APPENDICES
Appendix A: Communication Protocol and Wiring Considerations
A.1 Differences Between RS-485 and RS-422
Third-party manufacturers sometimes specify a device as either RS-485 or RS-422.
RS-485 and RS-422 terminology is interchangeable, but there is at least one wiring difference: Each can
have two-wire or four-wire connections, but an RS-422 connection needs four wires for two-way
communication.
An RS-422 four-wire network actually consists of two RS-422 two-wire, one-directional networks
communicating between the same devices.
The one-directional networks must be used because an RS-422 transmitter cannot turn off automatically
after transmission. An RS-422 transmitter that is On—even when not sending data—will prevent another
transmitter from effectively sending data.
An RS-485 transmitter, by contrast, can turn On and Off between transmissions. This makes a two-wire
connection preferable for many reasons, including the fact that two-wire networking allows for daisy-
chaining with no need to designate any devices as master, as four-wire requires.
The only advantage of four-wire over two-wire is that four-wire allows full duplex communications.
However, few protocols need full duplex communications, meaning there is little practical reason to run
four-wire RS-485. There are voltage and driver load differences between RS-485 and RS-422, but the
differences are negligible in normal use.
Liebert recommends using a two-wire connection unless:
• you are communicating with a device that uses RS-422 or
• the third-party device does not support two-wire RS-485.
Use of the two terms—RS-422 and RS-485—can be confusing in engineering drawings. When drawings
include Liebert SiteLink-E wiring details, Liebert suggests labeling any occurrences of RS-422 as “RS-422
(RS-485 four-wire).”
Vertiv | Liebert® SiteLink-E™ Installation Manual | 23
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