The standardized input circuit used by NCE is:
5VDC with a 10k pullup resistor resulting in a 500uA/.5 mA current draw
momentary normally open contact shorted to ground.
The AIU is really a very simple device. It is not aware of what you have it connected to. The board supplies +5v on one side and a ground on the other. It watches for the individual channels to be grounded. You can even just ground them by shorting them with a piece of wire. Any type of sensor, set of contacts, or relay can be used to close and trigger an AIU channel. In the simplest terms it is a 5V DC data line with a standard (+/-) connection. The circuit is normally open. When the circuit is closed / completed, the channel is considered active.