Specifications

Chapter 3 - Selecting RS-422 and RS-485 Cabling RS-422 And RS-485 Applications Ebook
Page 44 of 137 Manual Documentation Number: <DocNumber>
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or both conductors for the signal ground. A two-wire system then
requires two twisted pairs, and a four-wire system requires three
twisted pairs.
Figure 30: RS-422 Point-to-Point Connection
Shielding
It is often difficult to make a clear determination as to whether shielded
cable is required in an application or not. Since the added cost of
shielded cable is usually minimal in most cases it is worth installing the
first time. If shielded cable is used, the shield should be grounded at
one end only, preferably to earth ground.
It is not recommended to use a shield drain wire as the signal ground
between RS-422/485 transceivers.
Figure 31: RS-422 Point-to-Point Connection
Note: The preceding diagram shows the use of a shield around the signal cable. Although there is
some disagreement regarding the need for shielded cable, if a shield is used the drain wire
should be connected to earth ground a one end only. Do not use the shield drain wire as the
signal ground between RS-422/485 devices.