800XA 5.1 has revision A
800XA HAS revisions A, B, C, D, E also ther is a feature pack PF4, for PM851 controller FP4 ( say in manual ) add more memory.
but someone knows what exactly version begins with this change.
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You can find the answer in this presentation: https://library.e.abb.com/public/867684fca45e3e8ec1257b57001537c7/3BSE074081_en
FP4 is extra on top of rev c and the first version to include the extra memory option for the A versions of the small controllers. You have to check if your license also includes the feature pack.