def get_usb_serial(self, port_num):
    """Get the device serial number

    Args:
      port_num: port number on the Cambrionix unit

    Return:
      usb device serial number
    """

    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
Get the device serial number

    Args:
      port_num: port number on the Cambrionix unit

    Return:
      usb device serial number