code: def _send(self, data):
          
          # check link
          if self.__sock is None:
              self.__debug_msg('call _send on close socket')
              return None
          # send
          data_l = len(data)
          try:
              send_l = self.__sock.send(data)
          except socket.error:
              send_l = None
          # handle send error
          if (send_l is None) or (send_l != data_l):
              self.__last_error = const.MB_SEND_ERR
              self.__debug_msg('_send error')
              self.close()
              return None
          else:
              return send_l
docs: Send data over current socket
  
          :param data: registers value to write
          :type data: str (Python2) or class bytes (Python3)
          :returns: True if send ok or None if error
          :rtype: bool or None