What happens when a voltage from another source is applied to one of the digital outputs? My system works on 24 V, but whilst the output is on, it may be connected to another potential which will be higher, up to 28 V. Will this be a problem for the Controllino?
The best thing to do is to isolate the two voltage potentials. If they are different you do NOT want to have them tied together – isolation is best. I would suggest either use one of your RELAY outputs which are just allowing you to complete a circuit for your 28 VDC source or if you need to drive the 28 VDC source by one of the digital pins you can do so by using a simple MOSFET or BJT transistor to act as a digital “switch” (you can get these as an optically isolated version as well).