet cable thousands feet earth between grounding systems, during transient event there hundreds thousands volts potential between different "grounds". longer assume that points connected wire will same voltage potential. system designer this means that although RS-422/485 uses differential signaling, remote node signal superimposed transient hundreds thousands volts with respect that nodes local ground. more intuitive refer what commonly called "signal ground" "signal reference". connect system nodes knowing that these large potential differences between grounds exist? first step towards successful protection assure that each device system referenced only ground, eliminating path through device surge currents searching return. There approaches creating this idyllic ground state. first approach isolate data ground from host device ground, this typically done with transformers optical isolators shown Figure 4.4. second approach each grounds device together (typically power ground data ground) with impedance connection shown Figure 4.5. These techniques lead basic methods transient protection.
Device
Isolated Power
Port
Data Lines
Optical Isolation
Figure Isolated RS-485 Device
RS-422/485 Application Note
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Device
Port Data Lines
Ground line
Local Chassis Ground Connection Figure RS-485 Device with Signal Ground Connected Chassis Ground
Transient protection using Isolation
Isolation Theory most universal approach protecting against transients galvanically isolate data port from host device circuitry. This method separates signal reference from fixed ground. Optical isolators, transformers fiber optics methods commonly used many types data networks isolate circuitry from host device. RS-422 RS485 applications, optical isolators most common. optical isolator integrated circuit that converts electrical signal light back, eliminating electrical continuity. With isolated port, entire isolated circuitry floats level transient without disrupting data communications. long floating level circuitry does exceed breakdown rating isolators (typically 1000 2500 volts) port will damaged. This type protection does attempt absorb shunt excess energy sensitive length transient. Even continuous potential differences will harm isolated devices. important note that isolators work common mode transients, they cannot protect against large voltage differences between conductors data cable such those caused short circuits between data power circuits.
RS-422/485 Application Note
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Isolation Devices Optical isolation implemented number ways. conversion from RS-232 RS-422 RS-485 being made, optically isolated converters available. Optically isolated serial cards replace existing ports systems. systems with existing RS-422 RS-485 ports, optically isolated repeater installed. Examples each these type devices found Electronics Data Communications catalog.
Transient Protection using Shunting
Shunting Theory Creating common ground host device provides safe place divert surge energy well voltage reference attach surge suppression devices Shunting harmful currents ground before they reach data port components such (often referred trade name Tranzorb), discharge tubes. These devices work "clamping" voltage, once clamp voltage been exceeded, devices provide impedance connection between terminals. Since this type device diverts large amount energy, cannot tolerate very long duration continuous transients. Shunting devices most often installed from each data line local earth ground, should selected begin conducting current voltage close possible above systems normal communications levels. RS-422 RS-485 systems, voltage rating selected typically volts. These devices typically some capacitive load data lines. This should considered when designing system compensated derating total line length compensate added load. Several hundred feet usually adequate figure. apply these type products correctly they should installed close port protected possible, user must provide extremely impedance connection local earth ground unit being protected. This ground connection crucial proper operation shunting device. ground connection should made with heavy gauge wire kept short possible. cable must longer than meter, copper strap braided cable intended grounding purposes must used protection device effective. addition high frequency nature transients, there enormous amount current present. Several thousand amps typically result from applications combination wave test ANSI specification.
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Connecting Signal Grounds Since local ground connection required each node implementing shunt type protection, consequences connecting remote grounds together must considered. During transient events high voltage potential exist between remote grounds. Only impedance wire connecting grounds limits current that results from this voltage potential. RS-422 RS-485 specification both recommend using resistors series with signal ground path order limit ground currents. Figure illustrates ground connection recommended specification.
Figure Signal Ground Connection between nodes with resistor Shunting Devices There types shunting devices choose from. least expensive type single stage, which usually consists single device each line. Three stage devices also available. first stage three stage device discharge tube, which handle extremely high currents, high threshold voltage slow protect solid state circuits. second stage small series impedance which limits current creates voltage drop between first third stage. final stage device which fast enough protect solid state devices brings clamping voltage down safe level data circuits.
Combining Isolation Shunting
Installing combination both types protection offer highest reliability system. Figures illustrate means implementing this level protection.
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Device
Isolated Power
Port
Shunting Device
Data Lines
Ground line
Earth Ground
Figure Isolated node with shunt protection earth ground
Device
Shunting Device
Isolated Power
Port
Data Lines
Signal Ground
Figure Isolated port with ungrounded shunt protection
method shown Figure recommended, this case isolation protects circuit from voltage drops earth ground connection. shunt devices will prevent surge from exceeding breakdown voltage isolators well handling differential surges cable. Figure illustrates method recommended cases where there make earth ground connection. Here, shunt device's function protect port from differential surges, differential surge will balanced between conductors shunting device, converted common mode. isolation provides protection from common mode transient remaining.
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Special Consideration Fault Conditions
Data systems that could exposed short circuits power conductors require extra measure protection. these cases recommended fuse type device addition shunting type suppression, shown Figure 4.9. When short circuit occurs, shunt suppression will begin conducting, shunting itself cannot withstand steady state currents this type surge. small enough fuse value should chosen that fuse will open before shunt device damaged. typical fuse value
Device
Data Lines Signal Ground Fuse
Earth Ground
Figure Fused port protection
Choosing right protection your system
While hard predict what type level isolation correct system, educated guess should made based electrical environment, physical conditions cost failures downtime repair costs. Systems connected between power sources, such building building, office factory floor, system covering long distances should require some level transient protection. Table comparison transient protection techniques.
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Table Comparison Protection Techniques Optical Isolation Shunting
Requires ground reference Adds loading data lines Higher complexity Effective common mode transients dependent installation quality Requires external power source affected long term continuous transients Must have impedance ground path Presents additional capacitive loading data lines Lower complexity, uses passive components Effective both common differential mode transients improperly installed user power required Subject damage long duration transients
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Chapter Software Introduction
RS-422 RS-485 hardware specifications. Software protocol discussed either specification. system designer define protocol suitable their system. This chapter will attempt define protocol standard, will explain some issues that should considered system designer, whether writing purchasing software.
RS-422 Systems
RS-422 system software differs little from familiar point-to-point RS232 communication systems. RS-422 often used simply extend distance between nodes over capabilities RS-232. RS-422 also used master node four wire master-slave network described later this chapter. When selecting writing software RS-422 systems designer should aware signals being used hardware system. Many RS-422 systems implement hardware handshake lines often found RS-232 systems cost running additional conductors over long distances.
RS-485 Driver Control
principle difference between RS-422 RS-485 that RS-485 driver into high impedance, tristate mode, which allows other drivers transmit over same pair wires. There methods tristating RS-485 driver. first method control line, often handshake line, enable disable driver. This requires that host software raise line before beginning transmission enable driver, then lower line after completion transmission. Since only single RS-485 driver enabled network time important that driver disabled quickly possible after transmission avoid drivers trying control lines simultaneously, condition called line contention. Under some operating systems difficult lower timely manner this method driver control should avoided altogether. second method RS-485 driver control refer Automatic Send Data Control. This type control involves special circuitry that senses when data being transmitted automatically enables driver well disabling driver within character length transmission. This preferred method driver control since reduces software overhead number potential pitfalls programmer.
RS-422/485 Application Note
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RS-485 Receiver Control
RS-485 receiver also enable signal. Since RS-485 systems using wire configuration connect driver receiver loopback fashion, this feature often used disable receiver during transmission prevent echo local data. Another approach leave RS-485 receiver enabled monitor loopback data errors which would indicate that line contention occurred. Although good loopback signal does guaranty data integrity does offer degree error detection.
Master-Slave Systems
master-slave type system node that issues commands each "slave" nodes processes responses. Slave nodes will typically transmit data without request from master node, communicate with each other. Each slave must have unique address that addressed independent other nodes. These type systems configured wire four wire. Four wire systems often RS-422 master (the driver always enabled) RS-485 slaves reduce system complexity. Four Wire Master-Slave Systems This configuration reduces software complexity host since driver receiver always enabled, expense installing extra conductors system. Master node simply prefixes commands with appropriate address slave. There data echo turn around delays consider. Since each slave transmitters share same pair wires, care must taken that master never requests data from multiple nodes simultaneously data collisions will result. Wire Master-Slave Systems wire configurations small amount complexity system. RS-485 driver must tristated when allow other nodes shared pair wires. time delay between transmission tristate condition becomes very important parameter this type system. slave attempts reply before master tristated line, collision will occur data will lost. system designer must know response time turn around delay each slave nodes assure that master will tristate driver within that amount time. Electronics' Automatic Send Data control circuits tristate driver within character length transmission.
RS-422/485 Application Note
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Multi-Master RS-485 Systems
Each node multi-master type RS-485 system initiate transmission creating potential data collisions. This type system requires designer implement more sophisticated method error detection, including methods such line contention detection, acknowledgement transmissions system resending corrupted data.
Systems with Port Powered Converters
RS-232 RS-422 RS-485 converters that derive their power from RS-232 port becoming more common data systems. good programming practice unused handshake outputs high voltage state systems using type RS-232 RS-422 RS-485 converter. This will assure best possible operating conditions converters used.
RS-422/485 Application Note
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Chapter Selecting RS-485 Devices
When purchasing devices RS-485 system many pitfalls avoided determining device's communications characteristics before system design complete. Knowing what questions front save troubleshooting field. following device characteristics things that should answered system design stage. device configured wire four wire systems? signal ground connection available? device isolated? Does contain surge suppression? What value bias resistors any) used device? they accessible modification? device terminated? accessible modification? What device's response time (turn around delay)? What programmable address range device? What baud rate, range baud rates, supported?
possible often useful have schematic serial port each device system. schematic provide additional information that useful troubleshooting repairing problems data system.
RS-422/485 Application Note
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Chapter Sources Further Information
Standards Publications purchased from: GLOBAL ENGINEERING DOCUMENTS 7730 Carondelet Avenue Clayton, 63105 Phone: (800) 854-7179 FAX: (314) 726-6418 GLOBAL ENGINEERING DOCUMENTS Inverness East Englewood, 80112 Phone: (800) 854-7179 FAX: (303) 397-2740 Global Engineering Documents site found http://global.ihs.com. Related data interface standards are: EIA-232-E Interface between data terminal equipment date circuitterminating equipment employing serial binary data interchange (ANSI/IEA-232-D) EIA-422-A Electrical characteristics balanced voltage digital interface circuits EIA-423-A Electrical characteristics unbalanced voltage digital interface circuits EIA-485 Standard electrical characteristics generators receivers balanced digital multipoint systems EIA-449 General purpose 37-position 9-position interface data terminal equipment data circuit-terminating equipment. EIA-530 High speed 25-position interface data terminal equipment data circuit-terminating equipment EIA/TIA-562 Electrical characteristics unbalanced digital interface Manufactures integrated circuit data transceivers often offer practical application information RS-422 RS-485 systems. National Semiconductor's Interface Data Book includes number excellent applications notes. These notes also available online http://www.national.com/. search engine provided search text available application notes. Entering "422" "485" search criteria current list related application notes.
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Appendix Specification Summary
RS-422 Specification Summary
Parameter Driver Output Voltage Open Circuit Driver Output Voltage Loaded Driver Output Resistance Driver Output Short-Circuit Current Driver Output Rise Time Driver Common Mode Voltage Receiver Sensitivity Receiver Common-Mode Voltage Range Receiver Input Resistance Differential Receiver Voltage Conditions output common 4000 Operational: Withstand: Units Width
±150 ±200
RS-485 Specification Summary
Parameter Driver Output Voltage Open Circuit Driver Output Voltage Loaded Driver Output ShortCircuit Current Driver Output Rise Time Driver Common Mode Voltage Receiver Sensitivity Receiver Common-Mode Voltage Range Receiver Input Resistance Conditions -1.5 -1.5 ±250 ±200 Units Width
RLOAD output +12V RLOAD CLOAD RLOAD
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
RS-232 Specification Summary
Parameter Driver Output Voltage Open Circuit Driver Output Voltage Loaded Driver Output Resistance, Power Driver Output Short-Circuit Current Driver Output Slew Rate Maximum Load Capacitance Receiver Input Resistance Receiver Input Threshold Output Mark Output Space Conditions 2500 7000 Units V/µs
3000
RS-423 Specification Summary
Parameter Driver Output Voltage Open Circuit Driver Output Voltage Loaded Driver Output Resistance Driver Output ShortCircuit Current Driver Output Rise Fall Time Receiver Sensitivity Receiver Input Resistance Conditions Baud Rate Baud Baud Rate Baud 4000 ±150 ±200 Units Unit Interval
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
Appendix Standard RS-423 Data Transmission
RS-423 (EIA-423) another standard used point point communications. RS-423 data transmission uses unbalanced line driver that connects RS-422 type balanced line receiver shown Figure B.1. RS-423 line driver unique this system. produces voltage similar RS-232 slew rate control input that used limit rise times cross talk data lines. Typical adjustment slew rate control from This done proper selection resistor wave shape control input.
Figure
RS-422/485 Application Note
Electronics 1040 Ottawa, 61350 (815) 433-5100 (815) 434-7094
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