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The analog, digital, and counter I/O channels on DAQ devices in NI VeriStand systems perform single-point acquisition or generation. Analog input (AI) channels also can perform waveform acquisitions. You can configure the AI channels on a DAQ device to perform single-point or waveform acquisitions, but not both. This topic discusses considerations for acquiring waveform data.
In single-point acquisitions, channels acquire a single point at a time and return the value directly to the system. A single-point acquisition is an immediate, non-buffered operation that occurs at the rate at which the system runs. Single-point acquisitions are useful for implementing closed-loop control, as the system reads one value per iteration of the Primary Control Loop and can produce appropriate outputs during the same iteration.
During waveform acquisitions, channels acquire signals over a period of time as waveforms. You might want to acquire data as waveforms when you need to read at a rate faster than the rate at which the system runs. For example, when you need to monitor or log a value that changes quickly, such as the pressure in an engine cylinder, acquiring the signal as a waveform allows you to achieve the high sampling rate required to represent the signal adequately. When you configure AI channels to perform waveform acquisitions, you can achieve rates up the maximum sampling rate for the DAQ device without being restricted by the system rate.
You specify whether AI channels will perform single-point or waveform acquisitions when you add the channels to a system definition.
As part of setting up an AI channel(s) to acquire waveforms, you specify the rate and size of the acquisitions you want to perform, as well as other properties. You set these properties in a group called a task, and you assign the task back to the AI channel(s). You must assign one task to each DAQ device that performs waveform acquisitions. Tasks also control how NI VeriStand logs waveform data to file and define triggers that start and stop acquisitions.