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Chapter 4 Signal Connections
PCI-4451/4452 User Manual 4-16
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National Instruments Corporation
Pretriggered data acquisition allows you to view data that is acquired before
the trigger of interest in addition to data acquired after the trigger.
Figure 4-7 shows a typical pretriggered acquisition sequence. The
description for each signal shown in these figures is included later in this
chapter.
Figure 4-7.
Typical Pretriggered Acquisition
PFI1
PFI0/TRIG1 (EXT_TRIG) Signal
Any PFI pin can externally input the PFI0/TRIG1 (EXT_TRIG) signal,
which is available as an output on the PFI0/TRIG1 (EXT_TRIG) pin.
Refer to Figures 4-6 and 4-7 for the relationship of PFI0/TRIG1 to the
acquisition sequence.
As an input, the PFI0/TRIG1 signal is configured in the edge-detection
mode. You can select any PFI pin as the source for PFI0/TRIG1 and
configure the polarity selection for either rising or falling edge. The
selected edge of the PFI0/TRIG1 signal starts the data acquisition sequence
for both posttriggered and pretriggered acquisitions. The PCI-4451/4452
supports analog level triggering on the PFI0/TRIG1 pin. See Chapter 3,
Hardware Overview, for more information on analog level triggering.
As an output, the PFI0/TRIG1 signal reflects the action that initiates an
acquisition sequence. This is true even if the acquisition is externally
triggered by another PFI signal. The output is an active high pulse with a
pulse width of 50 to 100 ns. This output is set to tri-state at startup.
The device also uses the PFI0/TRIG1 signal to initiate pretriggered
acquisition operations. In most pretriggered applications, the PFI0/TRIG1
signal is generated by a software trigger. Refer to the PFI1/TRIG2 signal
description for a complete description of the use of PFI0/TRIG1 and
PFI1/TRIG2 in a pretriggered acquisition operation.
Don't Care
01231 0222
TRIG1
TRIG2
CONVERT*
Sample Counter
Don't Care
User.book Page 16 Tuesday, April 14, 1998 10:20 AM