Detect Direction in S7

I need to to compare to REAL values in an S7 PLC and Generate an alarm if one of the values is falling while the other is rising. I was intending to do this in a FB probably in LAD as this is what most electricians on site are comfortable with using for fault finding. I have a quick look thru the existing FB's and FC's and haven't found anything that will suit my purpose. So before I go too far down this track I thought I would just check here if any one here knows of a pre-existing FB/FC that can do this or if you haver any betetr idea's /suggestions. Thanks in Advance