First, you need to know that the block FC105 "SCALE" taka a Integer value (convertion of A/D) in the input "IN" and convert into REAL Value. For example: if you have a 4 to 20 ma level meter, that have a range betwind 0 to 1000 Lts
4ma----->0Lts--------->0 INT (Input of block) Unipolar!!
12ma---->500Lts------->13824 INT
20ma---->1000Lts------>27648 INT
Now you know how to force the analog input I hope!!
You need to be more specific in what you are trying to do...
If you are trying to write to an analog input, your value gets overwritten on every scan by the PLC. You should change your code to a temporary variable, like MWxxx, for troubleshooting.