The Industrial Feature Set: Programmable Input Filters

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 are Programmable Input Filters?

Programmable Input Filters remove chatter, noise, glitches, and spikes on inputs, and also provide debouncing for digital switches and relays. This is important for applications in noisy industrial environments to prevent false readings based on noise.

Recommended Products that Feature Programmable Input Filters

NI 6509
NI 6514
NI 6515
NI 6528

How Programmable Input Filters Work

The programmable input filter for each digital line can be configured by setting the filter time in seconds. Any digital noise, glitch, or spike that is faster than the filter time will be ignored by the digital I/O device, preventing invalid readings or false triggers.

If input filters are enabled on the NI device, it will discard the erroneous readings caused by the bouncing switch. It does this by ignoring any pulses that are too small in width. With programmable input filters, the user can programmatically select the minimum pulse width that is guaranteed to pass (figure 1).

Figure 1: Configuring Input Filters in NI-DAQmx

Any pulse that is greater than the specified minimum pulse is guaranteed to pass, and any pulse that is less than half of the specified pulse width is guaranteed to be filtered. Pulses that fall between half of the pulse width and one entire pulse width may or may not pass.

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