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RS-422 and RS-485 Applications eBook

RS-422 and RS-485 Applications eBook - A Practical Guide to Using RS-422 and RS-485 Serial Interfaces

The purpose of this ebook is to describe the main elements of RS-422 and RS-485 data communications systems. The authors have attempted to cover enough technical details so that personnel will have the necessary information to be successful in designing, modifying or 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 RS-485 are used when referring to equipment and systems based on the EIA/TIA-422 and EIA/TIA-485 Standards.

Table of Contents

Overview....................................................................................................5
     Assumptions ........................................................................................ 6
     What is RS-422/RS-485? ....................................................................... 6
     Data Transmission Signals .................................................................... 8
          Unbalanced Data Transmission ......................................................... 9
          Balanced Data Transmission ........................................................... 11
     RS-422 and RS-485 Terminals and Designations .................................... 13
     Common Mode Voltage......................................................................... 14
     Tri-State Control .................................................................................. 16
          RTS Control ................................................................................... 17
          Send Data Control........................................................................... 18
          Bit-wise Enable Timing Control......................................................... 20
          Other Tri-State Control Techniques................................................... 21
System Configuration ................................................................................23
      RS-422 Point-to-Point Connections....................................................... 24
      Network Topologies ............................................................................ 25
      Two-Wire Systems.............................................................................. 25
      Four-Wire Systems ............................................................................ 27
      Termination ....................................................................................... 27
          Deciding whether to use termination................................................. 28
          Example calculation to determine whether termination is necessary.... 29
          Methods of Termination .................................................................. 29
     Biasing an RS-485 Network.................................................................. 32
          Example 1: Biasing when termination resistors are used.................... 34
          Example 2: Biasing when termination resistors are not used .............. 36
     Other Biasing and Terminating Considerations ....................................... 38
     Using Repeaters ................................................................................. 39
          Using Repeaters in a Master-Slave Network ..................................... 40
          Using Repeaters in Star, Ring and Tree Networks ............................. 41
     Using Fractional Load Receivers to Extend Distance and Number of Nodes .42
Selecting RS-422 and RS-485 Cabling ........................................................43
     Number of Conductors ..........................................................................43
     Shielding............................................................................................. 44
     Cable Characteristics........................................................................... 45
          Cable Length vs Data Rate ............................................................. 45
          Attenuation vs Frequency................................................................ 46
     Using Cat 5 for RS-422/485 .................................................................. 47
Transient Protection of RS-422 and RS-485 Systems ...................................49
     What does a surge look like?................................................................. 49
          Surge Specifications ....................................................................... 50
          Common Mode versus Differential Mode ........................................... 52
     When Ground is not really Ground ......................................................... 53
     Transient Protection using Isolation........................................................ 55
          Isolation Theory .............................................................................. 55
          Isolation Devices ............................................................................ 57
     Transient Protection using Shunting ...................................................... 58
          Shunting Theory ............................................................................. 58
          Connecting Signal Grounds............................................................... 59
          Shunting Devices ............................................................................ 60
     Combining Isolation and Shunting........................................................... 60
     Special Consideration for Fault Conditions .............................................. 62
     Choosing the right protection for your system........................................... 63
Software ....................................................................................................65
     RS-422 Systems................................................................................... 66
     RS-485 Driver Control............................................................................. 66
     RS-485 Receiver Control......................................................................... 68
     Master-Slave Systems........................................................................... 68
          Four Wire Master-Slave Systems....................................................... 68
          Two Wire Master-Slave Systems ....................................................... 70
     Multi-Master RS-485 Systems ................................................................ 70
     Systems with Port Powered Converters ................................................... 71
RS-485 Devices ...........................................................................................73
      RS-232 to RS-485 Converters ................................................................. 74
      PC Serial Interface Cards ....................................................................... 75
      USB to RS-422/485 Converters ............................................................... 76
      Port Powered Converters ........................................................................ 77
      Repeaters.............................................................................................. 77
      Optically Isolated RS-422/485 Devices .................................................... 79
Sources of Additional Information....................................................................81
      Global Engineering...................................................................................81
      List of EIA/TIA Standards ........................................................................82
      Integrated Circuit Manufacturers................................................................82
      Other sources of information.................................................................... 83
EIA Specification Summary ...........................................................................85
      EIA-422 Specifications ........................................................................... 85
      EIA-485 Specifications ........................................................................... 86
      EIA-232 Specifications ........................................................................... 86
      EIA-423 Specifications ........................................................................... 87
      Comparison of RS-232, RS-423, RS-422 and RS-485 Specifications ........... 87
EIA Standard RS-423 Data Transmission.........................................................89
Testing and Troubleshooting RS-422/485 Systems...........................................91
      Selecting RS-485 Devices ...................................................................... 91
      Troubleshooting Checklist ...................................................................... 92
      Troubleshooting Flowchart ...................................................................... 93
      RTS Control Timing Diagrams.................................................................. 95
      Send Data Control Timing Diagrams ........................................................ 96
      Bit-Wise Enable Timing Diagrams............................................................ 97
Glossary of Terms ........................................................................................99
Index .........................................................................................................130