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NI 9775 Considerations

Last Modified: October 17, 2018

The NI 9775 module operates in two different modes, Continuous Mode and Record Mode. You can set the NI 9775 operation to be Record Mode by setting the DataXferReqCond attribute/property to When Acquisition Complete. Any other supported value of the DataXferReqCond attribute/property will force the NI 9775 into Continuous Mode. If a value is not set for DataXferReqCond attribute/property, the module will coerce a value based on other task settings.

Continuous Mode

While in Continuous Mode, the NI 9775 returns data as the data is acquired from the source.

The module does not use the module's onboard buffer when in Continuous Mode. NI-DAQmx will automatically put the NI 9775 in Continuous Mode if the task is set for continuous acquisition or if the specified rate is less than or equal to 4 MS/s aggregate for each module.

Continuous Mode Triggering

In Continuous Mode, the NI 9775 cannot be the source for Analog Edge or Analog Multi Edge Triggering.

The NI 9775 also does not support Analog Triggering or Retriggering, unless in a task with the NI 9205 or NI 9206. While in Continuous Mode, Reference and Start triggers can be configured on the same task.

Record Mode

While in Record Mode, the data is stored on a buffer in the module until the entire acquisition is complete. The data on the buffer is stored in records based on a trigger signal sent to the device.

Acquisition is considered complete when any of the following conditions are satisfied:

  • The maximum number of triggers are detected and the data for all detected triggers have been acquired.

    By default, the NI 9775 acquires one record. To acquire more than one record, you can set the Retriggerable attribute/property to true, which will give you as many records that can fit in the buffer, up to 32 records. Or, you can use the Maximum Number of Triggers to Detect attribute/property to specify how many records you want, up to 32 records. Any ignored triggers do not count towards the maximum of 32 records.

  • The Trigger Window has elapsed.

    This time setting can be configured through the Trigger Window attribute/property. The Trigger Window begins when the modules receive a sync pulse, which synchronizes modules in a task. This sync pulse is sent during the commit stage of the task which happens in the Start Task function/VI. If the Trigger Window expires, the module finishes acquiring post-trigger data. If the module is already finished acquiring post-trigger data, the module returns all of the acquired data.

  • The Retrigger Window has elapsed.

    This time setting can be configured through the Retrigger Window attribute/property. The Retrigger Window begins once the module has detected a trigger and resets every time it detects another trigger. If the Retrigger Window expires, the module finishes acquiring post-trigger data. If the module is already finished acquiring post-trigger data, the module returns all of the acquired data.

NI-DAQmx will automatically put the NI 9775 in Record Mode if the channels on the module are used for trigger sources or if the rate is specified to be greater than 4 MS/s aggregate for finite acquisitions. When in Record Mode, the NI 9775 may only be in the same task as other NI 9775 modules in Record Mode.

Record Mode Triggering

In Record Mode, the NI 9775 supports the following triggers and configurations:
  • Digital Edge Triggering.
  • A single channel configured as the source when using Analog Edge Triggering.
  • Up to 4 channels configured as the source, but all of the source channels must be on the same device when using Analog Multi Edge Triggering.
  • Configuration of a single trigger type. You can have Start or Reference triggers configured, but not both.
  • Retriggering on any edge trigger type: Digital Edge, Analog Edge, and Analog Multi Edge.
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    Note  

    The module will ignore triggers when acquiring pre-trigger buffer data or post-trigger data.

Onboard Memory Behavior

The retrieval of data from the onboard memory of the module may be faster than the chassis can transfer it to the host. If this occurs, the chassis FIFO will overflow, erroring the task and causing a loss of data. To prevent this, set the Data Transfer Maximum Rate attribute/property to a rate lower than the system's bandwidth.


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