Use duplicate symtable instead of symtable for enum constant value table (this will later allow us to detect semantic errors in IEC 61131-3 source code)
FUNCTION_BLOCK DIFFEQ
VAR_INPUT
RUN : BOOL ; (* 1 = run, 0 = reset *)
XIN : REAL ;
A : ARRAY[1..] OF REAL ; (* Input coefficients *)
M : INT ; (* Length of input history *)
B : ARRAY[0..] OF REAL ; (* Output coefficients *)
N : INT ; (* Length of output history *)
END_VAR
VAR_OUTPUT XOUT : REAL := 0.0 ; END_VAR
VAR (* NOTE : Manufacturer may specify other array sizes *)
XI : ARRAY [0..128] OF REAL ; (* Input history *)
XO : ARRAY [0..128] OF REAL ; (* Output history *)
I : INT ;
END_VAR
XO[0] := XOUT ; XI[0] := XIN ;
XOUT := B[0] * XIN ;
IF RUN THEN
FOR I := M TO 1 BY -1 DO
XOUT := XOUT + A[I] * XO[I] ; XO[I] := XO[I-1];
END_FOR;
FOR I := N TO 1 BY -1 DO
XOUT := XOUT + B[I] * XI[I] ; XI[I] := XI[I-1];
END_FOR;
ELSE
FOR I := 1 TO M DO XO[I] := 0.0; END_FOR;
FOR I := 1 TO N DO XI[I] := 0.0; END_FOR;
END_IF ;
END_FUNCTION_BLOCK