Global variable (RT)

Use a global variable to communicate between two parallel process loops contained within different VIs under the same target, and use a global variable to stop parallel loops with one "stop" button.

LabVIEW RT block diagrams: VI #1 writes a global variable, VI #2 reads the same global variable

Use cases

Communicate data, control, and status between two or more parallel process loops contained within the same target (“target-scoped”) and possibly in different VIs
Make the latest value of a variable available to other process loops
Stop multiple parallel loops from a single control

Features

Stores values in global variables
Tag-based (latest value) communication
Easy to code
Supports multiple clock domains: timed loops and conventional loops

Keep in mind

Because this is tag-based (latest value) communication, rapid changes of a variable in one process can be missed in the other process
Ensure that control signals remain active long enough for the other process to detect them, or devise a hand-shaking scheme
Consider local variables if you only need to communicate between process loops in the same VI

LabVIEW block diagram elements

Locate these elements with "Quick Drop" (press Ctrl+Space and start typing the name); click on an icon to see more sample code that uses that element:

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_global-variable.zip (for use with NI myRIO 1900) or the NIELVISIII-rt_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 all four VIs: “RT Main”, “RT subVI-1”, “RT subVI-2”, and “RT global variables”
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
- The “RT global variables” front panel displays the state of the three global variables
Stop all VIs by clicking the “Stop” button to set the “all stop” global variable

Expected results

https://youtu.be/G-kH06h3_Ck (1:15)

Developer walk-through

https://youtu.be/-r7Q8HCxpas (4:37)

Outline

Review overall structure:
- RT Main
  - Run two VIs
  - Poll the “Stop” button
  - Set the global variable “all stop” when the “Stop” button is clicked
- RT subVI-1: “fast counter”
  - Displays up-counter value as front-panel indicator
  - Writes counter value to global variable “fast counter”
  - Resets counter to zero when global variable “reset” is true
- RT subVI-2: “slow counter”
  - Displays up-counter displayed as front-panel indicator
  - Increments counter when the “fast counter” global variable equals 10
  - Activates “reset” indicator and corresponding global variable when the “fast counter” value equals 10
Create a global variable from scratch
- A special VI without a block diagram
- Place a new global variable
- Double-click it to open
- Add indicators and controls as needed
- Save the VI
- Select the desired variable
Update an existing global variable VI
- Double-click it to open
- Add/delete/modify indicators and controls as needed
- Save the VI
- Select the desired variable
Stop both loops from a single front-panel “Stop” button
- “RT Main” polls “Stop” button and writes its value to “all stop” global variable
- The two VIs read “all stop” global variable for the loop conditional terminal
- “RT Main” clears the “all stop” global variable before running the two VIs to cover the case where “all stop” remains set after the VIs have stopped

For more information

Stopping Parallel While Loops in LabVIEW with One Stop Button (http://digital.ni.com/public.nsf/allkb/267704CDE91156D186256F6D00711AAE)
Stop multiple process loops with channel wires, local variables, property nodes, notifiers, and queues.

RT code example