National Instruments Eight-slot USB Chassis NI cDAQ-9172 - Manual (Page 27)

© National Instruments Corporation 27 NI cDAQ-9172 User Guide and Specifications
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. The
first point is acquired when the task is committed. The second point is
acquired after the start trigger.
Figure 11. 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.
Refer to the KnowledgeBase document, C Series Modules Supported in the
NI cDAQ-9172 CompactDAQ, for more information. To access this
KnowledgeBase, go to
ni.com/info and enter the info code rdcdaq .
ai/StartTrigger
Data from
A/D Conversion
(Slow Module)
ai/SampleClock
Data Returned
to AI Task
1st A/D Conversion 2nd A/D Conversion 3rd A/D Conversion
A B C
A A A B B B C