Common Concepts - Digital Inputs


Digital Matter devices commonly have digital inputs. These are inputs are designed to read on or off. This article discusses connecting the DM inputs to external devices with dry contact style outputs.

Dry contact, open collector, and open drain outputs all refer to similar interfaces, although the implementation on the external device will differ. Effectively there are 2 states: high impedance (disconnected), and driven (connected, usually driven low).

In the example of a magnetic reed switch, consider this a dry contact. When off it is open circuit, and when on it is a closed circuit.

The easiest way to connect these inputs is to use the DM Bias Resistors

Consider the example of the reed switch. Set the input on the DM device to "pull up" with the bias resistor. Then connect the reed switch between ground and the input. When open circuit, the pull up will read 1, and the logic can be inverted on the device or on the server to be "off". Then when the switch is closed, the input is pulled to ground and will read 0. Again, invert the logic on the device or the server to be "on".

Consider an open drain output from a PLC. Ensure the DM device and the PLC have common grounds. Set the DM input to pull up. When the PLC output is off (high impedance), the DM input will read 1. When the PLC input is on (driving low), the DM input will read 0.

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

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

When the button is ON it

  1. Is pressed
  2. closes the loop.
  3. 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:

  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.

Results of different configurations. 

Bias ResistorActive HighInputResult
 Pull DownTrueLowInactive

Pull DownFalseLowActive

Pull UpTrueLowInactive

Pull UpFalseLowActive


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