Many digital inputs in the Digital Matter product range have a settable 'Active Level' and a configurable Bias Resistor. 


The Active Level determines the physical line level (high or low) at which the input is considered on. 

The Bias Resistor essentially determines the state of the input when nothing is connected to the input. 



One use case example is for pull-to-ground inputs like panic buttons. 

Typically they are set up using a Normally Open push button switch, one side is connected to the input, and the other to ground. 

In this configuration, when the button is not pressed, it is open circuit, and there is no complete path from the input to ground. When the button is pushed, the switch closes the loop. Connecting the input to a low signal (GND)


The configuration would be: bias resistor set to pull up. Then when the button is not pressed and nothing is connected to the input, the input is held high


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.


For Bias Resistors, there are usually 3 options:

  1. Pull up: pull the input high. 
  2. Pull down: pull the input low
  3. Disable: do not bias the input.
These options are useful when one of the states being read is open-circuit (disconnected/high impedance). In this case, enable the bias resistor setting in the opposite state to the level being switched to.  This is commonly used when working with an "open collector" output. The pull up/down is a "weak" pull up/down.


The default options for Active Level and Bias Resistor on DM devices is usually "active low" and 'Pull Up" respectively.


Results of different configurations. 


Bias ResistorActive HighInputResult
 Pull DownTrueLowInactive


HighActive
Pull DownFalseLowActive


HighInactive
Pull UpTrueLowInactive


HighActive
Pull UpFalseLowActive


HighInactive