Chapter 4
Analog Input
©
National Instruments Corporation
4-13
Pretriggered data acq
u
isition allows yo
u
to view data that is acq
u
ired before
the trigger of interest, in addition to data acq
u
ired after the trigger.
Fig
u
re 4-7 shows a typical pretriggered DAQ seq
u
ence. AI Start Trigger
(ai/StartTrigger) can be either a hardware or software signal. If AI Start
Trigger is set
u
p to be a software start trigger, an o
u
tp
u
t p
u
lse appears on
the AI Start Trigger line when the acq
u
isition begins. When the AI Start
Trigger p
u
lse occ
u
rs, the sample co
u
nter is loaded with the n
u
mber of
pretriggered samples, in this example, fo
u
r. The val
u
e decrements with
each p
u
lse on AI Sample Clock,
u
ntil the val
u
e reaches zero. The sample
co
u
nter is then loaded with the n
u
mber of posttriggered samples, in this
example, three.
Figure 4-7.
Pretriggered Data Acquisition Example
If an AI Reference Trigger (ai/ReferenceTrigger) p
u
lse occ
u
rs before the
specified n
u
mber of pretrigger samples are acq
u
ired, the trigger p
u
lse is
ignored. Otherwise, when the AI Reference Trigger p
u
lse occ
u
rs, the
sample co
u
nter val
u
e decrements
u
ntil the specified n
u
mber of posttrigger
samples have been acq
u
ired.
USB-621
x
devices feat
u
re the following analog inp
u
t timing signals:
•
•
AI Sample Clock Timebase Signal
•
•
AI Convert Clock Timebase Signal
•
•
•
•
n/
a
0 1
2
3
1 0
2 2
2
AI
S
t
a
rt Trigger
AI Reference Trigger
AI
Sa
mple Clock
AI Convert Clock
S
c
a
n Co
u
nter
