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.

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 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.

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.

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.

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.

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 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.

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.

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.