National Instruments Data Acquisition Device NI USB-621x - Manual (Page 91)

Chapter 7 PFI
NI USB-621x User Manual 7-4 ni.com
Connecting PFI Input Signals
All PFI input connections are referenced to D GND. Figure 7-3 shows this
reference, and how to connect an external PFI 0 source and an external
PFI 2 source to two PFI terminals.
Figure 7-3. PFI Input Signals Connections
PFI Filters
You can enable a programmable debouncing filter on each PFI signal.
When the filters are enabled, your device samples the input on each rising
edge of a filter clock. USB-621x devices use an onboard oscillator to
generate the filter clock with a 40 MHz frequency.
Note NI-DAQmx only supports filters on counter inputs.
The following is an example of low to high transitions of the input signal.
High to low transitions work similarly.
PFI 0
Source
PFI 2
Source
USB-621x Device
D GND
PFI 2
PFI 0
I/O Connector