Get or set unit ID field

        :param unit_id: unit ID (0 to 255) or None for get value
        :type unit_id: int or None
        :returns: unit ID or None if set fail
        :rtype: int or None

def unit_id(self, unit_id=None):
        """Get or set unit ID field

        :param unit_id: unit ID (0 to 255) or None for get value
        :type unit_id: int or None
        :returns: unit ID or None if set fail
        :rtype: int or None
        """
        if unit_id is None:
            return self.__unit_id
        if 0 <= int(unit_id) < 256:
            self.__unit_id = int(unit_id)
            return self.__unit_id
        else:
            return None