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Chapter 7 Buffering Your Way through Waveform Acquisition

LabVIEW Data Acquisition Basics Manual 7-2

National Instruments Corporation

you can use a data buffer in computer memory as your shopping bag with

which you acquire data.

With buffered I/O, LabVIEW transfers data taken at timed intervals from

a DAQ device to a data buffer in memory. Figure 7-1 illustrates how the

data fills up the buffer only once, however the overall size of the buffer is

specified in your VI. In this illustration, think of

as the number of scans

or updates the buffer can hold, and

as the trigger occurrence whether

the trigger is because of an external signal or the start of the execution of

your VI. Refer to Chapter 8,

Controlling Your Acquisition with Triggers

for more descriptions on triggering your acquisition from another signal.

Figure 7-1.

How Buffers Work

In your VI, you must specify the number of samples to be taken and the

number of channels from which LabVIEW will take the samples. From this

information, LabVIEW allocates a buffer in memory to hold a number of

data points equal to the number of samples per channel multiplied by the

number of channels. As the data acquisition continues, the buffer fills with

the data; however, the data may not actually be accessible until LabVIEW

acquires all the samples (N). Once the data acquisition is complete, the data

that is in the buffer can be analyzed, stored to disk, or displayed to the

screen by your VI.

Acquiring a Single Waveform

You can acquire a waveform from a single channel by using the

AI Acquire Waveform VI, shown in Figure 7-2. You can find this VI in

Functions»DAQ»Analog Input

. Because AI Acquire Waveform is an

Easy Analog Input VI, it has the minimal number of inputs needed to

acquire a waveform from a single channel. These minimal inputs are the

device

channel string

number of samples

from the channel, and the

sample rate

. You can programmatically set the

gain

by setting the

high limit

and the

low limit

. Using only the minimal set of inputs makes

programming the VI easier, but the VI lacks more advanced capabilities,

such as triggering. Built-in error handling is another useful feature of the

Easy VIs. If an error occurs, the program stops running and notifies you

with a dialog box explaining the error.

writing and reading