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Chapter 2
Analog Input
Slow Sample Rate Modules
Some C Series analog input modules are specifically designed for measuring signals that vary
slowly, such as temperature. Because of their slow rate, it is not appropriate for these modules
to constrain the AI Sample Clock to operate at or slower than their maximum rate. When using
such a module in the cDAQ chassis, the maximum Sample Clock rate can run faster than the
maximum rate for the module. When operating at a rate faster than these slow rate modules can
support, the slow rate module returns the same point repeatedly, until a new conversion
completes. In a hardware-timed task, the first point is acquired when the task is committed. The
second point is acquired after the start trigger as shown in Figure 2-2.
Figure 2-2.
Sample Clock Timing Example
For example, if running an AI task at 1 kHz using a module with a maximum rate of 10 Hz, the
slow module returns 100 samples of the first point, followed by 100 samples of the second point,
etc. Other modules in the task will return 1,000 new data points per second, which is normal.
When performing a single-point acquisition, no points are repeated. To avoid this behavior, use
multiple AI timing engines, and assign slow sample rate modules to a task with a rate at or slower
than their maximum rate.
For more information about which C Series modules are compatible with the cDAQ chassis, go
to
and enter the Info Code
rdcdaq
.
AI Start Trigger Signal
Use the Start Trigger signal to begin a measurement acquisition. A measurement acquisition
consists of one or more samples. If you do not use triggers, begin a measurement with a software
command. Once the acquisition begins, configure the acquisition to stop in one of the following
ways:
•
When a certain number of points has been sampled (in finite mode)
•
After a hardware reference trigger (in finite mode)
•
With a software command (in continuous mode)
An acquisition that uses a start trigger (but not a reference trigger) is sometimes referred to as a
posttriggered acquisition. That is, samples are measured only after the trigger.
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