How to diagnostic the HSBY "Offline" reason

1.1LCD / Keypad
When a CPU switches to offline, the simple way to get the reason why the CPU entered this state is to use the LCD/Keypad:
Using the Keypad, select “PLC Operations “ Menu Then select “Hot StandBy”
The LCD will display:
State: OFFLINE
Select the “UP” arrow till getting the Diag Menu
The Diag Menu provides the reason for the CPU State.
1.2 Diagnostic Viewer
Another way to get some Diagnostic in an HSBY system is to use the Diagnostic Viewer in Unity Pro:
Select the “Tools” menu in Unity Pro
Select “Diagnostic viewer”
The Following screen appears:
It provides information about the history regarding the CPU and the failure reason.