I had assumed that all APU fire detection systems were Kidde systems. Kidde systems are wired in series.
I recently worked on a Systron-Donner fire detection system on a APU.
I didn't verify resistance readings of each loop (two loops in each system) to see what the fire card might be reading. For a Kidde, we're looking at less than 20 ohms. I would guess 1.5K ohms for the Systron-Donner, but I would need to check one first.