A front panel contains the user interfaces of a VI shown in a block diagram. VI inputs are represented by controls such as knobs, pushbuttons and dials. VI outputs are represented by indicators such as graphs, LEDs (light indicators) and meters. As a VI runs, its front panel provides a display or user interface of controls (inputs) and indicators (outputs).

A block diagram contains terminal icons, nodes, wires and structures. Terminal icons, or interfaces through which data are exchanged between a front panel and a block diagram, correspond to controls or indicators that appear on a front panel. Whenever a control or indicator is placed on a front panel, a terminal icon gets added to the corresponding block diagram. A node represents an object or block that has input and/or output connectors and performs a certain function. SubVIs and functions are examples of nodes. Wires establish the flow of data in a block diagram, and structures control the flow of data such as repetitions or conditional executions. Figure 1 shows front panel and block diagram windows.

Figure 1: LabVIEW Windows: Front Panel and Block Diagram

A VI icon is a graphical representation of a VI. It appears in the top right corner of a block diagram or a front panel window. When a VI is inserted into a block diagram as a subVI, its icon is displayed.

A connector pane defines VI inputs (controls) and outputs (indicators). One can change the number of inputs and outputs by using different connector pane patterns. In Figure 1, a VI icon is shown at the top right corner of the block diagram, and its corresponding connector pane, with two inputs and one output, is shown at the top right corner of the front panel.

The Functions palette (see Figure 2) provides various function VIs or blocks to build a system. View this palette by right-clicking on an open area of a block diagram. Note that this palette can be displayed only in a block diagram.

Figure 2: Functions Palette

The Controls palette (see Figure 3) features front panel controls and indicators. View this palette by right-clicking on an open area of a front panel. Note that this palette can be displayed only in a front panel.

Figure 3: Controls Palette

The Tools palette offers various mouse cursor operation modes for building or debugging a VI. The Tools palette and the frequently used tools are shown in Figure 4.

Figure 4: Tools Palette

Each tool is used for a specific task. For example, use the wiring tool to wire objects in a block diagram. If one enables the automatic tool selection mode by clicking on the Automatic Tool Selection button, LabVIEW selects the best matching tool based on a current cursor position.

Controls make up the inputs to a VI. Controls grouped in the Numeric Controls palette(Controls → Express → Num Ctrls) are used for numerical inputs, controls grouped in the Buttons & Switches palette(Controls → Express → Buttons) are used for Boolean inputs, and controls grouped in the Text Controls palette(Controls →Express →Text Ctrls) are used for text and enumeration inputs. These control options are displayed in Figure 5.

Figure 5: Control Palettes

Indicators make up the outputs of a VI. Indicators grouped in the Numeric Indicators palette(Controls→ Express → Numeric Inds) are used for numerical outputs, indicators grouped in the LEDs palette(Controls → Express → LEDs) are used for Boolean outputs, indicators grouped in the Text Indicators palette(Controls → Express → Text Inds) are used for text outputs, and indicators grouped in the Graph Indicators palette(Controls → Express → Graph Indicators) are used for graphical outputs. These indicator options are displayed in Figure 6.

Figure 6: Indicator Palettes

The menu items on the front panel toolbar (see Figure 7) provide options to align and orderly distribute objects on the front panel. Normally, after one places controls and indicators on a front panel, these options can be used to tidy up their appearance.

Figure 7: Menu to Align, Distribute, Resize and Reorder Objects

Express VIs denote higher-level VIs configured to incorporate lower-level VIs or functions. These VIs are displayed as expandable nodes with a blue background. Placing an Express VI in a block diagram opens a configuration dialog window to adjust the Express VI parameters. As a result, Express VIs demand less wiring. The configuration window can be opened by double-clicking on its Express VI.

Basic operations such as addition or subtraction are represented by functions. Figure 8 shows three examples corresponding to three block diagram objects (VI, Express VI and function).

Figure 8: Block Diagram Objects: (a) VI, (b) Express VI, (c) Function

One can display subVIs or Express VIs as icons or expandable nodes. If a subVI is displayed as an expandable node, the background appears yellow. Icons can be used to save space in a block diagram and expandable nodes can be used to achieve easier wiring or better readability. One can resize expandable nodes to show their connection nodes more clearly. Three appearances of a VI/Express VI are shown in Figure 9.

Figure 9: Icon versus Expandable Node

Front panel objects are displayed as terminal icons in a block diagram. A terminal icon exhibits an input or output as well as its data type. Figure 10 shows two terminal icon examples consisting of a double precision numerical control and indicator. As shown in this figure, one can display terminal icons as data type terminal icons to conserve space in a block diagram.

Figure 10: Terminal Icon Examples Displayed in a Block Diagram

Wires transfer data from one node to another in a block diagram. Based on the data type of a data source, the color and thickness of its connecting wires change.

Wires for the basic data types used in LabVIEW are shown in Figure 11. In addition to the data types shown in this figure, there are some other specific data types. For example, the dynamic data type is always used for Express VIs, and the waveform data type, which corresponds to the output from a waveform generation VI, is a special cluster of waveform components incorporating trigger time, time interval and data value.

Figure 11: Basic Wire Types

A structure is represented by a graphical enclosure. The graphical code enclosed in the structure gets repeated or executed conditionally. A loop structure is equivalent to a for loop or a while loop statement in text-based programming languages, while a case structure is equivalent to an if-else statement.

For Loop

A for loop structure is used to perform repetitions. As illustrated in Figure 12, the displayed border indicates a for loop structure, where the count terminal represents the number of times the loop is to be repeated. It is set by wiring a value from outside of the loop to it. The iteration terminal denotes the number of completed iterations, which always starts at zero.

Figure 12: For Loop

While Loop

A while loop structure allows repetitions depending on a condition (see Figure 13). The conditional terminal initiates a stop if the condition is true. Similar to a for loop, the iteration terminal provides the number of completed iterations, always starting at zero.

Figure 13: While Loop

Case Structure

A case structure (see Figure 14) allows the running of different sets of operations depending on the value it receives through its selector terminal, which is indicated by . In addition to Boolean type, the input to a selector terminal can be of integer, string, or enumerated type. This input determines which case to execute. The case selector shows the status being executed. Cases can be added or deleted as needed.

Figure 14: Case Structure

VIs can be debugged as they run by checking values on wires with the Probe tool. Note that the Probe tool can be accessed only in a block diagram window.

With the Probe tool, breakpoints and execution highlighting, one can identify the source of an incorrect or an unexpected outcome. To visualize the flow of data during program execution, a breakpoint can be used to pause the execution of a VI at a specific location.

Timing and memory usage information – in other words, how long a VI takes to run and how much memory it consumes – can be gathered with the Profile tool. It is required to make sure that a VI is stopped before setting up a Profile window.

An effective way to become familiar with LabVIEW programming is to review examples. In the lab that follows, we explore most of the key LabVIEW programming features by building simple VIs.