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

Updated Jan 24, 2018

Reported In

Hardware

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

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?

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.

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