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

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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

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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

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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

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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

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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

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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

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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

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Top Tier Examples
Nonbuffered, software triggered ADC
Buffered, hardware triggered ADC
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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)

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Middle Tier Example
Buffered DAQ
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Example
Continuous, Circular Buffered DAQ