- DAQ M Series NI USB-621x User Manual

Chapter 4 Analog Input
NI USB-621x User Manual 4-14 ni.com
Figure 4-7. Pretriggered Data Acquisition Example
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.
USB M Series 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
AI Sample Clock Signal
Use the AI Sample Clock (ai/SampleClock) signal to initiate a set of
measurements. Your M Series device samples the AI signals of every
channel in the task once for every ai/SampleClock. A measurement
acquisition consists of one or more samples.
You can specify an internal or external source for ai/SampleClock. You also
can specify whether the measurement sample begins on the rising edge or
falling edge of ai/SampleClock.
n/a
0 1 2
3
1 0 2 2 2
ai/StartTrigger
ai/ReferenceTrigger
ai/SampleClock
ai/ConvertClock
Scan Counter