Chapter 4 Analog Input
NI USB-621x User Manual 4-14 ni.com
AI Sample Clock Signal
Use the AI Sample Clock (ai/SampleClock) signal to initiate a set of
measurements. Your USB-621x device samples the AI signals of every
channel in the task once for every AI Sample Clock. A measurement
acquisition consists of one or more samples.
You can specify an internal or external source for AI Sample Clock. You
also can specify whether the measurement sample begins on the rising edge
or falling edge of AI Sample Clock.
Using an Internal Source
One of the following internal signals can drive AI Sample Clock:
•Counter n Internal Output
• AI Sample Clock Timebase (divided down)
•A pulse initiated by host software
A programmable internal counter divides down the sample clock timebase.
Using an External Source
Use any input PFI line as the source of AI Sample Clock.
Routing AI Sample Clock to an Output Terminal
You can route AI Sample Clock out to any output PFI terminal. This pu lse
is always active high.
You can specify the output to have one of two behaviors. With the pulse
behavior, your DAQ device briefly pulses the PFI terminal once for every
occurrence of AI Sample Clock.
With level behavior, your DAQ device drives the PFI terminal high during
the entire sample.
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 on your device internally generates AI Sample Clock unless you
select some external source. AI Start Trigger starts this counter and either
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