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

Updated Feb 14, 2019

Reported In

Hardware

  • NI-9401
  • NI-9402
  • NI-9403
  • NI-9425
  • NI-9426

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 (input/output) 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 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, the current limit specifications for the NI Digital Input Modules are defined below, using the NI 9426 as an example. The Specifications Explained document explains these two current limits in more detail.
  • Maximum Sourcing Current :: OFF State Input Current: ≤150 μA from DI pin
  • Minimum Sinking Current :: ON State Input Current: ≥330 μA from DI pin

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