User Guide

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

B C

A A A B B B C