Table Of Contents

Configuring Data Rates

Last Modified: August 20, 2020

You can configure the data rate at which your DAQ device acquires data by customizing the data ranges and setting the values for analog (slow, medium, and fast) and digital data rate levels. For devices that support downsampling, you can use an anti-aliased filter to downsample at 100 Hz.

Complete the following steps to configure the available data rates for your DAQ device:

  1. Click the Configure Data Rates button on the toolbar, or click Configure from the Data Rate Level pull-down selector.
  2. Specify each frequency in Hz or s (seconds, for intervals) for the desired slow, medium, fast, counter, or digital rates. Available data rate types vary by device.
  3. Click Done to apply the rates for the module.

You can configure any analog input device to use Slow, Medium, or Fast data rates. Counter input modules, however, require the use of the data rate resource that is assigned to the Medium data rate level. If a counter input module is added to a chassis and any module(s) are using the Medium data rate level, you will be prompted to change the data rate on the affected module(s) in the chassis to use the Slow or Fast data rate.

Why is my module not sampling at the rate I specified?

The data rate for your module may be different than the rate you configure for one of the following reasons:
  • Modules support specific rates—You can configure your module to sample at almost any rate, but the limitations of the hardware itself may prevent it from sampling at that rate. If your module does not support the rate you configure, FlexLogger coerces it to the closest supported rate.

    The coerced rate is faster than the rate you specify. For example, if you configure 11 Hz for a module that supports 10 Hz and 20 Hz, FlexLogger uses 20 Hz.

  • Modules sample at the lowest common rate—If you configure multiple modules in the same chassis to the same rate and they support different data rates, FlexLogger sets a common, supported rate. Using a common rate ensures data is not missed and is synchronized. For example, if you have a NI-9219 and NI-9236 set to Medium (200 Hz), the NI-9236 cannot sample slow enough to meet the 200 Hz configuration. This results in both modules using the actual rate of about 793 Hz because it is the common rate closest to the rate you configured.

To ensure a module uses a supported rate you configure, you must either set your modules in the same chassis to different timing rates or separate the modules into different chassis.

Why does my module say the rate is downsampled?

Many modules support downsampling in software to meet a lower data rate than the modules support. Downsampling occurs when all of the modules set to the same rate support downsampling. The available downsampling rate is 100 Hz.

Why is my module rate not downsampled?

If your module meets the requirements, but is not downsampling, check the following items:
  • An existing downsampling preview feature interferes with the default, anti-aliased downsampling filter—Navigate to File»Preferences»Preview features and ensure the Enable downsampling from a coerced hardware sample rate to slower sample rate feature is disabled.

    The existing preview feature supports arbitrary data rates, but does not provide anti-aliasing.

  • C Series Slow Sample Devices do not support downsampling—Refer to C Series Device Groupings in the NI-DAQmx Manual for a list of slow sample devices. If you have one slow sample device set to the same rate as modules that support downsampling, it prevents downsampling for all of the modules.

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