I'm fitting a Cerbo GX to an off grid camper build and looking to use the digital inputs to monitor the alarm system, open door, etc. But I can't find specs on what can trigger the inputs. Can I use 12v ish triggers from the vehicle or do I need to run relays to isolate and use a 0v negative trigger? Also what is the current, voltage limit on the relays?
These inputs have 3.3V weak pull up resistors. They are designed to be pulled low with potential free (dry) contacts or an open collector opto-coupler.
Yes, I would use the contacts from a 12 volt relay in that application. Considering wkirby's answer above, including information about weak (I infer high resistance) pull up resistors, I also recommend that wiring to the relay contacts be shielded if it's a long run, or if you have any antennas nearby. Ground the shield at only one end - at or near the cerbo, to DC negative or connect it in parallel to the minus (-) input. Otherwise, you may get spurious inputs when transmitting.
Using shielded lines for all high impedance/resistance sources also makes them less vulnerable to lightning damage from induced currents. I hope there are electrostatic discharge (ESD) diodes on all of the cerbo inputs, but there's no way to confirm it.
