© National Instruments Corporation 3-1 NI 660x User Manual
3
Signal Connections
This chapter describes how to make input and output signal connections to
the NI 660x device by way of the device I/O connector and the RTSI
connector.
Programmable Function Interfaces (PFIs)
The 40 PFI pins are connected to the signal routing multiplexer for each
timing signal, and software can select a PFI as the external source for a
given timing signal. Any PFI pin can be used as an input by any timing
signal and multiple timing signals can simultaneously use the same PFI pin.
This flexible routing scheme reduces the need to change physical
connections to the I/O connector for different applications. You also can
individually enable each PFI pin to output a specific internal timing signal.
You can individually enable many of the PFI pins to output a specific
internal timing signal. For example, if you need the Counter 0 Source signal
as an output on the I/O connector, software can turn on the output driver for
the PFI 39/CTR 0 SRC pin.
Caution Do not drive a PFI signal externally when it is configured as an output.
When using the PFI pin as an input, you can individually configure each
PFI for edge or level detection and for polarity selection. You can use the
polarity selection for any of the timing signals, but the edge or level
detection depends upon the particular timing signal being controlled.
The detection requirements for each timing signal are listed within the
section that discusses that signal.
Digital Filtering
Each PFI line coming from the I/O connector can be passed through a
simple digital debouncing filter. The filter operates off a filter clock and a
fast internal sampling clock. The filter circuit samples the signal on the PFI
line on each rising edge of the sampling clock. A change in the signal is
propagated only if it maintains its new state for at least the duration
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