def backward(self, speed=1):
        """
        Drive the motor backwards.

        :param float speed:
            The speed at which the motor should turn. Can be any value between
            0 (stopped) and the default 1 (maximum speed).
        """
        if isinstance(self.enable_device, DigitalOutputDevice):
            if speed not in (0, 1):
                raise ValueError(
                    'backward speed must be 0 or 1 with non-PWM Motors')
        self.enable_device.off()
        self.phase_device.on()
        self.enable_device.value = speed