code: def _send_mbus(self, frame):
          
          # for auto_open mode, check TCP and open if need
          if self.__auto_open and not self.is_open():
              self.open()
          # send request
          bytes_send = self._send(frame)
          if bytes_send:
              if self.__debug:
                  self._pretty_dump('Tx', frame)
              return bytes_send
          else:
              return None
docs: Send modbus frame
  
          :param frame: modbus frame to send (with MBAP for TCP/CRC for RTU)
          :type frame: str (Python2) or class bytes (Python3)
          :returns: number of bytes send or None if error
          :rtype: int or None