Chapter 4 Analog Input
© National Instruments Corporation 4-13 NI USB-621x User Manual
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 DAQ sequence. AI Start Trigger
(ai/StartTrigger) can be either a hardware or software signal. If AI Start
Trigger is set up to be a software start trigger, an output pulse appears on
the AI Start Trigger line when the acquisition begins. When the AI Start
Trigger pulse occurs, the sample counter is loaded with the number of
pretriggered samples, in this example, four. The value decrements with
each pulse on AI Sample Clock, until the value reaches zero. The sample
counter is then loaded with the number of posttriggered samples, in this
example, three.
Figure 4-7. Pretriggered Data Acquisition Example
If an AI Reference Trigger (ai/ReferenceTrigger) pulse occurs before the
specified number of pretrigger samples are acquired, the trigger pulse is
ignored. Otherwise, when the AI Reference Trigger pulse occurs, the
sample counter value decrements until the specified number of posttrigger
samples have been acquired.
USB-621x devices feature the following analog input timing signals:
• AI Sample Clock Signal
• AI Sample Clock Timebase Signal
• AI Convert Clock Signal
• AI Convert Clock Timebase Signal
• AI Hold Complete Event Signal
• AI Start Trigger Signal
• AI Reference Trigger Signal
• AI Pause Trigger Signal
n/a
0 1 2 3 1 0 2 2 2
AI Start Trigger
AI Reference Trigger
AI Sample Clock
AI Convert Clock
Scan Counter
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