Retrieves the FTDI device serial number.

        :returns: string containing the device serial number

def _get_serial_number(self):
        """
        Retrieves the FTDI device serial number.

        :returns: string containing the device serial number
        """
        return usb.util.get_string(self._device.usb_dev, 64, self._device.usb_dev.iSerialNumber)