Common Concepts - Digital Inputs

Elliott Mace

Modified on: Thu, 2 Apr, 2020 at 3:52 PM

Externally powered devices in the Digital Matter range feature Digital Inputs. These inputs detect the voltage at the input and then record this as on "1" or off "0" and report to the server. This can be used to determine the state of an asset for example:

Door open/close
Ignition on/off
Pump on/off
Button presses

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.

Use Case Example - Panic Button

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

not pressed
an open circuit,
and there is no complete path from the input to ground.

When the button is ON it

Is pressed
closes the loop.
Connects the input to a low signal (GND)

The configuration would be:

Bias resistor set to 'pull up.'
- When the button is not pressed (OFF) and nothing is connected to the input, the input is held high.
The Active Level is set to low
- When the button is pressed (ON) the input is pulled to ground.
- The digital input is considered on when the button is pushed.

Uploads and Logging

If you wish to have vision of when the Digital Input is Active/Inactive you can set the related logging settings to Yes, if you want to enable a connection to the server as soon as the digital input state changes, set Upload settings to Yes.
This is critical for functions such as Duress buttons where alerts are mapped to the change in state. If Upload on Active and Upload on inactive are set to No, then the logs (if enabled) will be sent in the next upload session.

Bias Resistors

There are 3 options:

Pull up: pull the input high.
Pull down: pull the input low
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.

Results of different configurations.

Bias Resistor	Active High	Input	Result
Pull Down	True	Low	Inactive
		High	Active
Pull Down	False	Low	Active
		High	Inactive
Pull Up	True	Low	Inactive
		High	Active
Pull Up	False	Low	Active
		High	Inactive