INTRODUCTION
Industrial and instrumentation applications (I&I) require transmission of data between multiple systems often over very long distances. The RS-485 bus standard is one of the most widely used physical layer bus designs in I&I applications. The key features of RS-485 that make it ideal for use in I&I communications applications are
•  Long distance links—up to 4000 feet.
•  Bidirectional communications possible over a single pair of twisted cables.
•  Differential transmission increases noise immunity and decreases noise emissions.
•  Multiple drivers and receivers can be connected on the same bus.
•  Wide common-mode range allows for differences in ground potential between the driver and receiver.
•  TIA/EIA-485-A allow for data rates of up to 10 Mbps. Devices meeting the TIA/EIA-485-A specifications do not have to operate over the entire range and are not limited to 10 Mbps.