Specifications

26 RS-422/485 Application Note
 Copyright B&B Electronics -- Revised October 1997
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
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Ground Differences
Realizing that transient energy can be high frequency in nature leads to
some disturbing observations. At frequencies of this magnitude, it is difficult to
make a low impedance electrical connection between two points due to the
inductance of the path between them. Whether that path is several feet of cable
or thousands of feet of earth between grounding systems, during a transient event
there can be hundreds or thousands of volts potential between different
“grounds”. We can no longer assume that two points connected by a wire will
be at the same voltage potential. To the system designer this means that
although RS-422/485 uses 5V differential signaling, a remote node may see the
5V signal superimposed on a transient of hundreds or thousands of volts with
respect to that nodes local ground. It is more intuitive to refer to what is
commonly called “signal ground” as a “signal reference”.
How do we connect system nodes knowing that these large potential
differences between grounds may exist? The first step towards successful
protection is to assure that each device in the system is referenced to only one
ground, eliminating the path through the device for surge currents searching for a
return. There are two approaches to creating this idyllic ground state. The first
approach is to isolate the data ground from the host device ground, this is
typically done with transformers or optical isolators as shown is Figure 4.4. The
second approach is to tie each of the grounds on a device together (typically
power ground and data ground) with a low impedance connection as shown in
Figure 4.5. These two techniques lead us to the two basic methods of transient
protection.
Figure 4.4 Isolated RS-485 Device
Device
Isolated Power
Vcc
Optical
Isolation
Port
Data Lines Out