A brief overview of labview data acquisition daq
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A Brief Overview of LabVIEW Data Acquisition (DAQ). J. Carroll 10/14/03. Overview of LabVIEW DAQ. Two characteristics help classify the type of DAQ operation performed Whether you use a buffer Whether you use an external trigger to start, stop, or synchronize an operation . Buffers.

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Overview of labview daq
Overview of LabVIEW DAQ

  • Two characteristics help classify the type of DAQ operation performed

    • Whether you use a buffer

    • Whether you use an external trigger to start, stop, or synchronize an operation


Buffers
Buffers

  • A buffer is an area of PC memory reserved for data, DMA allows data to be acquired directly into computer memory

    • Not using a buffer means you must handle each data point one at a time, as it is acquired

  • Use buffered I/O when:

    • Many samples are acquired at a rate faster than is practical to display, store, or analyze in real-time

    • Data is acquired/displayed continuously on the fly

    • The sampling period must be precise and uniform throughout the data samples


Buffers1
Buffers

  • Use nonbuffered I/O when:

    • The data set is small and short (e.g., acquiring one data point every 100ms)

    • Reduced memory overhead is required (since a buffer takes up memory)

  • There are separate LabVIEW VIs for both buffered and nonbuffered I/O


Triggering
Triggering

  • Triggering is any method which initiates, terminates, or synchronizes a DAQ event

  • A trigger is usually an analog or digital signal whose condition is analyzed to determine a course of action

    • Software triggering is the easiest and most intuitive

    • Hardware triggering lets the circuitry of the DAQ board take control, adding more precision and control


Triggering1
Triggering

  • Use software triggering when:

    • The user needs to have explicit control over all DAQ operations

    • The timing of an event does not need to be precise

  • Use hardware triggering when:

    • Timing a DAQ event needs to be precise

    • You want to reduce software overhead, i.e., to reduce the need for a While Loop)

    • DAQ events need to be synchronized to external events


Analog i o definitions
Analog I/O Definitions

  • A device is the “number” that NI-DAQ assigns to an I/O board

  • A sample is one A/D conversion (one data point)

  • Channels specify the physical source of the data

  • A scan is a sample taken from each channel

    • represents data versus channel number

  • A waveform is a set of samples from one channel, collected over a period of time

    • represents data versus time


The daq palette
The DAQ Palette

  • The DAQ palette has three VI “tiers”

  • Top tier VIs are easiest to use but least flexible

    • these VIs are synchronous with the DAQ data, meaning that they do not finish executing until all of the data is read/written from the board

    • one fundamental limitation with these VIs is that every time the VI is called the hardware is “setup” for the sampling operating (adding excessive overhead)

    • multiple sample points acquired using a While Loop, which adds additional overhead

    • see class web site for more examples


Top tier examples
Top Tier Examples

Nonbuffered, software triggered ADC

Buffered, hardware triggered ADC


The middle bottom vi tiers
The Middle/Bottom VI Tiers

  • Middle tier VIs offer more functionality, flexibility and efficiency

    • allows buffered acquisition that is hardware controlled (see web for more examples)

    • allows continuous or real-time acquisition using “circular” buffers

    • returns data from an acquisition in progress without interrupting the acquisition

  • Bottom tier VIs offer the most functionality, flexibility and efficiency, at the cost of complexity (see web examples)


Middle tier example
Middle Tier Example

Buffered DAQ


Example
Example

Continuous, Circular Buffered DAQ


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