SQL> create or replace
  2  FUNCTION fn_convertascii(iv_ascii_string VARCHAR2)
  3      RETURN VARCHAR2
  4    AS
  5      v_string    VARCHAR2(32767);
  6      v_delimiter CONSTANT VARCHAR2(1):=',';
  7    BEGIN
  8      execute immediate REPLACE('select chr('||iv_ascii_string||') from dual',v_delimiter,')||chr(') INTO v_string;
  9      RETURN(v_string);
 10    EXCEPTION
 11    WHEN OTHERS THEN
 12    --DBMS_OUTPUT.PUT_LINE(lv_string);
 13      RETURN(iv_ascii_string);
 14    END;
 15  
 16  /
 
Function created
 
SQL> show errors;
No errors for FUNCTION SCOTT.FN_CONVERTASCII
 
SQL> select fn_convertascii('65,66,67,68,69,70') from dual;
 
FN_CONVERTASCII('65,66,67,68,6
--------------------------------------------------------------------------------
ABCDEF
 
SQL> select fn_convertascii('69,70') from dual;
 
FN_CONVERTASCII('69,70')
--------------------------------------------------------------------------------
EF
 
SQL>

SQL> create or replace
  2   FUNCTION fn_convertascii(iv_ascii_string VARCHAR2)
  3       RETURN VARCHAR2
  4     AS
  5       v_string    VARCHAR2(32767);
  6       v_delimiter CONSTANT VARCHAR2(1):=',';
  7     BEGIN
  8  --     execute immediate REPLACE('select chr('||iv_ascii_string||') from dual',v_delimiter,')||chr(') INTO v_string;
  9       dbms_output.put_line(REPLACE('select chr('||iv_ascii_string||') from dual',v_delimiter,')||chr('));
 10       RETURN(v_string);
 11  /*
 12     EXCEPTION
 13     WHEN OTHERS THEN
 14     --DBMS_OUTPUT.PUT_LINE(lv_string);
 15       RETURN(iv_ascii_string);
 16  */
 17      END;
 18  /

Function created.

SQL> set serveroutput on
SQL> select fn_convertascii('65,66,67,68,69,70') from dual;
FN_CONVERTASCII('65,66,67,68,69,70')
------------------------------------------------------------------------------------------------------------------------


1 row selected.

select chr(65)||chr(66)||chr(67)||chr(68)||chr(69)||chr(70) from dual

create or replace
  function fn_convertascii(
                           iv_ascii_string varchar2
                          )
    return varchar2
    as
        v_string    varchar2(32767);
        v_delimiter constant varchar2(1):=',';
        v_next_pos  number;
        v_pos       number := 1;
    begin
        loop
           v_next_pos := instr(iv_ascii_string,v_delimiter,v_pos);
           exit when v_next_pos = 0;
           v_string := v_string || chr(substr(iv_ascii_string,v_pos,v_next_pos - v_pos));
           v_pos := v_next_pos + 1;
        end loop;
        v_string := v_string || chr(substr(iv_ascii_string,v_pos));
        return v_string;
end;
/