code: def get_serialnumber(self):
          
          command = const.CMD_OPTIONS_RRQ
          command_string = b'~SerialNumber\x00'
          response_size = 1024
          cmd_response = self.__send_command(command, command_string, response_size)
          if cmd_response.get('status'):
              serialnumber = self.__data.split(b'=', 1)[-1].split(b'\x00')[0]
              serialnumber = serialnumber.replace(b'=', b'')
              return serialnumber.decode() # string?
          else:
              raise ZKErrorResponse("Can't read serial number")
docs: :return: the serial number