MS-2443 Datasheet, PDF (1/10 Pages) Analog Devices – Safeguard Your RS-485 Communication Networks from Harmful EMC Events

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Safeguard Your RS-485
Communication Networks
from Harmful EMC Events
by James Scanlon, Senior Evaluation Engineer, Analog
Devices, Inc., and Koenraad Rutgers, Senior Field
Applications Engineer, Bourns, Inc.
IDEA IN BRIEF
In real industrial and instrumentation (I&I) applications,
RS-485 interface links must work in harsh electromagnetic
environments. Large transient voltages caused by lightning
strikes, electrostatic discharge, and other electromagnetic
phenomenon can damage communications ports. To ensure
that these data ports can survive in their final installation
environments, they must meet certain electromagnetic
compatibility (EMC) regulations.
These requirements include three main transient immunity
standards: electrostatic discharge, electrical fast transients,
and surge.
Many EMC problems are not simple or obvious, so they must
be considered at the start of the product design. Leaving
these considerations to the end of the design cycle can lead
to overruns in engineering budget and schedule.
This article describes each of these main transient types and
presents and demonstrates three different EMC compliant
solutions for three different cost/protection levels on RS-485
communication ports.
Analog Devices, Inc., and Bourns, Inc., have partnered
to extend their offering of system oriented solutions by
co-developing the industry’s first EMC compliant RS-485
interface design tool that provides up to Level 4 protection
levels for IEC61000-4-2 ESD, IEC61000-4-4 EFT, and
IEC61000-4-5 surge. It gives designers the design options
depending on the level of protection required and available
budgets. These design tools allow designers to reduce risk of
project slippage due to EMC problems by considering them
at the start of the design cycle.
Technical Article
MS-2443
RS-485 STANDARD
I&I applications require data transmission between multiple
systems, often over very long distances. The RS-485 electrical
standard is one of the most widely used physical layer
specifications in I&I applications, such as industrial
automation, process control, motor control, and motion
control; remote terminals; building automation, such as
heating, ventilation, and air conditioning (HVAC); security
systems; renewable energy.
Some of the key features of the 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 common-mode
noise immunity and decreases noise emissions
• Multiple drivers and receivers can be connected on the
same bus
• Wide common-mode range (–7 V to +12 V) allows for
differences in ground potential between the driver and
receiver
• TIA/EIA-485-A allows for data rates of up to 10’s
of Mbps
TIA/EIA-485-A 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.
In real applications, however, lightning strikes, power
induction and direct contact, power source fluctuations,
inductive switching, and electrostatic discharge can damage
RS-485 transceivers by generating large transient voltages.
Designers must ensure that equipment does not only work
under ideal conditions but that it will also work in the “real
world”. To ensure that these designs can survive in electrically
harsh environments, various government agencies and
regulatory bodies have imposed EMC regulations. Compliance
with these regulations gives the end user assurance that
designs will operate as desired in these harsh environments.
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