Navio provides RC (receiver) input via a connector on it's servo rail mapped to GPIO pin 4. Navio+ has a logic level converter and you can connect receivers which generate both 3.3V and 5V signals. The older Navio model only has a built-in voltage divider in PPM Input that lowers the voltage level from 5V to 3.3V. So if you connect a 3.3V PPM device (which is rare) to the original Navio, no signal will not be detected.