def read_word_data(self, addr, cmd):
        """read_word_data(addr, cmd) -> result

        Perform SMBus Read Word Data transaction.
        """
        self._set_addr(addr)
        result = SMBUS.i2c_smbus_read_word_data(self._fd, ffi.cast("__u8", cmd))
        if result == -1:
            raise IOError(ffi.errno)
        return result