Specifications

Chapter 4 - Transient Protection of RS-422 and RS-485 Systems RS-422 And RS-485 Applications Ebook
Page 62 of 137 Manual Documentation Number: <DocNumber>
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The following diagram shows an isolated RS-232 to RS-485 converter
with TVS devices connected across the signal lines, as well as from the
signal lines to the signal ground. This method is 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, which will be balanced between conductors by the shunting
device, converted to common mode. The isolation provides protection
from any common mode transient remaining.
Figure 46: Isolated port with ungrounded shunt protection
Note: Sometimes when shunting protection is used at both ends of a run between two buildings,
data communications are lost as soon as the second shunting device is grounded. This can
happen when there are AC or DC voltages between the buildings. Usually it is
recommended that isolation should be added at one end and shunting at the other, or
isolation and shunting at one end and isolation at the other. The best solution is usually RS-
485 to fiber converters between buildings. A few meters of fiber provides more than 2,000
volts of isolation.
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