RRSS--442222 AANNDD RRSS--448855 - B&B Elec
RS-422 AND RS-485APPLICATIONS EBOOKA Practical Guide to Using RS-422 and RS-485 SerialInterfacesv. 1.0
International HeadquartersMfg. Co. Inc.707 Dayton RoadOttawa, IL 61350 USAPhone: (815) 433-5100 General Fax: (815) 433-5105Website: www.bb-smartworx.comEuropean HeadquartersWestlink Commercial ParkOranmore, Co. Galway, IrelandPhone: ( 353) 91-792444 Fax: ( 353) 91-792445Website: www.bb-smartworx.comRevision – Original –October 2010 2010 No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photography,recording, or any information storage and retrieval system without written consent. Information in this manual is subject to change without notice,and does not represent a commitment on the part .shall not be liable for incidental or consequential damages resulting from the furnishing, performance, or use of this manual.All brand names used in this manual are the registered trademarks of their respective owners. The use of trademarks or other designations in thispublication is for reference purposes only and does not constitute an endorsement by the trademark holder.This document was created using Author-it, Microsoft Word, Adobe Acrobat, SnagIt and other software.
Table of ContentsRS-422 and RS-485 Applications eBook .1Overview .5Assumptions . 6What is RS-422/RS-485? . 6Data Transmission Signals . 8Unbalanced Data Transmission . 9Balanced Data Transmission . 11RS-422 and RS-485 Terminals and Designations . 13Common Mode Voltage . 14Tri-State Control. 16RTS Control. 17Send Data Control . 18Bit-wise Enable Timing Control . 20Other Tri-State Control Techniques . 21System Configuration . 23RS-422 Point-to-Point Connections . 24Network Topologies . 25Two-Wire Systems . 25Four-Wire Systems . 27Termination . 27Deciding whether to use termination . 28Example calculation to determine whether termination is necessary . 29Methods of Termination . 29Biasing an RS-485 Network . 32Example 1: Biasing when termination resistors are used . 34Example 2: Biasing when termination resistors are not used . 36Other Biasing and Terminating Considerations . 38Using Repeaters . 39Using Repeaters in a Master-Slave Network . 40Using Repeaters in Star, Ring and Tree Networks . 41Using Fractional Load Receivers to Extend Distance and Number of Nodes . 42Page 1 of 137Manual Documentation Number: RS-422 and RS-485 Applications eBookwww.bb-smartworx.com/
Selecting RS-422 and RS-485 Cabling . 43Number of Conductors . 43Shielding . 44Cable Characteristics . 45Cable Length vs Data Rate . 45Attenuation vs Frequency . 46Using Cat 5 for RS-422/485 . 47Transient Protection of RS-422 and RS-485 Systems . 49What does a surge look like? . 49Surge Specifications . 50Common Mode versus Differential Mode . 52When Ground is not really Ground . 53Transient Protection using Isolation . 55Isolation Theory . 55Isolation Devices . 57Transient Protection using Shunting . 58Shunting Theory . 58Connecting Signal Grounds . 59Shunting Devices . 60Combining Isolation and Shunting . 60Special Consideration for Fault Conditions . 62Choosing the right protection for your system . 63Software. 65RS-422 Systems . 66RS-485 Driver Control . 66RS-485 Receiver Control. 68Master-Slave Systems . 68Four Wire Master-Slave Systems . 68Two Wire Master-Slave Systems . 70Multi-Master RS-485 Systems . 70Systems with Port Powered Converters. 71RS-485 Devices . 73RS-232 to RS-485 Converters . 74PC Serial Interface Cards . 75USB to RS-422/485 Converters . 76Page 2 of 137Manual Documentation Number: RS-422 and RS-485 Applications eBookwww.bb-smartworx.com/
Port Powered Converters . 77Repeaters . 78Optically Isolated RS-422/485 Devices . 79Sources of Additional Information. 81Global Engineering . 81List of EIA/TIA Standards . 82Integrated Circuit Manufacturers . 82Other sources of information . 83EIA Specification Summary . 85EIA-422 Specifications . 85EIA-485 Specifications . 86EIA-232 Specifications . 86EIA-423 Specifications . 87Comparison of RS-232, RS-423, RS-422 and RS-485 Specifications . 87EIA Standard RS-423 Data Transmission. 89Testing and Troubleshooting RS-422/485 Systems . 91Selecting RS-485 Devices . 91Troubleshooting Checklist . 92Troubleshooting Flowchart . 93RTS Control Timing Diagrams . 95Send Data Control Timing Diagrams . 96Bit-Wise Enable Timing Diagrams . 97Glossary of Terms . 99Index . 130Page 3 of 137Manual Documentation Number: RS-422 and RS-485 Applications eBookwww.bb-smartworx.com/
Chapter 1 - OverviewRS-422 And RS-485 Applications EbookCHAPTER 1OverviewThe purpose of this ebook is to describe the main elements of RS-422and RS-485 data communications systems. The authors haveattempted to cover enough technical details so that personnel will havethe necessary information to be successful in designing, modifying ortroubleshooting an RS-422 or RS-485 data communication system.Since both RS-422 and RS-485 are data transmission systems that usebalanced differential signals, it is appropriate to discuss both systemsin the same publication. Throughout this document the generic termsof RS-422 and RS-485 are used when referring to equipment andsystems based on the EIA/TIA-422 and EIA/TIA-485 Standards.1 In This ChapterAssumptions . 6What is RS-422/RS-485? . 6Data Transmission Signals . 8RS-422 and RS-485 Terminals and Designations . 13Common Mode Voltage . 14Tri-State Control . 16Page 5 of 137Manual Documentation Number: RS-422 and RS-485 Applications eBookwww.bb-smartworx.com/www.bb-europe.com/
Chapter 1 - OverviewRS-422 And RS-485 Applications EbookAssumptionsThis writers of this ebook have based the content and level of detail onseveral assumptions. The first assumption is that the personnel readingand using the information in it will have an understanding of basicelectricity and electronics. This includes an understanding of transistorinputs and outputs, current sourcing and sinking, the differencesbetween analog and digital circuits, signal propagation, signalinversion, digital rise and fall times, input and output impedance,single-ended and differential voltage measurements, and other datacommunication-related electronic concepts. Some explanation of theseconcepts is included, but if the reader is unfamiliar with these areas,some additional study may be required.The writers also assume that the users of this ebook will come fromseveral different perspectives. Some may be using the material fordesigning new communications systems; others will be using it in thecontext of installing and commissioning systems; still others will beinvolved in maintaining, expanding and troubleshooting problemsrelated to existing systems. This ebook may also be used in trainingand upgrading personnel.What is RS-422/RS-485?RS-422 and RS-485 are the common names for two serialcommunications standards. The standards, defined by the ElectronicsIndustry Association, are more correctly named EIA/TIA-422 andEIA/TIA-485.Communications systems based on RS-422 and RS-485 communicatedigital information over twisted pair wire from transmitters toreceivers. Devices can be up to 4000 feet (1220 meters) apart beforerepeaters are required. RS-422 drivers can transmit to up to 10receivers. Up to 32 RS-485 transceivers can occupy a bus. RS-422/485systems can communicate at rates up to 10 Mbps (though mostsystems operate at lower bit rates). Both systems utilize balancedoutputs and differential inputs, which provide better noise immunitythan single-ended systems such as RS-232. This results in the ability toPage 6 of 137Manual Documentation Number: RS-422 and RS-485 Applications eBookwww.bb-smartworx.com/www.bb-europe.com/
Chapter 1 - OverviewRS-422 And RS-485 Applications Ebookoperate over longer distances at higher bit rates than links using RS232.RS-422's capabilities make it a good choice for extending the distanceand speed of point-to-point connections. In situations where data mustbe communicated over long distances and through electrically noisyareas, RS-422 provides a higher reliability replacement for standardssuch as RS-232 .Figure 1: RS-422 Point-to-Point LinkRS-422 also makes it possible to create point-to-multipointconnections. In a point-to-multipoint arrangement the node originatingthe data (master) can broadcast data to several (slave) nodes at once.However, for the master to receive data back from each slave, RS-485devices are required.RS-485 can be used in two and four-wire networked systems. The tristate capabilities of RS-485 transmitters enable multiple transceivers(transmitter/receiver pairs) to be connected to a two-wire multidropbus. Alternately, a four-wire system can be used in which the masternode driver is connected to all slave receivers and all slave drivers areconnected to the master receiver.Figure 2: RS-485 Daisy Chain NetworkPage 7 of 137Manual Documentation Number: RS-422 and RS-485 Applications eBookwww.bb-smartworx.com/www.bb-europe.com/
Chapter 1 - OverviewRS-422 And RS-485 Applications EbookRS-485 is used as the basis for many commercial and industrial datacommunications systems. Industry systems such as PROFIBUS,INTERBUS, and others are built on RS-485 technology. RS-422 washistorically used in older Apple computers and printers, and in videoediting equipment.Often, end-users build systems themselves using RS-232 to RS422/485 converters, RS-485 smart switches, RS-485 repeaters, andother off-the-shelf products. These devices are commonly used to linkprogrammable logic controllers (PLC), supervisory control and dataacquisition (SCADA) systems, remote terminal units (RTU) and otherequipment in custom networked systems.The EIA/TIA-422 and EIA/TIA-485 standards do not define protocols.They are simply physical layer standards (and even then, do notspecify connectors or pin-outs). This means that RS-422 and RS-485can be implemented in many systems and applications. Often thestandards are used to extend serial links, or to network equipment thatwas not previously interconnected. Systems using the ubiquitousMODBUS protocol are often interconnected using RS-485 multidropnetworks.Data Transmission SignalsThere are several inescapable facts about data communications signalsthat must be considered in any data communications system. Data, inthe form of electromagnetic signals, must be created, sent along somekind of medium (which travels through a variety of environments) anddetected at its destination. For the purposes of this document thesignal is digital pulses and the medium is wire pairs.Digital pulses are voltages that represent binary states - ones andzeros. Voltages change from state to state, which creates rising andfalling edges. The number of pulses per second (baud rate) determineshow much data can be sent in a given period. The transmitters, mediaand receivers must be able to create, transfer and detect the pulsesreliably at the baud rate at which the system is operating. The noisepicked up in transit can affect the receiver's ability to reliably detectPage 8 of 137Manual Documentation Number: RS-422 and RS-485 Applications eBookwww.bb-smartworx.com/www.bb-europe.com/
Chapter 1 - OverviewRS-422 And RS-485 Applications Ebookreal data. Characteristics such as the bandwidth and amount ofattenuation in the media, the amount and type of noise, and thesensitivity of the receiver to the signal and noise, affect the maximumpossible data rate and transmission distance.Unbalanced Data TransmissionTo understand the advantages of balanced data transmission used inRS-422 and RS-485 systems it is helpful to review the characteristicsand features of unbalanced data transmission.RS-232 is an unbalanced transmission standard. Each signal thattransmits in an RS-232 system appears on the interface connector as avoltage with reference to a signal ground. For example, the transmitteddata from a DTE device appears on TD with respect to GND. (Whenusing the connector specified in the EIA-232 standard (DB-25) thesesignals are found on pin 2 and pin 7 respectively.)Page 9 of 137Manual Documentation Number: RS-422 and RS-485 Applications eBookwww.bb-smartworx.com/www.bb-europe.com/
Chapter 1 - OverviewRS-422 And RS-485 Applications EbookWhen the TD line is in the idle state (not transmitting), this voltage isnegative (representing a logic 1 or mark). A logic 0 or space isrepresented by a positive voltage. When transmitting data the voltagealternates between that negative level and a positive level. Typicallydata pulses are transmitted with a magnitude somewhere between 5and 15 volts. Although the EIA-232 standard allows a range of 3volts to 25 volts, an RS-232 receiver typically operates within thevoltage range of 3 volts and 12 volts.Figure 3: RS-232 Transmission SignalsUnbalanced data transmission works well over short distances, atlimited baud rates, especially in low noise environments. Unfortunately,unbalanced systems allow capacitively and inductively coupled noise tobe superimposed onto the signal (TD) line. If the noise attains levelsthat compete with the data signal, errors will occur. In environmentsw
troubleshooting an RS-422 or RS-485 data communication system. Since both RS-422 and RS-485 are data transmission systems that use balanced differential signals, it is appropriate to discuss both systems in the same publication. Throughout this document the generic terms of RS-422 and
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