Many digital inputs in the Digital Matter product range have a settable "active level". This determines the physical line level (high or low) at which the input is considered on. It can be thought of as inverting the polarity.


This is useful for pull-to-ground inputs like panic buttons. These would have a bias resistor set to pull up. When the button is pushed (on) the input is pulled to ground. The active level is set to low, meaning the input is on when the button is pushed.


The default option on DM devices is usually "active low"


Also see the note on bias resistors here.