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Block Diagram

Module by: National Instruments

Summary: In this section, you will learn about block diagrams.

After you build the front panel, you add code using graphical representations of functions to control the front panel objects. The block diagram contains this graphical source code. Front panel objects appear as terminals, on the block diagram. Block diagram objects include terminals, subVIs, functions, constants, structures, and wires, which transfer data among other block diagram objects.

The VI in Figure 1 shows several primary block diagram objects-nodes, terminals, and wires.

Figure 1: 1. Nodes, 2. Indicator Terminals, 3. Wires, 4. Control Terminals
Figure 1 (nodterm.png)

Functions Palette

The Functions palette is available only on the block diagram. The Functions palette contains the VIs and functions you use to build the block diagram. Select Window>>Show Functions Palette or right-click the block diagram workspace to display the Functions palette. Tack down the Functions palette by clicking the thumbtack on the top left corner of the palette. By default, the Functions palette starts in the Express view.

Express VIs, VIs, and Functions

LabVIEW uses colored icons to distinguish between Express VIs, VIs, and functions on the block diagram. By default, icons for Express VIs appear on the block diagram as expandable nodes with icons surrounded by a blue field. Icons for VIs have white backgrounds, and icons for functions have pale yellow backgrounds.

By default, most functions and VIs on the block diagram appear as icons that are not expandable, unlike Express VIs.

Express VIs

Use Express VIs for common measurement tasks. Express VIs are nodes that require minimal wiring because you configure them with dialog boxes. You can save the configuration of an Express VI as a subVI. Refer to Building the Block Diagram, of the LabVIEW User Manual for more information about creating subVIs from Express VIs.

VIs

When you place a VI on the block diagram, LabVIEW considers the VI to be a subVI. When you double-click a subVI, its front panel and block diagram appear, rather than a dialog box in which you can configure options. The front panel includes controls and indicators. The block diagram includes wires, front panel icons, functions, possibly subVIs, and other LabVIEW objects.

The upper right corner of the front panel and block diagram displays the icon for the VI. This is the icon that appears when you place the VI on the block diagram.

You can create a VI to use as a subVI. Refer to Modular Programming for more information about creating VIs and configuring them as subVIs.

Functions

Functions are the fundamental operating elements of LabVIEW. Functions do not have front panels or block diagrams but do have connector panes. Double-clicking a function only selects the function.

Nodes

Nodes are objects on the block diagram that have inputs and/or outputs and perform operations when a VI runs. They are analogous to statements, operators, functions, and subroutines in text-based programming languages. Nodes can be functions, subVIs, or structures. Structures are process control elements, such as Case structures, For Loops, or While Loops. The Add and Subtract functions in Figure 1 are function nodes.

Expandable Nodes versus Icons

You can display VIs and Express VIs as icons or as expandable nodes. Expandable nodes appear as icons surrounded by a colored field. SubVIs appear with a yellow field, and Express VIs appear with a blue field. Use icons, such as the Basic Function Generator VI icon if you want to conserve space on the block diagram. Use expandable nodes, such as the Basic Function Generator VI expandable node to make wiring easier and to aid in documenting block diagrams. By default, subVIs appear as icons on the block diagram, and Express VIs appear as expandable nodes.

To display a subVI or Express VI as an expandable node, right-click the subVI or Express VI and select View As Icon from the shortcut menu to remove the checkmark.

You can resize the expandable node to make wiring even easier, but it also takes a large amount of space on the block diagram. Complete the following steps to resize a node on the block diagram.

  1. Move the Positioning tool over the node. Resizing handles appear at the top and bottom of the node.
  2. Move the cursor over a resizing handle to change the cursor to the resizing cursor.
  3. Use the resizing cursor to drag the border of the node down to display additional terminals.
  4. Release the mouse button.

To cancel a resizing operation, drag the node border past the block diagram window before you release the mouse button.

Figure 2 shows the Basic Function Generator VI as a resized expandable node.

Figure 2
Figure 2 (funcgenexpnode2.png)

note:

If you display a subVI or Express VI as an expandable node, you cannot display the terminals for that node and you cannot enable database access for that node.

Terminals

iconterminal.png cdbl.png Front panel objects appear as terminals on the block diagram. The terminals represent the data type of the control or indicator. You can configure front panel controls or indicators to appear as icon or data type terminals on the block diagram. By default, front panel objects appear as icon terminals. For example, a knob icon terminal, shown at left, represents a knob on the front panel. The DBL at the bottom of the terminal represents a data type of double-precision, floating-point numeric. To display a terminal as a data type on the block diagram, right-click the terminal and select View As Icon from the shortcut menu to remove the checkmark. A DBL data type terminal, shown above at left, represents a double-precision, floating-point numeric control or indicator.

addsubterm.png Terminals are entry and exit ports that exchange information between the front panel and block diagram. Terminals are analogous to parameters and constants in text-based programming languages. Types of terminals include control or indicator terminals and node terminals. Control and indicator terminals belong to front panel controls and indicators. Data you enter into the front panel controls (a a and b b in Figure 1) enter the block diagram through the control terminals. The data then enter the Add and Subtract functions. When the Add and Subtract functions complete their internal calculations, they produce new data values. The data flow to the indicator terminals, where they exit the block diagram, reenter the front panel, and appear in front panel indicators ( a+b a b and a-b a b in Figure 1). The terminals in Figure 1 belong to four front panel controls and indicators. The connector panes of the Add and Subtract functions, shown in Media 5, have three node terminals. To display the terminals of the function on the block diagram, right-click the function node and select Visible Items>>Terminals from the shortcut menu.

Wires

You transfer data among block diagram objects through wires. Wires are analogous to variables in text-based programming languages. In Figure 1, wires connect the control and indicator terminals to the Add and Subtract functions. Each wire has a single data source, but you can wire it to many VIs and functions that read the data. Wires are different colors, styles, and thicknesses, depending on their data types. A broken wire appears as a dashed black line with a red X in the middle. The examples in Table 1 are the most common wire types.

Wire Type Scalar 1D Array 2D Array Color
Numeric wirenum.png wireintnum.png wirenumary.png wireintary.png wirenumary2.png wireintary2.png Orange (floating-point), Blue (integer)
Boolean wirebool.png wireboolary.png wireboolary2.png Green
String wirestrg.png wirestrgary.png wirestrgary2.png Pink

In LabVIEW, you use wires to connect multiple terminals together to pass data in a VI. The wires must be connected to inputs and outputs that are compatible with the data that is transferred with the wire. For example, you cannot wire an array output to a numeric input. In addition the direction of the wires must be correct. The wires must be connected to only one input and at least one output. For example, you cannot wire two indicators together. The components that determine wiring compatibility include the data type of the control and/or indicator and the data type of the terminal.

Data Types

Data types indicate what objects, inputs, and outputs you can wire together. For example, a switch has a green border so you can wire a switch to any input with a green label on an Express VI. A knob has an orange border so you can wire a knob to any input with an orange label. However, you cannot wire an orange knob to an input with a green label. Notice the wires are the same color as the terminal.

dynamicdt.png The dynamic data type stores the information generated or acquired by an Express VI. The dynamic data type appears as a dark blue terminal, shown at left. Most Express VIs accept and/or return the dynamic data type. You can wire the dynamic data type to any indicator or input that accepts numeric, waveform, or Boolean data. Wire the dynamic data type to an indicator that can best present the data. Indicators include a graph, chart, or numeric indicator.

Most other VIs and functions in LabVIEW do not accept the dynamic 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.

cddt.png Use the Convert from Dynamic Data Express VI, shown at left, to convert the dynamic data type to numeric, Boolean, 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, the Configure Convert from Dynamic Data dialog box appears. The Configure Convert from Dynamic Data 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 a dynamic data type to an array indicator, LabVIEW automatically places the Convert from Dynamic Data Express VI on the block diagram. Double-click the Convert from Dynamic Data Express VI to open the Configure Convert from Dynamic Data dialog box to control how the data appears in the array.

Use the Convert to Dynamic Data Express VI to convert numeric, Boolean, waveform, and array data types to the dynamic data type for use with Express VIs. When you place the Convert to Dynamic Data Express VI on the block diagram, the Configure Convert to Dynamic Data dialog box appears. Use this dialog box to select the kind of data to convert to the dynamic data type.

Automatically Wiring Objects

LabVIEW automatically wires objects as you place them on the block diagram. You also can automatically wire objects already on the block diagram. LabVIEW connects the terminals that best match and leaves terminals that do not match unconnected.

As you move a selected object close to other objects on the block diagram, LabVIEW draws temporary wires to show you valid connections. When you release the mouse button to place the object on the block diagram, LabVIEW automatically connects the wires.

Toggle automatic wiring by pressing the spacebar while you move an object using the Positioning tool. You can adjust the automatic wiring settings by selecting Tools>>Options and selecting Block Diagram from the top pull-down menu.

Manually Wiring Objects

When you pass the Wiring tool over a terminal, a tip strip appears with the name of the terminal. In addition, the terminal blinks in the Context Help window and on the icon to help you verify that you are wiring to the correct terminal.

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