You can configure channels of a parallel digital module as Programmable Function Interface (PFI) terminals. Up to two digital modules can be used to access PFI terminals in a single chassis.

You can configure each module PFI individually as the following:

  • Timing input signal for AI, AO, DI, DO, or counter/timer functions
  • Timing output signal from AI, AO, DI, DO, or counter/timer functions

PFI Filters

You can enable a programmable debouncing filter on each PFI signal. When the filter is enabled, the chassis samples the inputs with a user-configured Filter Clock derived from the chassis timebase. This is used to determine whether a pulse is propagated to the rest of the circuit. However, the filter also introduces jitter onto the PFI signal.

The following is an example of low-to-high transitions of the input signal. High-to-low transitions work similarly.

Assume that an input terminal has been low for a long time. The input terminal then changes from low to high, but glitches several times. When the Filter Clock has sampled the signal high on N consecutive edges, the low-to-high transition is propagated to the rest of the circuit. The value of N depends on the filter setting, as shown in the following table.

Table 18. Selectable PFI Filter Settings
Filter Setting Filter Clock Jitter Min Pulse Width* to Pass Max Pulse Width* to Not Pass
112.5 ns (short) 80 MHz 12.5 ns 112.5 ns 100 ns
6.4 μs (medium) 80 MHz 12.5 ns 6.4 μs 6.3875 μs
2.56 ms (high) 100 kHz 10 μs 2.56 ms 2.55 ms
Custom User-
configurable 1 Filter Clock period Tuser Tuser - (1 Filter Clock period)
* Pulse widths are nominal values; the accuracy of the chassis timebase and I/O distortion will affect these values.

On power up, the filters are disabled. The following figure shows an example of a low-to-high transition on an input that has a custom filter set to N = 5.

Figure 41. PFI Filter Example