Siemens ENCO Fucntion

I am testing with Siemens S7-300 FC96 ENCO fuctions. The CPU error pops out whenever I tried to read the return value of this fuction. Please advise.

A I 0.4
JCN OUT
Call "ENCO"
IN:= MD 20
RET_VAL:= MW 30

OUT: NOP 0

Questions:

1) What values are you inputting for MD20?
2) When you call the module diagnostics (ctrl-d), what error is displayed?
3) Are you sure the problem is in this function and not some other code that might be using this return value?

Thanks for your reply.

I have just found out the root cause.
I have not download this function FC96 into the PLC, all the while I only download the OB1 only.

Thanks a lot for your support.