The interface on the domotica system (EIB KNX) has a voltage out of 5V but low current not enough to power the arduino UNO.
You do not power an Arduino through an input pin.
You might drive an input from your system but that is not the same thing. If you device has not enough power to drive an Arduino input it will not have enough power to drive an optical isolator as that in effect is an LED. Therefore you will need to amplify this signal before you apply it to the optical isolator with a transistor, thus giving you the inversion you need to cancel out the inversion from the isolator.