How to trigger warnings on an unplugged aural warning unit? I got an old 80-0557-11 Rev A manufactured by Grimmes from an EMB-120 Brasilia, and I was wondering how to make it speak.

I powered the component with a 28VDC power supply. It only said "Aural unit Ok" while I was expecting a lot of warnings, due the lack of connected sensors.

Checking an old wiring manual, I realized that most of the sensor inputs have a single connection, not a "HI/LO (+/-)" lines.

Does someone have some idea about how make it play alarms?

Thank you in advance for anyone who can explain me the logic of the system. Please, keep in mind that I'm just a enthusiast trying to better understand how this system works.
Answering my own question based on tips posted on comments:

As @Gerry suggested on comments, they are OPEN/GROUND, if i ground any of sensor cables it'll trigger the alarms. Those lines are 5V, and i think that most of sensors are just simple switches, except for the Fire Alarms, that i  couldn't trigger and has 6.8V over its line.

For now i got my AWU speaking.

Thank everybody who commented with their suggestions. Those were very very very useful,  you guys are awesome!