NI High-Speed Digitizers Help (NI-SCOPE)

Edition Date: January 2017

Part Number: 370592AB-01

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NI-SCOPE (English | Japanese)

Some NI digitizers support multiple-record acquisitions, also known as retriggerable acquisitions, which allow you to capture multiple, triggered waveforms without software intervention. NI-SCOPE stores each record in separate memory locations on the digitizer. Refer to Features Supported by Device for a listing of digitizers that support multiple-record acquisitions.

The main benefit of multiple-record acquisitions is that you can acquire numerous triggered waveforms quickly. Multiple-record acquisitions allow hardware rearming of the digitizer. The digitizer hardware must transition between several states (given by the digitizer's Acquisition Engine State Diagram) before it is ready to accept a subsequent trigger. If two triggers arrive, one shortly after the other, the digitizer may or may not detect the second trigger.

• If the second trigger arrives before the first record completes, it will not be accepted. Only one trigger can be accepted in any one record in the NI-SCOPE driver.
• If the second trigger arrives after the first record completes, but before the dead time has elapsed, it will not be accepted.
• If the second trigger arrives after the first record completes and after the dead time has elapsed, but before the minimum pre-reference trigger sampling for the subsequent record is complete, it will not be accepted.
• If the second trigger arrives after the first record completes, the dead time has elapsed, and the minimum pre-reference trigger sampling is finished, the trigger will be accepted.

Note  The information above assumes that there are no holdoff values set (for example, trigger holdoff).

Minimum Time Between Reference Triggers

The minimum time between triggers can be calculated with the dead time and the time per record as follows:

Minimum Time Between Triggers = Time Per Record + Dead Time

The maximum trigger rate can be calculated as follows:

Maximum Trigger Rate (Hz) = 1/(Minimum Time Between Triggers)

When the reference trigger type is set to immediate and all holdoff values = 0 seconds, the digitizer will trigger at the maximum trigger rate regardless of any external signal.

Some digitizers specify a minimum rearm time. Minimum rearm time is the minimum time between reference triggers as the record length approaches a minimum (for example, record length = 1 sample). Therefore, when the record length is 1 sample, Maximum Trigger Rate (Hz) = 1/(minimum rearm time).

Multiple-Record Example

For an introduction to multiple-record acquisitions, refer to the Multi Record example. Modify the niScope_ConfigureHorizontalTiming function in your application, setting the numRecords parameter to the number of records you want to acquire. The digitizer acquires an additional record each time a trigger is accepted until all the requested records have been acquired.

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