RS-485/RS-422 Circuit Implementation Guide: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 applica-tions. The key features of RS-485 that make it ideal for use in I&I communications applications are
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Long distance links—up to 4000 feet.
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Bidirectional communications possible over a single pair of twisted cables.
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Differential transmission increases noise immunity and decreases noise emissions.
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Multiple drivers and receivers can be connected on the same bus.
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Wide common-mode range allows for differences in ground potential between the driver and receiver.
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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.
The purpose of this application note is to discuss the imple-mentation of RS-485/RS-422 in an industrial environment. Applications for RS-485/RS-422 include process control networks; industrial automation; remote terminals; building automation, such as heating, ventilation, air conditioning (HVAC), security systems; motor control; and motion control.
TIA/EIA-485-A, the telecommunication industry’s most widely used transmission line standard, describes the physical layer of the RS-485 interface and is normally used with a higher-level protocol, such as Profibus, Interbus, Modbus, or BACnet. This allows for robust data transmission over relatively long distances. The RS-422 physical layer is described in TIA/EIA-422-B. The TIA/EIA-485-A standards are similar to those described in TIA/EIA-422-B, and the values used to specify the drivers and receivers in TIA/EIA-485-A standards are specified so that it can meet both standards.