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

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.