FUNCTION CHECKSUM: STRING[2]  // FC1012

VAR_INPUT

S:STRING[58];              //INPUT  STRING

L:INT;                     //INPUT 

END_VAR

VAR_OUTPUT

CSB:BYTE;

//AS1O, AS2O : STRING[1];               // OUTPUT 

END_VAR

VAR_TEMP

i,j,f,A1,A2: INT;

CS: BYTE;

AS1,AS2:STRING[1] ;

SS:STRING[58];

AST:STRING[2];

END_VAR

BEGIN

SS:=S;

CS:=0;

f:=L;   //BYTE_TO_INT(db1004.db205); //

FOR i:=1 TO f BY 1 DO

CS:=CS XOR CHAR_TO_BYTE(STRING_TO_CHAR(MID(in:=SS, L:=1, P:=i)));

CSB:=CS;

END_FOR;

A1:=BYTE_TO_INT(CSB) DIV 16;

A2:=BYTE_TO_INT(CSB) MOD 16;

CASE A1 OF

0: AS1:='0';

1: AS1:='1';

2: AS1:='2';

3: AS1:='3';

4: AS1:='4';

5: AS1:='5';

6: AS1:='6';

7: AS1:='7';

8: AS1:='8';

9: AS1:='9';

10: AS1:='A';

11: AS1:='B';

12: AS1:='C';

13: AS1:='D';

14: AS1:='E';

15: AS1:='F';

END_CASE;

CASE A2 OF

0: AS2:='0';

1: AS2:='1';

2: AS2:='2';

3: AS2:='3';

4: AS2:='4';

5: AS2:='5';

6: AS2:='6';

7: AS2:='7';

8: AS2:='8';

9: AS2:='9';

10: AS2:='A';

11: AS2:='B';

12: AS2:='C';

13: AS2:='D';

14: AS2:='E';

15: AS2:='F';

END_CASE;

AST:=CONCAT(IN1:=AS1,IN2:=AS2);

CHECKSUM:=AST;//"COMM".CS;

END_FUNCTION

 But, when i call this CHECKSUM in my MAIN function i have no results.