Chapter 4
Analog Input
©
National Instruments Corporation
4-21
complete description of the
u
se of AI Start Trigger and AI Reference
Trigger in a pretriggered DAQ operation.
AI Reference Trigger Signal
Use a reference trigger (ai/ReferenceTrigger) signal to stop a meas
u
rement
acq
u
isition. To
u
se a reference trigger, specify a b
u
ffer of finite size and
a n
u
mber of pretrigger samples (samples that occ
u
r before the reference
trigger). The n
u
mber of posttrigger samples (samples that occ
u
r after the
reference trigger) desired is the b
u
ffer size min
u
s the n
u
mber of pretrigger
samples.
After the acq
u
isition begins, the DAQ device writes samples to the b
u
ffer.
After the DAQ device capt
u
res the specified n
u
mber of pretrigger samples,
the DAQ device begins to look for the reference trigger condition. If the
reference trigger condition occ
u
rs before the DAQ device capt
u
res the
specified n
u
mber of pretrigger samples, the DAQ device ignores the
condition.
If the b
u
ffer becomes f
u
ll, the DAQ device contin
u
o
u
sly discards the oldest
samples in the b
u
ffer to make space for the next sample. This data can be
accessed (with some limitations) before the DAQ device discards it. Refer
to the KnowledgeBase doc
u
ment,
Can a Pretriggered Acquisition be
Continuous?
, for more information. To access this KnowledgeBase, go
to
ni.com/info
and enter the info code
rdcanq
.
When the reference trigger occ
u
rs, the DAQ device contin
u
es to write
samples to the b
u
ffer
u
ntil the b
u
ffer contains the n
u
mber of posttrigger
samples desired. Fig
u
re 4-15 shows the final b
u
ffer.
Figure 4-15.
Reference Trigger Final Buffer
Reference Trigger
Pretrigger
Sa
mple
s
Complete B
u
ffer
Po
s
ttrigger
Sa
mple
s
