Texas Instruments 4Q 2006
Interface Selection Guide
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.
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
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
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.
RS-485/422 Family of Products
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
Resources
For a complete list of resources (evaluation modules, data sheets and application notes), visit
interface.ti.com
RS-485/422
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