Digital I/O Sinking and Sourcing

This document is part of a comprehensive tutorial on industrial Digital I/O and Counter/Timer hardware. Learn about National Instruments product offerings for digital and timing I/O, the Industrial Feature Set including watchdog timers and isolation, complementary devices like relays, solenoids and encoders, concepts like sinking and sourcing, and see how these devices can be used in your industrial application.

For more information return to the Complete Industrial Digital I/O and Counter/Timer Tutorial.

What is Sinking and Sourcing?

Sinking and Sourcing are terms used to define the control of direct current flow in a load. The concept of sourcing and sinking is independent of the component (transistor, mechanical relay) that implements the operation. While this concept applies to any DC circuitry; the component that implements the circuitry may vary.

Sinking Devices

A sinking device provides a path for the current to ground and is not responsible for powering the device. Terms used to describe sinking devices include NPN, Open Collector, Normally High, and IEC Negative Logic.

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Figure 1: Sinking

Sourcing Devices

A sourcing device provides the power or a positive potential. Sourcing devices 'push' the current through the load. Terms used to describe sourcing devices include PNP, Open Emitter, Normally Low, and IEC Positive Logic.

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Figure 2: Sourcing

Recommended NI Products that Connect to Sinking and Sourcing Devices

NI 6509
NI 6514
NI 6515
NI 6528

Reader Comments | Submit a comment »

Not an Adequate Explanation
This did not help me understand how a DI/O is use with a sinking or sourcing device.
- Jul 22, 2008

Sometimes, like with the UMi-7774 you can only connect sourcing devices, but with help of a resistor you can change a sinking device into a sourcing devices. I was looking for details on how exactly to do this, and haven't found it here.
- Nils Gokemeijer, Seagate Technology. nils.gokemeijer@seagate.com - Jan 28, 2005