code: def get_usb_serial(self, port_num):
      
  
      port = self.port_map[str(port_num)]
      arg = ''.join(['DEVICE INFO,', self._addr, '.', port])
      cmd = (['esuit64', '-t', arg])
      info = subprocess.check_output(cmd, stderr=subprocess.STDOUT)
      serial = None
      if "SERIAL" in info:
        serial_info = info.split('SERIAL:')[1]
        serial = serial_info.split('\n')[0].strip()
        use_info = info.split('BY')[1].split(' ')[1]
        if use_info == 'NO':
          cmd = (['esuit64', '-t', 'AUTO USE ALL'])
          subprocess.check_output(cmd, stderr=subprocess.STDOUT)
          time.sleep(50.0/1000.0)
      else:
        raise ValueError('No USB device detected')
      return serial
docs: Get the device serial number
  
      Args:
        port_num: port number on the Cambrionix unit
  
      Return:
        usb device serial number