function BCDToNumString(const inStr: string): string;
 procedure UnPack(ch: Char; var ch1, ch2: Char);
 begin
  ch1 := Chr((Ord(ch) and $F) + $30);
  ch2 := Chr(((Ord(ch) shr 4) and $F) + $30);
  Assert((ch1 >= '0') and (ch1 <= '9'));
  Assert((ch2 >= '0') and (ch2 <= '9'));
 end;
var
 i: Integer;
begin
 SetLength(Result, Length(inStr) * 2);
 for i := 1 to Length(inStr) do
  UnPack(inStr[i], Result[2 * i - 1], Result[2 * i]);
end;
procedure TForm1.Button1Click(Sender: TObject);
var
 S1, S2: string;
begin
 S1 := '15151515151515151515';
 S2 := NumStringToBCD(S1);
 memo1.lines.add('S1: ' + S1);
 memo1.lines.add('Length(S2): ' + IntToStr(Length(S2)));
 memo1.lines.add('S2 unpacked again: ' + BCDToNumString(S2));
end;