How to monitor state of 50 Digital Inputs
I had a project that used 16 rotary encoders and I used the MUX Shield which uses 74HC4067 analog multiplexers. They port to A0 - A3 on the shield which is fine as you can set these pins to Digital Inputs else you can get teh breakout version and connect to any input pins.
I also used a Due since ALL pins can be interrupt enabled.
Reading was a bit of a pain but worked like this...
The method is.. Continually address each input in turn in the loop or an interrupt is not detected, when an interrupt fires, address each input in turn again to find out which pin changed state.
Hope that helps.