Specifications

RS-422/485 Application Note 25
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax [phone]
0 5 10 15 20 25 30
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time, us
V(t)/Vp
100kHz Ring Wave
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.
26 RS-422/485 Application Note
© Copyright B&B Electronics -- Revised 2506
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax [phone]
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph 353-91-792444 – Fax [phone]
Ground ≠ 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
Device
Isolated Power
Vcc
Optical
Isolation
Port
Data Lines Out