RS422 RS485技术标准

RS-422/485 Application NoteCover Page BB Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH 815 433-5100 -- FAX 815 434-7094 RS-422 and RS-485 Application Note BB Electronics Mfg. Co. Inc. P.O. Box 1040 -- Ottawa, IL 61350 PH 815 433-5100 -- FAX 815 434-7094 Internet Addresses BB Home Pagehttp//www.bb- Customer Servicesalesbb- Technical Supporttechsuptbb-  BB Electronics - Revised October 1997 RS-422/485 Application NoteTable of Contentsi BB Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH 815 433-5100 -- FAX 815 434-7094 Table of Contents CHAPTER 1 OVERVIEW.1 INTRODUCTION.1 DATA TRANSMISSION SIGNALS1 Unbalanced Line Drivers.1 Balanced Line Drivers.1 Balanced Line Receivers3 EIA STANDARD RS-422 DATA TRANSMISSION3 EIA STANDARD RS-485 DATA TRANSMISSION6 TRISTATE CONTROL OF AN RS-485 DEVICE USING RTS9 SEND DATA CONTROL OF AN RS-485 DEVICE11 CHAPTER 2 SYSTEM CONFIGURATION13 NETWORK TOPOLOGIES.13 TWO WIRE OR FOUR WIRE SYSTEMS13 TERMINATION.16 BIASING AN RS-485 NETWORK17 EXTENDING THE SPECIFICATION19 CHAPTER 3 SELECTING RS-422 AND RS-485 CABLING20 NUMBER OF CONDUCTORS20 SHIELDING.20 CABLE CHARACTERISTICS.20 CHAPTER 4 TRANSIENT PROTECTION OF RS-422 AND RS-485 SYSTEMS23 WHAT DOES A SURGE LOOK LIKE23 Surge Specifications.23 Common Mode vs. Differential Mode.25 GROUND ≠ GROUND26 TRANSIENT PROTECTION USING ISOLATION27 Isolation Theory.27 Isolation Devices .28 TRANSIENT PROTECTION USING SHUNTING28 Shunting Theory.28 Connecting Signal Grounds .29 Shunting Devices .29 COMBINING ISOLATION AND SHUNTING.29 SPECIAL CONSIDERATION FOR FAULT CONDITIONS.31 CHOOSING THE RIGHT PROTECTION FOR YOUR SYSTEM.31 iiTable of ContentsRS-422/485 Application Note BB Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH 815 433-5100 -- FAX 815 434-7094 CHAPTER 5 SOFTWARE.33 INTRODUCTION.33 RS-422 SYSTEMS.33 RS-485 DRIVER CONTROL33 RS-485 RECEIVER CONTROL.34 MASTER-SLAVE SYSTEMS.34 Four Wire Master-Slave Systems34 Two Wire Master-Slave Systems.34 MULTI-MASTER RS-485 SYSTEMS35 SYSTEMS WITH PORT POWERED CONVERTERS35 CHAPTER 6 SELECTING RS-485 DEVICES36 CHAPTER 7 SOURCES OF FURTHER INATION37 APPENDIX A EIA SPECIFICATION SUMMARY38 APPENDIX B EIA STANDARD RS-423 DATA TRANSMISSION40 RS-422/485 Application Note1 BB Electronics -- PO Box 1040 -- Ottawa, IL 61350 PH 815 433-5100 -- FAX 815 434-7094 Chapter 1 Overview Introduction The purpose of this application note is to describe the main elements of an RS-422 and RS-485 system. This application note attempts to cover enough technical details so that the system designer will have considered all the important aspects in his data system design. 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 application note. Throughout this application note the generic terms of RS-422 and RS-485 will be used to represent the EIA/TIA-422 and EIA/TIA-485 Standards. Data Transmission Signals Unbalanced Line