Block Diagram Objects

FlexRIO Help

Edition Date: November 2015

Part Number: 372614J-01

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Objects on the block diagram include terminals and nodes. You build block diagrams by connecting the objects with wires. The color and symbol of each terminal indicate the data type of the corresponding control or indicator. Constants are terminals on the block diagram that supply fixed data values to the block diagram.

Block Diagram Terminals

Front panel objects appear as terminals on the block diagram. Double-click a block diagram terminal to highlight the corresponding control or indicator on the front panel.

Terminals are entry and exit ports that exchange information between the front panel and block diagram. Data values you enter into the front panel controls enter the block diagram through the control terminals. During execution, the output data values flow to the indicator terminals, where they exit the block diagram, reenter the front panel, and appear in front panel indicators.

LabVIEW has control and indicator terminals, node terminals, constants, and specialized terminals on structures. You use wires to connect terminals and pass data to other terminals. Right-click a block diagram object and select Visible Items»Terminals from the shortcut menu to view the terminals. Right-click the object and select Visible Items»Terminals again to hide the terminals. This shortcut menu item is not available for expandable VIs and functions.

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 as follows, represents a knob control on the front panel.

The DBL at the bottom of the terminal represents a data type of double-precision, floating-point numeric. A DBL terminal, shown as follows, represents a double-precision, floating-point numeric control.

Right-click a terminal and remove the checkmark next to the View As Icon shortcut menu item to display the data type for the terminal. Use icon terminals to display the types of front panel objects on the block diagram, in addition to the data types of the front panel objects. Use data type terminals to conserve space on the block diagram.

Note  Icon terminals are larger than data type terminals, so you might unintentionally obscure other block diagram objects when you convert a data type terminal to an icon terminal.

Control terminals have a thicker border than indicator terminals. Also, arrows indicate whether the terminal is a control or an indicator. An arrow appears on the right if the terminal is a control, and an arrow appears on the left if the terminal is an indicator.

Control and Indicator Data Types

The following table shows the symbols and uses for the different types of control and indicator terminals.

The color and symbol of each terminal indicate the data type of the corresponding control or indicator. Many data types have a corresponding set of functions that can manipulate the data, such as the String functions on the String palette that correspond to the string data type.

Refer to Numeric Data Types for more information about using numeric data types.

Control Indicator Data Type Use Default Values
Single-precision, floating-point numeric Saves memory and does not overflow the range of the numbers. 0.0
Double-precision, floating-point numeric Is the default format for numeric objects. 0.0
Extended-precision, floating-point numeric Performs differently depending on the platform. Use only when necessary. 0.0
Complex single-precision, floating-point numeric Same as single-precision, floating-point, with a real and an imaginary part. 0.0 + 0.0i
Complex double-precision, floating-point numeric Same as double-precision, floating-point, with a real and an imaginary part. 0.0 + 0.0i
Complex extended-precision, floating-point numeric Same as extended-precision, floating-point, with a real and an imaginary part. 0.0 + 0.0i

Fixed-point numeric Stores values that fall within a user-defined range. This data type is useful when you do not need the dynamic range of floating-point representation or when you want to save resources when working with a target on which floating-point arithmetic is expensive in terms of FPGA resource usage, such as an FPGA target. 0.0
8-bit signed integer numeric Represents whole numbers and can be positive or negative. 0
16-bit signed integer numeric Same as above. 0
32-bit signed integer numeric Same as above. 0
64-bit signed integer numeric Same as above. 0
8-bit unsigned integer numeric Represents only non-negative integers and has a larger range of positive numbers than signed integers because the number of bits is the same for both representations. 0
16-bit unsigned integer numeric Same as above. 0
32-bit unsigned integer numeric Same as above. 0
64-bit unsigned integer numeric Same as above. 0
<64.64>-bit time stamp Stores absolute time with high precision. 12:00:00.000 AM 1/1/1904 (Universal Time)
Enumerated type Gives users a list of items from which to select.
Boolean Stores Boolean (TRUE/FALSE) values. FALSE
String Provides a platform-independent format for information and data, which you can use to create simple text messages, pass and store numeric data, and so on. empty string

Array Encloses the data type of its elements in square brackets and takes the color of that data type. As you add dimensions to the array, the brackets become thicker.

A matrix of complex elements. The wire pattern differs from that of an array of the same data type.

A matrix of real elements. The wire pattern differs from that of an array of the same data type.

Cluster Encloses several data types. Cluster data types appear brown if all elements in the cluster are numeric or pink if any elements in the cluster are of non-numeric types. Error code clusters appear dark yellow, while LabVIEW class clusters are crimson by default or teal green for Report Generation VIs.
Path Stores the location of a file or directory using the standard syntax for the platform you are using. empty path
Dynamic (Express VIs) Includes data associated with a signal and the attributes that provide information about the signal, such as the name of the signal or the date and time the data was acquired.
Waveform Carries the data, start time, and t of a waveform.
Digital waveform Carries start time, x, the digital data, and any attributes of a digital waveform.
Digital Encloses data associated with digital signals.
Reference number (refnum) Acts as a unique identifier for an object, such as a file, device, or network connection.
Variant Includes the control or indicator name, the data type information, and the data itself.
I/O name Passes resources you configure to I/O VIs to communicate with an instrument or a measurement device.
Picture Includes a set of drawing instructions for displaying pictures that can contain lines, circles, text, and other types of graphic shapes.

Select consistent and simple data types to optimize memory usage.

Symbolic Numeric Values for Floating-Point Data

Undefined or unexpected data invalidate all subsequent operations. Floating-point operations return the following two symbolic values that indicate faulty computations or meaningless results:

  • NaN (not a number) represents a floating-point value that invalid operations produce, such as taking the square root of a negative number.
  • Inf (infinity) represents a floating-point value outside of the range for that data type. For example, dividing 1 by zero produces Inf. LabVIEW can return +Inf or –Inf.

LabVIEW does not check for overflow or underflow conditions on integer values.

Overflow and underflow for floating-point numbers is in accordance with IEEE 754, Standard for Floating-Point Arithmetic.

Floating-point operations propagate NaN and Inf reliably. However, integers and fixed-point numbers do not support symbolic numeric values. When you explicitly or implicitly convert +Inf to an integer or fixed-point number, the value becomes the largest value for that data type. For example, converting +Inf to a 16-bit signed integer produces the value 32,767, the largest possible value for a 16-bit signed integer. LabVIEW converts –Inf to the smallest value for that data type.

When you explicitly or implicitly convert NaN to an integer or a fixed-point value, the value becomes the largest value for that data type.

Before you convert data to integer or fixed-point data types, use the Probe tool to check intermediate floating-point values for validity. Check for NaN programmatically by wiring the Comparison function Not A Number/Path/Refnum? to the value you suspect is invalid. You also can use the In Range and Coerce function to keep values within a range you specify.

Note   (FPGA Module) The bit pattern of NaN outputs on the FPGA may differ from outputs on the development computer.


Constants are terminals on the block diagram that supply fixed data values to the block diagram. Universal constants are constants with fixed values, such as pi (π) and infinity (∞). User-defined constants are constants you define and edit before you run a VI.

Label a constant by right-clicking the constant and selecting Visible Items»Label from the shortcut menu. Universal constants have predetermined values for their labels that you can edit by using the Operating tool or the Labeling tool.

Most constants are located at the bottom or top of their palettes.

Universal Constants

Use universal constants for mathematical computations and formatting strings or paths. LabVIEW includes the following types of universal constants:

  • Universal numeric constants—A set of high-precision and commonly used mathematical and physical values, such as the natural logarithm base (e) and the speed of light. The universal numeric constants are located on the Math & Scientific Constants palette.
  • Universal string constants—A set of commonly used non-displayable string characters, such as line feed and carriage return. The universal string constants are located on the String palette.
  • Universal file constants—A set of commonly used file path values, such as Not a Path, Not a Refnum, and Default Directory. The universal file constants are located on the File Constants palette.

User-Defined Constants

The Functions palette includes constants organized by type, such as Boolean, numeric, ring, enumerated type, color box, string, array, cluster, and path constants.

Create a user-defined constant by right-clicking an input terminal of a VI or function and selecting Create»Constant from the shortcut menu.

You cannot change the value of user-defined constants when the VI is running.

You also can create a constant by dragging a front panel control to the block diagram. LabVIEW creates a constant that contains the value of the front panel control at the time you dragged it to the block diagram. The front panel control remains on the front panel. Changing the value of the control does not affect the constant value and vice versa.

Use the Operating or Labeling tool to click the constant and edit its value. If automatic tool selection is enabled, double-click the constant to switch to the Labeling tool and edit the value.

Constant Folding

LabVIEW uses constant folding to optimize the performance of VIs. With constant folding, LabVIEW computes constant values on first run of the VI, instead of calculating them during each run.

Note  If you enable debugging, LabVIEW does not use constant folding for structures.

You can display constant folding hash marks on the block diagram by selecting Tools»Options, selecting Block Diagram from the Category list, and placing checkmarks in the Show constant folding of wires and Show constant folding of structures checkboxes. When you place a checkmark in the Show constant folding of wires checkbox, hash marks appear on the wires attached to constants that are constant folded. When you place a checkmark in the Show constant folding of structures checkbox, gray hash marks appear inside structures that are wired to constants. The hash marks might not appear until after you run or save the VI. The hash marks also do not appear when you make an edit to the VI.

Note   (FPGA Module)The LabVIEW FPGA Module performs constant folding entirely at compile time rather than running the VI to compute constant values. The Show constant folding of wires and Show constant folding of structures checkbox options have no effect when used in an FPGA VI.

Block Diagram 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. LabVIEW includes the following types of nodes:

  • Functions—Built-in execution elements, comparable to an operator, function, or statement.
  • SubVIs—VIs used on the block diagram of another VI, comparable to subroutines.
  • Express VIs—SubVIs designed to aid in common measurement tasks. You configure an Express VI using a configuration dialog box.
  • Structures—Execution control elements, such as For Loops, While Loops, Case structures, Flat and Stacked Sequence structures, Timed structures, and Event structures.
  • Formula and Expression Nodes—Formula Nodes are resizable structures for entering equations directly into a block diagram. Expression Nodes are structures for calculating expressions that contain a single variable.
  • Property and Invoke Nodes—Property Nodes are structures for setting or finding properties of a class. Invoke Nodes are structures for executing methods of a class.
  • Call By Reference Nodes—Structures for calling a dynamically loaded VI.
  • Call Library Function Nodes—Structures for calling most standard shared libraries or DLLs.

Polymorphic VIs and Functions

Polymorphic VIs and functions can adjust to input data of different data types. Most LabVIEW structures are polymorphic, as are some VIs and functions.

Functions are polymorphic to varying degrees—none, some, or all of their inputs can be polymorphic. Some function inputs accept numeric values or Boolean values. Some accept numeric values or strings. Some accept not only scalar numeric values, but also arrays of numeric values, clusters of numeric values, arrays of clusters of numeric values, and so on. Some accept only one-dimensional arrays, although the array elements can be of any type. Some functions accept all types of data, including complex numeric values.


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