def _send(self, data):
        """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
        """
        # 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
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