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
• 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
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 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
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
• 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 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
Nonbuffered, software triggered ADC
Buffered, hardware triggered ADC
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
Buffered DAQ
Example
Continuous, Circular Buffered DAQ