©
National Instruments
3-1
NI 660x User Manual
3
Signal Connections
This chapter describes how to make inp
u
t and o
u
tp
u
t signal connections to
NI 660
x
devices 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 ro
u
ting m
u
ltiplexer for each
timing signal, and software can select a PFI as the external so
u
rce for a
given timing signal. Any PFI pin can be
u
sed as an inp
u
t by any timing
signal and m
u
ltiple timing signals can sim
u
ltaneo
u
sly
u
se the same PFI pin.
This flexible ro
u
ting scheme red
u
ces the need to change physical
connections to the I/O connector for different applications. Yo
u
also can
individ
u
ally enable each PFI pin to o
u
tp
u
t a specific internal timing signal.
Yo
u
can individ
u
ally enable many of the PFI pins to o
u
tp
u
t a specific
internal timing signal. For example, if yo
u
need the Co
u
nter 0 So
u
rce signal
as an o
u
tp
u
t on the I/O connector, software can t
u
rn on the o
u
tp
u
t driver for
the PFI 39/CTR 0 SRC pin.
Caution
Do not drive a PFI signal externally when it is config
u
red as an o
u
tp
u
t.
When
u
sing the PFI pin as an inp
u
t, yo
u
can individ
u
ally config
u
re each
PFI for edge or level detection and for polarity selection. Yo
u
can
u
se the
polarity selection for any of the timing signals, b
u
t the edge or level
detection depends
u
pon the partic
u
lar timing signal being controlled.
The detection req
u
irements for each timing signal are listed within the
section that disc
u
sses that signal.
Digital Filtering
Use the digital filter option available on the NI 660
x
PFI lines to eliminate
glitches on inp
u
t data. The filter operates off a filter clock and a fast internal
sampling clock. The filter circ
u
it samples the signal on the PFI line on each
rising edge of the sampling clock. However, a change in the signal is
propagated only if the signal maintains its new state for at least two
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