Single-process shared variable (SPSV)

Use cases

• Exchange data between a deterministic process loopA special type of while-loop structure that executes with a precisely-defined time per loop iteration. and a nondeterministic process loopA conventional while-loop structure whose execution time may vary unexpectedly from one loop iteration to the next.
• Isolate elements that cause jitterUnpredictable variation in process loop execution time from one iteration to the next. such as network communications from the deterministic process

Features

• Relatively easy to code
• Similar behavior as a global variable

Keep in mind

• RT timed loops (deterministic process loops) should be free of all non-deterministic elements
• A global variable shared with a non-deterministic loop introduces non-determinism into the timed loop

LabVIEW block diagram elements

Follow this general procedure to create a network-published shared variable, but select “Single Process” as the desired variable type.

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_single-process-shared-variable.zip (for use with NI myRIO 1900) or the NIELVISIII-rt_single-process-shared-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"
• Run the “RT Main” VI:
  • This is a “headless” VI – once started, the PC host monitors and controls the VI through the network connection
  • The VI contains one non-deterministic loop and one deterministic loop
  • Polls the “LED0” NPSV every 100ms and sets the states of LEDs 0 and 3
  • Writes the “Button” NPSV according to the state of the Academic RIO Device onboard pushbutton
  • Reads the onboard accelerometer Z-axis every 10ms in the deterministic loop, and writes the “Accelerometer-Z” NPSV
  • Polls the “Stop” NPSV in the non-deterministic loop and copies this state to the “Local Stop” SPSV which in turn is polled within the deterministic loop
• Run the “PC Main” VI:
  • Writes the “LED0” NPSV every 10ms according to the LED0 front-panel button
  • Reads the “Button” NPSV and displays its state
  • Reads the “Accelerometer-Z” NPSV and displays its value as a chart; tip and rotate the Device to see changes
• Stop the “PC Main” VI to stop the “RT Main” VI, too

Expected results

Developer walk-through

Outline

• Review overall structure (“RT Main”):
  • Two process loops
    • Non-deterministic (conventional) loop handles most of the network interaction with “PC Main” VI
    • Deterministic (timed) loop reads accelerometer precision 100-Hz rate
  • Single-process shared-variable with FIFO enabled links the two loops without disrupting the deterministic loop, i.e., without adding jitter
• Place an existing single-process shared variable
• Create a single-process shared variable with FIFO enabled
  • Open the LabVIEW project
  • Right-click the RT target
  • Select “New | Variable”
  • “Variable” page: Enter the variable name, select “Single Process” as the variable type, and choose the desired data type
  • “Description” page: Enter a description for the variable, if desired
  • “RT FIFO” page: enable this option when the SPSV resides in a deterministic loop
  • Right-click the newly-created library and choose Save
  • Enter the name of the LabVIEW library (.lvlib) file; recommended that you store this file with the LabVIEW project file
  • Drag the variable from the LabVIEW project window to the VI block diagram window
  • Note the distinctive FIFO symbol on the variable
  • Right-click and select “Access Mode” and select “Read” or “Write” as needed
  • Add additional variables by right-clicking the LabVIEW library, and select New | Variable