@user_3257445 hello, I searched the ar550 manual, the manual said that the signal type which sent out of the DO interface when short circuit and open circuit is passive DO, it seems that no extra configurations are necessary for this function.
you said you testedd few of AR550 nothing happened when unplug one of the power supply, in fact, I'm not sure how the test is done, I googled that the passive DO signal needs the connected device is with power supply, maybe you can try this.