All redundancy will be configured and handled in the ControlLogix (CLX) equipment. Besides a redundant-capable CPU, each chassis will need a 1756-CRM module (this takes two slots in a CLX rack), plus an ENBT module in each chassis. This configuration uses all four slots in the local redundant CLX chassis, one for the CPU, two for the CRM, and one for the ENBT.
I/O modules will be placed in remote chassis and the PC56 should also be in a remote chassis. The ENBT cards of the two local chassis and the PC56-SLS in a remote chassis will all need to be peers on a local Ethernet spur. Then, whichever redundant CLX CPU is currently the primary CPU will talk to the remote chassis via ControlNet for regular I/O. However, the PC56-SLS will not be able to get data from the Primary CPU via ControlNet but will use Ethernet instead. That's why the Local chassis need to have an ENBT card; not for CPU redundancy but so that the PC56-SLS will be able to get data via Ethernet from whichever CPU is primary.