function HexToBin(Hexadecimal: string): string;
 const
  BCD: array [0..15] of string =
  ('0000', '0001', '0010', '0011', '0100', '0101', '0110', '0111',
  '1000', '1001', '1010', '1011', '1100', '1101', '1110', '1111');
 var
  i: integer;
 begin
  for i := Length(Hexadecimal) downto 1 do
  Result := BCD[StrToInt('$' + Hexadecimal[i])] + Result;
 end;
 procedure TForm1.Button1Click(Sender: TObject);
 begin
  ShowMessage(HexToBin('FFA1'));
  // Returns 1111111110100001
end;