Texas Instruments 4Q 2006 manual RS-485/422 Family of Products

Design Considerations

Interoperability — In general, RS-485 is a superset of RS-422. Compliance with the TIA/EIA standard will ensure reliable data communication in a variety of networks, including Modbus, INTERBUS, PROFIBUS, BACnet and a variety of proprietary protocols.

Robustness — RS-485 is a robust interface standard for use in industrial environments. It features a wide common mode range of –7 V to 12 V. Parts from TI are available with ESD protection up to 30 kV.

range allows data exchange at signaling rates of up to 50 Mbps or to distances of several kilometers at lower rates.

Line Loading — RS-422 is capable of support- ing one driver and up to 10 receivers on the bus line. Standard RS-485 is capable of supporting up to 32 unit loads or nodes on the bus line. However, there are reduced unit load devices available that can support up to 256 devices.

Termination — A multipoint bus architecture requires termination at both ends of the bus line. The termination resistors must be within

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20 percent of the characteristic impedance of the cable and can vary from 90 Ω to 120 Ω.

Technical Information

•The main difference between RS-422 and RS-485 is the multidrop and multipoint bus architecture—that is, one driver to many receivers and many drivers to many receivers, respectively.

•Typical signaling rates and distances for these standards are up to 10 Mbps or up to 1.2 km. TI offers devices capable of reaching signaling rates of up to 50 Mbps.

Reliability — Integrated fail-safe circuitry protects the bus from interpreting noise as valid data when short-circuit, open-circuit or idle line fault conditions occur.

Speed and Distance — Low noise coupling of differential signaling with twisted-pair cabling and wide common-mode voltage

RS-485/422 Family of Products

Resources For a complete list of resources (evaluation modules, data sheets and application notes), visit interface.ti.com

Literature Number	Description
Application Notes
SLLA036B	Interface Circuits for TIA/EIA-485 (RS-485)
SLLA070C	422 and 485 Standards Overview and System Configurations
SLLA112	RS-485 for E-Meter Applications
SLLA177	PROFIBUS Electrical-Layer Solutions
SLLA169	Use Receiver Equalization to Extend RS-485 Data Communications
SLLA143	RS-485 for Digital Motor Control Applications

Note: IBIS models are available at interface.ti.com

Texas Instruments 4Q 2006 Interface Selection Guide