Using the Dynamic Data Type
Most Express VIs accept and/or return the dynamic data type. The dynamic data type appears as a dark blue terminal, shown as follows.
The dynamic data type accepts data from and sends data to the following data types, where the scalar data type is a floating-point number or a Boolean value:
- 1D array of waveforms
- 1D array of scalars
- 1D array of scalars—most recent value
- 1D array of scalars—single channel
- 2D array of scalars—columns are channels
- 2D array of scalars—rows are channels
- Single scalar
- Single waveform
Wire the dynamic data type to an indicator that can best present the data. Indicators include a graph, chart, or numeric indicator. However, because dynamic data undergoes an automatic conversion to match the indicator to which it is wired, Express VIs can slow down the block diagram execution speed.
The dynamic data type is for use with Express VIs. Most other VIs and functions that ship with LabVIEW do not accept this data type. To use a built-in VI or function to analyze or process the data the dynamic data type includes, you must convert the dynamic data type.
In addition to the data associated with a signal, the dynamic data type includes attributes that provide information about the signal, such as the name of the signal or the date and time the data was acquired. Attributes specify how the signal appears on a graph or chart. For example, if you use the DAQ Assistant Express VI to acquire a signal and plot that signal on a graph, the name of the signal appears in the plot legend of the graph, and the x-scale adjusts to display timing information associated with the signal in relative or absolute time based on the attributes of the signal. If you use the Spectral Measurements Express VI to perform an FFT analysis on the signal and plot the resulting value on a graph, the x-scale automatically adjusts to plot the signal in the frequency domain based on the attributes of the signal. Right-click a dynamic data type output terminal of a VI or function on the block diagram and select Create»Graph Indicator from the shortcut menu to display the data in a graph or select Create»Numeric Indicator from the shortcut menu to display the data in a numeric indicator.
The following table lists indicators that accept the dynamic data type and the type of data the dynamic data type can contain. The table also describes how indicators handle the data.
| Data in Dynamic Data Type | Indicator | Result |
|---|---|---|
|
Single numeric value |
Graph |
Plots the single value, includes timestamp and attributes |
|
Single channel |
Graph |
Plots the whole waveform, includes timestamp and attributes |
|
Multiple channels |
Graph |
Plots all the data, includes timestamps and attributes |
|
Single numeric value |
Numeric indicator |
Displays the single value |
|
Single channel |
Numeric indicator |
Displays the last value of the data from the channel |
|
Multiple channels |
Numeric indicator |
Displays the last value of the data from the first channel |
|
Single numeric value |
Boolean indicator |
Displays a TRUE value if numeric value is greater than or equal to 0.5 |
|
Single channel |
Boolean indicator |
Displays a TRUE value if the last value of the data from the channel is greater than or equal to 0.5 |
|
Multiple channels |
Boolean indicator |
Displays a TRUE value if the last value of the data from the first channel is greater than or equal to 0.5 |
Retrieving and Setting Dynamic Data Attributes
Use the Get Dynamic Data Attributes Express VI to retrieve dynamic data attributes. When you add the Get Dynamic Data Attributes Express VI to the block diagram, a configuration dialog box appears. Use this dialog box to retrieve the attributes of a signal in the dynamic data you wire to this Express VI.
Use the Set Dynamic Data Attributes Express VI to set dynamic data attributes, such as the signal name, timestamp, time mode, and so on. When you add the Set Dynamic Data Attributes Express VI to the block diagram, a configuration dialog box appears. Use this dialog box to modify or set the attributes of a signal in the dynamic data you wire to this Express VI.
 |
Note The Get Dynamic Data Attributes Express VI and Set Dynamic Data Attributes Express VI refer to both channels and signals as signals. |
Refer to the Get DDT Attributes VI in the labview\examples\express directory for an example of retrieving dynamic data attributes.
Refer to the Set DDT Attributes VI in the labview\examples\express directory for an example of setting dynamic data attributes.
Converting Dynamic Data
Use the Convert to Dynamic Data Express VI to convert numeric, Boolean, waveform, and array data to dynamic data for use with Express VIs. When you place the Convert to Dynamic Data Express VI on the block diagram, a configuration dialog box appears. Use this dialog box to select the kind of data to convert to the dynamic data type.
Use the Convert from Dynamic Data Express VI to convert dynamic data to numeric, waveform, and array data types for use with other VIs and functions. When you place the Convert from Dynamic Data Express VI on the block diagram, a configuration dialog box appears. The configuration dialog box displays options that let you specify how you want to format the data that the Convert from Dynamic Data Express VI returns.
When you wire dynamic data to an array indicator, LabVIEW places the Convert from Dynamic Data Express VI on the block diagram. Double-click the Convert from Dynamic Data Express VI to open the configuration dialog box and control how the data appears in the array.