1. A Brief Overview of LabVIEW Data Acquisition (DAQ)
J. Carroll
10/14/03

2. 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

3. 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

4. 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

5. 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

6. Triggering Use software triggering when: Use hardware 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

7. 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

8. 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

9. Top Tier Examples Nonbuffered, software triggered ADC
Buffered, hardware triggered ADC

10. 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)

11. Middle Tier Example
Buffered DAQ

12. Example
Continuous, Circular Buffered DAQ