function THIStreamConvertor.Hex2Str;
const Convert: array['0'..'f'] of byte =
   (0, 1, 2, 3, 4, 5, 6, 7, 8, 9,16,16,16,16,16,16,
   16,10,11,12,13,14,15,16,16,16,16,16,16,16,16,16,
   16,16,16,16,16,16,16,16,16,16,16,16,16,16,16,16,
   16,10,11,12,13,14,15);
var len: Integer; ch:char;
begin
  FPos := 0;
  Check := 0;
  len := Length(S);
  SetLength(Result, len div 2);
  if len < 2 then Exit; {Too small}
  repeat
    ch := S[2*FPos+1];
    if (not(ch in['0'..'f']))or(Convert[ch]>15) then break;
    Result[FPos+1] := Chr((Convert[ch] shl 4));
    ch := S[2*FPos+2];
    if (not(ch in['0'..'f']))or(Convert[ch]>15) then break;
    inc(FPos);
    Result[FPos] := Chr(ord(Result[FPos])+Convert[ch]);
    Check := Check + ord(Result[FPos]);
  until false;
  SetLength(Result, FPos);
end;