Are Sealevel optically-isolated inputs active high or active low?
It depends on how the input is wired to your application. An input is triggered when the input terminal and local common terminal of the Sealevel device detects a voltage difference greater than or equal to the inputs turn-on voltage generally 3 VDC. The inputs are non-polarized so the voltage direction does not matter.
To wire an active high input, connect the local common terminal of the Sealevel device to the application Ground or negative voltage reference.
To wire an active low input, connect the local common terminal of the Sealevel device to the positive voltage reference.