SolutionSinking and Sourcing
are terms used to define the control of direct current flow in a load and is an Input/Output type for NI digital modules.
Sinking digital I/O provides a grounded connection to the load and sourcing digital I/O provides a voltage source to the load. Because both a voltage source and a ground reference are needed to create a complete circuit, you must have a sourcing input or output connected to a sinking output or input, respectively.Note:
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
There are a couple of hardware considerations you need to take:
- You must determine whether your hardware can either be a sink, a source, or both. Checking the datasheet of your NI hardware for schematic diagrams and information will help inform you.
- Consider the current limit specifications for NI 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.
Please refer to the Input/output type section in the Digital Subsystem Specifications
section of the Specifications Explained: C Series Modules document for more information on sinking and sourcing.
Now, for example, 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.