Functional global variable (FGV)

Use cases

• Communicate data, control, and status between two or more parallel process loops contained within the same target (“target-scoped”), either in the same VI or in different VIs
• Make the latest value of a variable available to other process loops
• Stop multiple parallel loops from a single control
• Perform additional operations (the “functional” aspect of the FGV acronym) beyond merely storing the data, e.g., counting and calculations

Features

• A functional global variable is subVI with these defining characteristics:
  • Uninitialized shift register – an uninitialized shift register on a while-loop causes LabVIEW to allocate storage for a single value that persists as long as the calling VI remains in memory
  • Single-iteration while-loop – the while-loop conditional terminal is wired to a “true” constant, therefore it only iterates once; the while-loop is merely a mechanism to hold the shift register
  • Case structure with enumerated control – the case structure selects an operation to perform on the stored value; “read” and “write” are the most basic operations, and additional cases can implement additional functions such as “increment”, “decrement”, etc.
  • Non-reentrant subVI – this execution mode ensures that only one instance of the subVI exists in the target; multiple instances of the subVI all refer to the same stored value
• Functional global variables offer two advantages over local variables and global variables:
  • Memory efficiency – the stored value exists in only one place in memory, whereas each instance of a local/global variable reader creates its own copy of the data
  • Avoids race conditions – the stored value cannot be written while it is being read because the subVI can only be executed by one process at a time

Keep in mind

• The subVI execution mode must be set to “non-reentrant” (open the subVI, press Ctrl+I, select “Execution” category, and choose “non-reentrant execution”); the other two reentrant execution modes create independent copies (“clones”) of the subVI, thereby eliminating the global variable aspect of the FGV

LabVIEW block diagram elements

The functional global variable is a design pattern as opposed to a set of built-in VIs. Refer to the example code below for details.

Example code

• Connect your Academic RIO Device to your PC using USBLAN, Ethernet, or Wi-Fi. NOTE: Not all Academic RIO Devices have Ethernet and Wi-Fi connectivity options.
• Download and unpack the rt_functional-global-variable.zip (for use with NI myRIO 1900) or the NIELVISIII-rt_functional-global-variable.zip (for use with NI ELVIS III) archive, and then double-click the ".lvproj" file to open the project. NOTE: This project was written for a NI myRIO 1900 or NI ELVIS III connected by USBLAN at IP address 172.22.11.2. If you are using a different IP address or another Academic RIO Device (Example: NI myRIO 1950 or NI RIO Control Module) do the following:
  • If using the NI myRIO 1950 or NI RIO Control Module start with the NI myRIO 1900 Archive.
  • Different IP address: Right-click on the "NI myRIO 1900" Device, choose "Properties", and then enter the new IP address
  • Different device:
    • Right-click on the top of the project hierarchy, select "New Targets and Devices", keep the "Existing target or device" option, and then find and select your particular device
    • Select all of the components under the "NI myRIO 1900" device: click the first one and then shift+click the last one
    • Drag the selected components to the new device
    • Right-click the "NI myRIO 1900" device and select "Remove from project"
• Open three VIs: “RT Main”, “RT subVI-1”, and “RT subVI-2”
• Run “RT Main”
  • The “fast counter” in Process Loop #1 of “VI one” increments once each loop cycle (100 ms)
  • The Boolean “reset” generated by Process Loop #2 of “VI two” resets the fast counter
  • The “slow counter” in Process Loop #2 increments each time the fast counter reaches 10 counts; this condition also enables the “reset” signal
• Open the three “fgv - ” subVIs to observe the state of the three functional global variables “fast counter”, “reset”, and “all stop”
• Stop all VIs by clicking the “Stop” button to set the “all stop” FGV

Expected results

Developer walk-through

Outline

• Review overall structure:
  • RT Main
    • Initialize the “all stop” functional global variable (FGV)
    • Run two subVIs, each containing FGVs
    • Poll the “Stop” button and indicate loop activity
    • Set the “all stop” FGV when the “Stop” button is clicked
    • The two VIs read “all stop” FGV for the loop conditional terminal
  • RT subVI-1: “fast counter”
    • Displays up-counter value as front-panel indicator
    • Writes counter value to “fast counter” FGV
    • Resets counter to zero when “reset” FGV is true
  • RT subVI-2: “slow counter”
    • Displays up-counter displayed as front-panel indicator
    • Increments counter when the “fast counter” FGV equals 10
    • Activates “reset” indicator and corresponding FGV when the “fast counter” value equals 10
• Functional global variable VIs
  • Outer case structure implements standard error behavior
  • While-loop structure:
    • Uninitialized shift register allocates persistent storage
    • Loop conditional wired to “T” means loop runs only once per call
  • Inner case structure:
    • Enumerated data type selects the operation (or “function”) to be performed on the variable
    • “Read” and “Write” are the most basic operations
    • Add additional operations with more case structure subdiagrams
  • Must use non-reentrant execution mode

For more information

1. Effective LabVIEW Programming (http://www.ntspress.com/publications/effective-labview-programming/)
   An excellent textbook by Dr. Thomas J. Bress covering all aspects of system design; Chapter 5 ("Action Engines" describes functional global variables in detail. Download all of the code examples from this textbook: follow the link above, select "User Resources", and then "All VIs".
   
2. The LabVIEW Style Book (https://www.pearson.com/us/higher-education/program/Blume-Lab-VIEW-Style-Book-The/PGM219404.html)
   A comprehensive textbook by Peter A. Blume that lays out everything you need to know about best practices for LabVIEW system design; Chapter 8 describes functional global variables.