def _wakeup(self):
        '''
        issue wakeup command to device to take out of standby mode.
        '''
        log.info("send: WAKEUP")
        for i in xrange(3):
            self.port.write('\n')  # wakeup device
            ack = self.port.read(len(self.WAKE_ACK))  # read wakeup string
            log_raw('read', ack)
            if ack == self.WAKE_ACK:
                return
        raise NoDeviceException('Can not access weather station')
issue wakeup command to device to take out of standby mode.