National Instruments DAQ X Series User Manual page 101

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A programmable internal counter divides down the sample clock timebase.
Several other internal signals can be routed to AI Sample Clock through
internal routes. Refer to Device Routing in MAX in the NI-DAQmx Help or
the LabVIEW Help for more information.
Using an External Source
Use one of the following external signals as the source of AI Sample Clock:
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Routing AI Sample Clock Signal to an Output Terminal
You can route AI Sample Clock out to any PFI <0..15>, RTSI <0..7>, or
PXIe-DSTARC terminal. This pulse is always active high.
All PFI terminals are configured as inputs by default.
Other Timing Requirements
Your DAQ device only acquires data during an acquisition. The device
ignores AI Sample Clock when a measurement acquisition is not in
progress. During a measurement acquisition, you can cause your DAQ
device to ignore AI Sample Clock using the AI Pause Trigger signal.
A counter/timing engine on your device internally generates AI Sample
Clock unless you select some external source. AI Start Trigger starts this
counter and either software or hardware can stop it once a finite acquisition
completes. When using the AI timing engine, you also can specify a
configurable delay from AI Start Trigger to the first AI Sample Clock
pulse. By default, this delay is set to two ticks of the AI Sample Clock
Timebase signal.
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AO Sample Clock (ao/SampleClock)
DO Sample Clock (do/SampleClock)
PFI <0..15>
RTSI <0..7>
PXI_STAR
PXIe-DSTAR<A,B>
Analog Comparison Event (an analog trigger)
4-55
Chapter 4 Analog Input
X Series User Manual