 RS422/485
Application Note
Chapter 4: Transient Protection of RS-422
and RS-485 Systems
The first step towards protecting an RS-422 or RS-485 system
from transients is understanding the nature of the energy
we are guarding against. Transient energy may come from several
sources, most typically environmental conditions or induced
by switching heavy inductive loads.
What does a surge look like?
Surge Specifications
While transients may not always conform to industry specifications,
both the Institute of Electrical and Electronics Engineers
(IEEE) and the International Electrotechnical Commission (IEC)
have developed transient models for use in evaluating electrical
and electronic equipment for immunity to surges. These models
can offer some insight into the types of energy that must
be controlled to prevent system damage.
Both IEC 1000-4-5: 1995 "Surge Immunity Test" and IEEE C62.41-1991
"IEEE Recommended Practice on Surge Voltages in Low-Voltage
AC Power Circuits" define a "1.2/50µs - 8/20µs
combination wave" surge which has a 1.2 µs voltage rise time
with a 50 µs decay across an open circuit. The specified current
waveform has an 8 µs rise time with a 20 µs decay into a short
circuit. Open circuit voltages levels from 1 to 6 kV are commonly
used in both the positive and negative polarities, although,
under some circumstances, voltages as high as 20 kV may be
applied. Figures 4.1 and 4.2 illustrate the combination wave
characteristics. In addition, IEEE C62.41 also specifies a
100 kHz "ring wave" test. The ring wave has a 0.5 µs rise
time and a decaying oscillation at 100 kHz with source impedance
of 12 ohms as shown in Figure 4.3. Typical amplitudes for
the 100 kHz ring wave also range from 1 - 6 kV.
Figure 4.1 - Combination Wave Voltage Waveform
Figure 4.2 Combination Wave Current Waveform
Figure 4.3 100 kHz Ring Wave
Common Mode vs. Differential Mode
Identifying the type of surges that may threaten a system
is an important part of selecting the appropriate levels and
methods of transient protection. Since each of the conductors
in a data cable travels through the same physical space, it
is reasonable to expect transients caused by environmental
or current switching to be "common mode" that is, present
on all data and ground conductors within the data cable. In
some installations, there may be another source of unwanted
energy to consider. If there are high voltage cables running
anywhere near the data cables, the potential for a fault condition
exists as a result of insulation failures or inadvertent contact
by an installer. This type of surge could contact any number
of conductors in the data cable, presenting a "differential"
surge to the data equipment. Although the voltages and currents
associated with this type surge are much lower than the types
of surges modeled by ANSI or IEC, they have a particularly
destructive quality of their own. Instead of dissipating within
several milliseconds, they can exist in a steady state condition
on the data network.
Ground Is Not Equal To Ground
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
Figure 4.5 RS-485 Device with Signal Ground Connected to Chassis
Ground
Transient Protection using
Isolation
Isolation Theory
The most universal approach to protecting against transients
is to galvanically isolate the data port from the host device
circuitry. This method separates the signal reference from
any fixed ground. Optical isolators, transformers and fiber
optics are all methods commonly used in many types of data
networks to isolate I/O circuitry from its host device. In
RS-422 and RS-485 applications, optical isolators are most
common. An optical isolator is an integrated circuit that
converts the electrical signal to light and back, eliminating
electrical continuity. With an isolated port, the entire isolated
circuitry floats to the level of the transient without disrupting
data communications. As long as the floating level of the
circuitry does not exceed the breakdown rating of the isolators
(typically 1000 - 2500 volts) the port will not be damaged.
This type of protection does not attempt to absorb or shunt
excess energy so it is not sensitive to the length of the
transient. Even continuous potential differences will not
harm isolated devices. It is important to note that isolators
work on common mode transients, they cannot protect against
large voltage differences between conductors of a data cable
such as those caused by short circuits between data and power
circuits.
Isolation Devices
Optical isolation can be implemented in a number of ways.
If a conversion from RS-232 to RS-422 or RS-485 is being made,
optically isolated converters are available. Optically isolated
ISA bus serial cards can replace existing ports in PC systems.
For systems with existing RS-422 or RS-485 ports, an optically
isolated repeater can be installed. Examples of each of these
type devices can be found in the B&B Electronics Data Communications
catalog.
Transient Protection using
Shunting
Shunting Theory
Creating one common ground at the host device provides a safe
place to divert surge energy as well as a voltage reference
to attach surge suppression devices to. Shunting harmful currents
to ground before they reach the data port is the job of components
such as TVS (often referred to by the trade name Tranzorb),
MOV or gas discharge tubes. These devices all work by "clamping"
at a set voltage, once the clamp voltage has been exceeded,
the devices provide a low impedance connection between terminals.
Since this type of device diverts a large amount of energy,
it cannot tolerate very long duration or continuous transients.
Shunting devices are most often installed from each data line
to the local earth ground, and should be selected to begin
conducting current at a voltage as close as possible above
the systems normal communications levels. For RS-422 and RS-485
systems, the voltage rating selected is typically 6 - 8 volts.
These devices typically add some capacitive load to the data
lines. This should be considered when designing a system and
can be compensated for by derating the total line length to
compensate for the added load. Several hundred feet is usually
adequate.
To apply these type products correctly they should be installed
as close to the port to be protected as possible, and the
user must provide an extremely low impedance connection to
the local earth ground of the unit being protected. This ground
connection is crucial to proper operation of the shunting
device. The ground connection should be made with heavy gauge
wire and kept as short as possible. If the cable must be longer
than one meter, copper strap or braided cable intended for
grounding purposes must be used for the protection device
to be effective. In addition to the high frequency nature
of transients, there can be an enormous amount of current
present. Several thousand amps typically result from applications
of the combination wave test in the ANSI and IEC specification.
Connecting Signal Grounds
Since a local ground connection is required at each node implementing
shunt type protection, the consequences of connecting remote
grounds together must be considered. During transient events
a high voltage potential may exist between the remote grounds.
Only the impedance in the wire connecting the grounds limits
the current that results from this voltage potential. The
RS-422 and RS-485 specification both recommend using 100 ohm
resistors in series with the signal ground path in order to
limit ground currents. Figure 4.6 illustrates the ground connection
recommended in the specification.
Figure 4.6 Signal Ground Connection between two nodes with
100 ohm resistor
Shunting Devices
There are two types of shunting devices to choose from. The
least expensive type is single stage, which usually consists
of a single TVS device on each line. Three stage devices are
also available. The first stage of a three-stage device is
a gas discharge tube, which can handle extremely high currents,
but has a high threshold voltage and is too slow to protect
solid state circuits. The second stage is a small series impedance
which limits current and creates a voltage drop between the
first and third stage. The final stage is a TVS device that
is fast enough to protect solid state devices and brings the
clamping voltage down to a safe level for data circuits.
Combining Isolation
and Shunting
Installing a combination of both types of protection can offer
the highest reliability in a system. Figures 4.7 and 4.8 illustrate
two means of implementing this level of protection.
Figure 4.7 Isolated node with shunt protection to earth ground
Figure 4.8 Isolated port with ungrounded shunt protection
The method shown in Figure 4.7 is recommended, in this case
isolation protects the circuit from any voltage drops in the
earth ground connection. The shunt devices will prevent a
surge from exceeding the breakdown voltage of the isolators
as well as handling any differential surges on the cable.
Figure 4.8 illustrates a method recommended for cases where
there is no way to make an earth ground connection. Here,
the shunt device's function is to protect the port from differential
surges, a differential surge will be balanced between conductors
by the shunting device, converted to common mode. The isolation
provides protection from the common mode transient remaining.
Special Consideration
for Fault Conditions
Data systems that could be exposed to short circuits to power
conductors require an extra measure of protection. In these
cases its recommended to add a fuse type device in addition
to shunting type suppression, as shown in Figure 4.9. When
a short circuit occurs, the shunt suppression will begin conducting,
but shunting by itself cannot withstand the steady state currents
of this type of surge. A small enough fuse value should be
chosen so that the fuse will open before the shunt device
is damaged. A typical fuse value is 125 mA.
Figure 4.9 Fused port protection
Choosing the right protection
for your system
While it is hard to predict what type and level of isolation
is correct for a system, an educated guess should be made
based on the electrical environment, physical conditions and
cost of failures in downtime and repair costs. Systems connected
between two power sources, such as building to building, office
to factory floor, or any system covering long distances should
require some level of transient protection. Table 4.1 is a
comparison of transient protection techniques.
| Optical Isolation |
Shunting |
| Requires no ground reference |
Must have low impedence ground path |
| Adds no loading to data lines |
Presents additional capacitive loading
to data lines |
| Higher complexity |
Lower complexity, uses passive components |
| Effective on common mode transients |
Effective on both common and differential
mode transients |
| Not dependent on installation quality |
Can be improperly installed by user |
| Requires an external power source |
No power required |
| Not affected by long term or continuous
transients |
Subject to damage by long duration
transients |
Table 4.1 Comparison
of Protection Techniques
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