Specifications

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Table of Contents
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RS-422 and RS-485 Applications eBook ...................................................... 1
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