Specifications

Chapter 1 - Overview RS-422 And RS-485 Applications Ebook
Page 15 of 137 Manual Documentation Number: <DocNumber>
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coupled equally to both signal lines, TDA(-) and TDB(+). At the
receiver any signal that occurs equally on both signal lines is
subtracted by the differential inputs. This effectively eliminates noise
common to both inputs, leaving the communications signal, which can
then be detected and recovered.
Figure 8: Common Mode Voltage on Differential Inputs
However, differences in ground potential (including steady state
voltages and transients created by ground currents) between the
transmitter and receiver can create common mode voltages that
exceed the specifications of the receiver's inputs. Exceeding the V
CM
voltage specification of the device can cause loss of data and even
damage to the component itself.
The specifications for most RS-422 and RS-485 devices indicate that
the device can withstand a maximum V
CM
of -7 volts to +12 volts. The
function of the GND connection is to tie the signal grounds of all nodes
on a network to one common ground potential. This ensures that the
common mode voltage cannot exceed the specified value.
Depending on the application, several different techniques are used to
ensure common mode voltages do not exceed these specifications.
Refer to Transient Protection of RS-422 and RS-485 Systems (on page
49).