Use cases

• PC-based user interface (UI) (also called human-machine interface or HMI) for a continually-running RT system controller application
• Connect to and disconnect from the remote RT application as part of its normal operation
• Use as a template for your own embedded control and monitoring application software architecture that promotes reliability, scalability, and maintainability

Features

• Dual-loop design with each loop referencing a common queue
• Producer loop:
  • Event structure handles button clicks
  • Each button enqueues a particular consumer-loop state as a string of the form “state:params” in which the optional “:params” provides state-specific parameters
  • The front-panel stop button enqueues the “Shutdown” state
  • Guard clause (Boolean case structure) traps errors generated within the Producer Loop and shuts down both loops
• Consumer Loop:
  • Queued state machine based on the common queue
  • Guard clause inspects error cluster on each loop iteration:
    • No error – dequeue the state inserted by the Producer Loop
    • Error – go directly to the “Shutdown” state
  • Separate the dequeued string into the state string and parameters string, if included
  • Case structure decodes the dequeued string from the Producer Loop:
    • “Initialize” – use “Invoke Node” to release the “connect” button and “Property Nodes” to disable the buttons that should only be used once the RT connection has been established
    • “Connect” – create the network stream writer endpoint, disable the “connect” button, and enable the system controller buttons; generate an error if the connection cannot be established within 5 seconds
    • “Send” – write the parameter to the network stream as the command message for the RT controller
    • “Disconnect” – send the “Disconnect” command message, flush the network stream to ensure that the message is received by the RT controller, destroy the network stream endpoint, and then return the buttons to their default states
    • “Shutdown” – signals the Producer Loop to stop by virtually clicking the front-panel “Stop” button via a “Property Node” and then stops itself by feeding a “true” constant to the loop conditional terminal
    • Default case – traps unrecognized state strings and inserts an error into the error cluster

See the complete Home Security System project to get an idea of how this producer-consumer state machine interacts with the RT system.

Keep in mind

• Stopping both loops is a rather involved yet still understandable process that has three possible causes:
  • “Stop” button is clicked:
    1. Producer Loop event structure performs two actions: (a) enqueues the “Shutdown” state at the front of the queue to ensure that this is the next state processed by the Consumer Loop, then (b) stops itself by feeding a “true” constant to its own loop conditional terminal
    2. Consumer Loop dequeues the “Shutdown” state which performs two actions: (a) signals the Producer Loop to stop by virtually clicking the front-panel “Stop” button (in this case a redundant action because the “Stop” button was already pressed), then (b) stops itself by feeding a “true” constant to its own loop conditional terminal
  • Error generated within Producer Loop:
    1. Producer Loop guard clause detects error and enqueues the “Shutdown” state at the front of the queue to ensure that this is the next state processed by the Consumer Loop
    2. Consumer Loop dequeues the “Shutdown” state which performs two actions: (a) signals the Producer Loop to stop by virtually clicking the front-panel “Stop” button, then (b) stops itself by feeding a “true” constant to its own loop conditional terminal
    3. Producer Loop event structure performs two actions: (a) enqueues the “Shutdown” state (in this case a redundant action because the Consumer Loop has already stopped), then (b) stops itself by feeding a “true” constant to its own loop conditional terminal
  • Error generated in Consumer Loop:
    1. Consumer Loop guard clause detects error and selects the “Shutdown” state
    2. Subsequent actions are the same as Steps 2 and 3 from the Producer Loop-generated error

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-pc_home-security-system.zip (for use with NI myRIO 1900) or the NIELVISIII-rt-pc_home-security-system.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 and run “RT Main”; see Home Security System for operating instructions
• Open “PC Main” and enter the correct IP address for your device (172.22.11.2 if connected as the only device by USB cable), otherwise use NI MAX to determine the network address of your device
• Run “PC Main”
  • Observe the indicators at the top of the “Remote Monitor” panel: the green flags designate these indicators as bound to network-published shared variables (NPSVs) hosted on the RT target. When properly connected these indicators will track the indicators on the RT system controller.
  • Click “Connect” to establish the network stream connection to the system controller; note the “PC host connected” indicator on the “RT Main” front panel
  • Click each of the now-active buttons to send a message to the controller:
    • “Valid Code” emulates a proper keypad sequence entry
    • “Reset” initializes the controller regardless of its current state
    • “PANIC” sends the controller directly to the “alarm” state
  • Click “Disconnect” to close the network stream connection
• “RT Main” should continue to run regardless of the run state of “PC Main”: observe the “PC host connected” indicator on “RT Main” as you stop “PC Main” while connected to the RT target and then restart and reconnect

Expected results

Developer walk-through

Outline

• Review overall structure: Producer Loop
  • Event structure blocks (waits) until a front-panel button is pressed
  • Each event case subdiagram enqueues the associated command for the Consumer Loop
  • Guard clause enqueues “Shutdown” state when error is detected
• Review overall structure: Consumer Loop
  • Guard clause enqueues “Shutdown” state when error is detected, otherwise blocks until a queued command is available from Producer Loop
  • “State & Args” subVI separates command and optional arguments (parameters) into two strings
  • Case structure subdiagram implements functionality for each command
• Review overall structure: indicators and queue setup
  • Front-panel indicators bound to network-published shared variables hosted on RT target
  • No special programming required, simply place them somewhere on the block diagram
  • Create the command queue and enqueue the “Initialize” state
  • Release the queue only when both loops have stopped
• Normal operation sequence: Producer Loop
  • Both loops block until a button is pressed – much more CPU efficient than polling, and ensures responsive user interface
  • Press a button to cause event structure to enqueue the appropriate Producer Loop command as a string
    • connect -> “Connect” command (connect to RT target)
    • disconnect -> “Disconnect” command (disconnect from RT target)
    • reset -> “Send” command with the RT controller message command “SysMgr/Initialize” as the optional argument
    • valid code -> “Send” command with the RT controller message “SysMgr/alert:valid”
    • panic -> “Send” command with the RT controller message “SysMgr/alarm”
    • [ESC] (Stop button) -> enqueue “Shutdown” state at the top (front) of the queue to ensure that it is executed as the next command, and select the “true” value to stop the Producer Loop
  • Guard clause takes no action
  • Producer Loop completes iteration and waits for next button press
• Normal operation sequence: Consumer Loop
  • Guard clause dequeues command from Producer Loop
  • “State & Args” subVI separates command and optional arguments (parameters) joined by colon “:” into two strings
  • Case structure implements the appropriate action for each command
    • Initialize -> Unstick and enable the “connect” button and disable the buttons which are only to be used when connected; use Invoke Node and Property Nodes
    • Connect -> Create the network stream writer endpoint (wait up to 5 seconds before timing out), and enable the three buttons
    • Disconnect -> Send the “Disconnect” message to the RT target, flush the stream to ensure that it is received successfully, destroy the network stream endpoint, and disable the three buttons
    • Send -> Send the arguments as the message to the RT target
    • Shutdown -> Virtually press the front-panel stop button with a Property Node, and select the “true” value to stop the Consumer Loop
    • Default -> Throw an error for unrecognized command from Producer Loop
  • Consumer Loop completes iteration and waits for next command in the queue
• Normal shutdown sequence: “Stop” button
  • Producer Loop enqueues the “Shutdown” state at the front of the queue to ensure that this is the next state processed by the Consumer Loop and then stops itself
  • Consumer Loop dequeues the “Shutdown” state and stops itself
  • Queue is released because both loops have stopped
• Error-induced shutdown sequence: Producer Loop
  • Guard clause detects error and enqueues the “Shutdown” state
  • Remaining sequence is the same as normal shutdown sequence
  • “Simple Error Handler” function reports the detected error
• Error-induced shutdown sequence: Consumer Loop
  • Guard clause detects error and selects the “Shutdown” state
  • Remaining sequence is the same as normal shutdown sequence
  • “Simple Error Handler” function reports the detected error
• Tips to modify the controller
  • Create a new button
  • Create associated event structure subdiagram
  • Create associated case structure subdiagram
  • Locate the Invoke Node and Property Node
  • Create a custom error message with Error Ring

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 27 describes the "Event-driven Producer-Consumer State Machine" 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)
   This comprehensive textbook by Peter A. Blume lays out everything you need to know about best practices for LabVIEW system design.
   

Related content

Tags:

• producer-consumer
• state machine
• queue
• application example
• system controller
• embedded control and monitoring
• ECM
• human-machine interface
• HMI

Targets:

• PC

Communications:

• message
• inter-process
• queue

LabVIEW VIs and block diagram elements:

• Event Structure
• Obtain Queue
• Release Queue
• Enqueue Element
• Enqueue Element at Opposite End
• Dequeue Element
• Invoke Node
• Property Node
• Format Into String
• Create Network Stream Writer Endpoint
• Write Single Element to Stream
• Flush Stream
• Destroy Stream Endpoint
• Error Ring