Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

function NumStringToBCD(const inStr: string): string;
   function Pack(ch1, ch2: Char): Char;
   begin
     Assert((ch1 >= '0') and (ch1 <= '9'));
     Assert((ch2 >= '0') and (ch2 <= '9'));
       {Ord('0') is $30, so we can just use the low nybble of the character 
       as value.}
     Result := Chr((Ord(ch1) and $F) or ((Ord(ch2) and $F) shl 4))
   end;
 var
   i: Integer;
 begin
   if Odd(Length(inStr)) then
     Result := NumStringToBCD('0' + instr)
   else begin
     SetLength(Result, Length(inStr) div 2);
     for i := 1 to Length(Result) do
       Result[i] := Pack(inStr[2 * i - 1], inStr[2 * i]);
   end;
 end;

 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;