What is the Difference Between Sinking and Sourcing Digital I/O?

Updated Oct 21, 2019

Reported In

Hardware

  • C Series Digital Output Device
  • C Series Digital Input Device

Issue Details

  • I am setting up digital inputs and outputs on my system, and I want to make sure that I have enough current to drive the correct digital level. How do I know whether to use the sinking or sourcing digital I/O?
  • Can my data acquisition (DAQ) device that is a sourcing digital input be utilized with a sinking digital output signal?

Solution

Sinking and Sourcing are terms used to define the control of direct current flow in a load.
  • Sinking digital I/O provides a grounded connection to the load.
  • Sourcing digital I/O provides a voltage source to the load. 

Consider a simple circuit that consists of one digital input connected to a digital output. The circuit needs a voltage source, a ground, and a load.
  • A sourcing digital I/O provides the voltage needed for the circuit.
  • A sinking digital I/O provides the ground needed in the circuit.
  • The digital input provides the load required for the circuit to work.

Figure 1 shows a sinking digital output that is connected to a sourcing digital input. In this circuit, the load is pulled down to ground because of the sinking digital input provided.
 
Figure 2 shows a sourcing digital output that is connected to a sinking digital input. In this circuit, the load is pulled up to receive voltage because the sourcing digital input has been provided.

Additional Information

Because both a voltage source and a ground reference are needed in order to create a complete circuit, you must have a sourcing input or output connected to a sinking output or input. If you wish to connect a sourcing input to a sourcing output or a sinking input to a sinking output, you will need to add an additional resistor. For further information on connecting two I/O of the same type, please refer to Connecting Two Sinking I/O or Two Sourcing I/O Together.

Also, consider the current limit specifications for digital I/O modules:
 
  • Digital Output (DO) modules typically have a maximum continuous output current defined; these modules don't typically have a minimum operating current defined.
    • Using the NI-9472 as example, this is 0.75 A maximum continuous output current per channel.
  • Digital Input (DI) modules differ, as their internal receiver circuitry draws a small amount of current when performing the measurement.
    • Using the NI-9426 sourcing input module as example, which is specified as follows:
      • OFF State: Up to 150 μA from the DI pin.
      • ON State: At least 330 μA from the DI pin.
  • The Digital Subsystem Specifications section of the Specifications Explained: C Series Modules document explains these two current limits in more detail.

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