Specifications

Chapter 2 - System Configuration RS-422 And RS-485 Applications Ebook
Page 26 of 137 Manual Documentation Number: <DocNumber>
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additional ground conductor should be used to tie the signal grounds
together.
Note: RS-422/485 systems may communicate successfully without the signal ground when nodes
are located close together and circuit grounds for all nodes are at the same potential--e.g., a
controlled lab environment. However, this practice is not recommended. If a signal ground is
not used when nodes are separated by distance, and there is the possibility of lightning
and/or other electrical noise, the common mode voltage can rise to levels that could
compromise communications, or even damage the transceivers in the system nodes.
Two wire systems operate in half-duplex, "taking turns" using the
media to alternately transmit and receive data. A protocol,
implemented by the system software, determines which node(s)
initiate communications, and how other nodes respond. Often, RS-485
networks use simple master-slave protocols such as Modbus. For more
detail refer to Software (on page 65).
Figure 17: RS-485 Multidrop Network