Specifications

RS-422/485 Application Note 25
 Copyright B&B Electronics -- Revised October 1997
B&B Electronics -- PO Box 1040 -- Ottawa, IL 61350
PH [phone] -- FAX [phone]
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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.