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AN-960. APPLICATION NOTE. One Technology Way Box 9106 Norwood, MA 02062-9106, Tel: Fax: RS-485/RS-422 Circuit Implementation Guide by Hein Marais INTRODUCTION WHY USE DIFFERENTIAL DATA TRANSMISSION? Industrial and instrumentation applications (I&I) require The main reason why RS-485 can communicate over long transmission of data between multiple systems often over distances is the use of differential or balanced lines. A com- very long distances. The RS-485 bus standard is one of the munication channel requires a dedicated pair of signal lines most widely used physical layer bus designs in I&I applica- to exchange information. The voltage on one line equals the tions. The key features of RS-485 that make it ideal for use inverse of the voltage on the other line. in I&I communications applications are TIA/EIA-485-A designates the two lines in this differential pair Long distance links up to 4000 feet. as A and B.
For reliable RS-485 and RS-422 communications, it is essential that the reflections in the transmission line be kept as small as possible. This can only be done by proper cable termination.
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