Chapter 2 Analog Input
E Series User Manual 2-32 ni.com
An acquisition with pretrigger data allows you to view data that is acquired
before the trigger of interest, in addition to data acquired after the trigger.
Figure 2-18 shows a typical pretrigger DAQ sequence. The ai/StartTrigger
signal can be either a hardware or software signal. If ai/StartTrigger is set
up to be a software start trigger, an output pulse appears on the AI START
TRIG line when the acquisition begins. When the ai/StartTrigger pulse
occurs, the sample counter is loaded with the number of pretrigger samples,
in this example, four. The value decrements with each pulse on
ai/SampleClock, until the value reaches zero. The sample counter is then
loaded with the number of posttrigger samples, in this example, three.
Figure 2-18. Typical Pretrigger Acquisition
If an ai/ReferenceTrigger pulse occurs before the specified number of
pretrigger samples are acquired, the trigger pulse is ignored. Otherwise,
when the ai/ReferenceTrigger pulse occurs, the sample counter value
decrements until the specified number of posttrigger samples have been
acquired. Refer to the
Analog Input Triggering section for more
information about start and reference triggers.
AI Start Trigger Signal
You can use the AI Start Trigger (ai/StartTrigger) signal to begin a
measurement acquisition. A measurement acquisition consists of one or
more samples. If you do not use triggers, you begin a measurement with a
software command. Once the acquisition begins, you can configure the
acquisition to stop when one of the following conditions apply:
• When a certain number of points are sampled (in finite mode)
• After a hardware reference trigger (in finite mode)
• With a software command (in continuous mode)
Don't Care
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ai/StartTrigger
ai/Reference Trigger
ai/SampleClock
Sample Counter
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