Tethered drone battery backup signal

I’m putting together a tethered drone and the tether has a battery backup system. The battery backup system will only fly the quad-copter for less than 2-minutes so the flight control needs to initiate an auto-land asap. The battery backup has a logic-level output indicating when it’s on battery. (logic HIGH for tether power, logic LOW for battery power) What is the best way to connect this status output to the Pixhawk 2? At minimum the pilot should get a obvious warning. Ideally it would warn the pilot, confirm the signal for 5-10 seconds, then auto-land.

It looks like I can wire the status line into aux 5 or 6 as long as BRD_PWM_CNT= 4. (or less) Then read in the logic level with “fmu” commands. I’m not clear how to use the input signal past this point. Skimmed some documents covering the failsafe systems, maybe the status signal should go to that? Also looks like the analog inputs can be configured to monitor extra batteries. Maybe I should configure a virtual battery with a low-voltage cut-off of 1.7 volts and connect the status pin to an analog input?

Tethered systems delivers constant voltage, so when your tether fails and the air unit switch to backup battery there will be a considerable voltage drop. You can setup your battery failsafe to detect this drop of voltage and initiate autoland.
Or you can put an small arduino onboard to check output from air unit and output whatever signal you can use (even can output a mavlink message to warn pilot on the ground)

Q: Is it ElistAIR ?

pixhawk 2 supports dual power modules, i would set one up on the battery and one on the teather to monitor the voltage and current from them separately. i would also have a diode between the teather and battery so if the teather fails you can see the voltage drop on it separate from the battery.

George, your solution assumes a common ground based tether system, which is not always the case, plus you will have to separate the power module from the current and voltage meter part. Tether do switch it’s power output to backup battery in case of tether failure.
And finally you can setup battery failsafe only for batt1, not for batt2.