The Ethernet backplane is only required if you plan on using any of the BME comm modules, Hart module, diagnostic info from the power supply, etc.
One key role of a rack is to provide a communication bus for the modules installed on the rack.
The X bus backplane is present on all Modicon X80 racks and all module slots have connections to the X bus backplane.
A subset of the Modicon X80 racks contains an additional Ethernet backplane. The number of module slots with connection to the Ethernet backplane is limited to 8. So, for the racks with more than eight module slots, some module slots have only connection to the X bus backplane.

Ethernet backplane is used for:
- eX80 I/O modules, which require an Ethernet bus on the rack in order to exchange data (for example, X80 HART modules)
- Third-party modules that require Ethernet
- Ethernet communication modules (interlinked to the CPU)

For any of these cases, use a Modicon X80 rack with dual Ethernet and X bus backplane. In other cases, a rack with only an X bus backplane is allowed. If you use an X Bus rack for any of the cases above, the Ethernet capabilities of the modules do not work, and the modules do not perform as expected.

A second key role of a rack is to provide power to the modules in the rack. A subset of the Modicon X80 racks contains an additional power supply slot to provide power redundancy.

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