'Retrieve latest state.'

def update(self):

    if (self._nad_receiver.main_power('?') == 'Off'):
        self._state = STATE_OFF
    else:
        self._state = STATE_ON
    if (self._nad_receiver.main_mute('?') == 'Off'):
        self._mute = False
    else:
        self._mute = True
    self._volume = self.calc_volume(self._nad_receiver.main_volume('?'))
    self._source = self._source_dict.get(self._nad_receiver.main_source('?'))
