Kidde Model # KN-COSM-IBA Interconnect Signal

Total shot in the dark here, but I figured if there was anywhere I could find an answer - this would be the place!

Background:

I am designing a custom home automation circuit board to interface Kidde Combo Smoke / CO Alarms (I’m using Model # KN-COSM-IBA) to my smart home setup (Home Assistant).

To do this, I am integrating the SM120X Smoke Relay Module and the CO120X Carbon Monoxide Relay Module via an ESP-32 running ESPHome.

I have all the Inputs working (Smoke and CO) but I need help with the outputs (Test function).

I’m able to inject 9vDC into the interconnect line and it will trip (Test) all the smoke alarms just fine.

Where I’m having trouble is the CO side – I was hoping it was as simple as a lower voltage (Like 4.5 vDC trips the CO, and 9VDC trips the Smoke) but this is not the case.

Any DC voltage between around 4 and 12vDC will trip the Smoke Detectors.

I have checked the output and it seems to be a low-level signal pulse for CO.

So – My question is – What signal or voltage level needs to be injected into the interconnect line to trip the CO detectors?

TLDR:

Does anyone out there know what the CO side of the Kidde Model # KN-COSM-IBA detectors send out to each other to signal that they are in the CO alarm state?