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What Is the Difference Between the Terms Sinking and Sourcing?

Hardware: FieldPoint, Digital I/O (DIO), Compact FieldPoint, Multifunction DAQ (MIO)

I am setting up the 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 the sourcing digital I/O?

Sinking and Sourcing are terms used to define the control of direct current flow in a load. A sinking digital I/O (input/output) provides a grounded connection to the load, whereas a 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 1: Sinking Digital Output Circuit

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.

Figure 2: Sourcing Digital Output Circuit

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 KnowledgeBase 2K0D6H5F: Connecting Two Sinking I/O or Two Sourcing I/O Together.

Related Links:
KnowledgeBase 2K0D6H5F: Connecting Two Sinking I/O or Two Sourcing I/O Together
KnowledgeBase 3E6AIAFL: M Series Maximum Sink/Source Current Specifications for PFI, DIO and Counters
KnowledgeBase 1EPBH9LK: Can I Connect a Coil Relay Directly to my FP-DO-400 or FP-DO-401?
White Paper: Digital I/O Sinking and Sourcing


Report Date: 01/25/2002
Last Updated: 05/19/2016
Document ID: 2HOE7K5F

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