1. EE 200 Design Tools Laboratory 14

2. Laboratory 14 Topics LabVIEW Programming
Properties of Front Panel Objects
Implicitly and Explicitly Linked Property Nodes
Control References
Event programming and the Val(Sgnl) property

3. Properties of Front Panel Objects
Front panel objects have properties
Properties are attributes that describe the object
Numerous properties describe appearance:
Color
Visibility
Position
Numeric display format
Value
Property nodes alter the properties of objects

4. Property Nodes
Property nodes allow you to programmatically set the properties of front panel objects
Property nodes can be configured to read or write a given property
Property nodes can be explicitly or implicitly linked to front panel objects
Implicitly linked property nodes are tied to a specific front panel object
Explicitly linked property nodes must be linked to front panel objects using control references

5. Implicitly Linked Property Nodes
Right click front a panel object and select Create >> Property Node
In the above example, the implicitly linked property node allows us to read the value of the control

6. Property Node Attributes
Right click on the property node
Click on Change to Write
In the above example, the implicitly linked property node allows us to write a value to the control

7. Exercise 1 Open Exercise_1.vi Use implicitly linked property nodes to
Set LED Control/Indicator visible or invisible
Set LED Control status: enabled, disabled, or disabled and grayed out
Set the LED Indicator to blink, or not to blink
Change the LED Indicator color

8. Exercise 1

9. Exercise 1
Set the LED Color using a Color Box Control

10. Exercise 1 Set the default colors for the LED
Left-click the middle of the color constant box to open the color palette; use the values shown

11. Property Node Execution Order
The property node does not execute if the status of the input error cluster is true
The node executes from top to bottom
By default, if an error occurs on a terminal, the node stops at that terminal, returns an error, and does not execute any further terminals
To override this behavior, right-click and select Ignore Errors Inside Node

12. Exercise 2 Open Exercise_2.vi
The default values for both indicators are zero
After the VI executes, what do the indicators display?
Verify by executing Exercise_2.vi
Why does Numeric Indicator 1 display 2 rather than 3?

13. Control Reference
A control reference (refnum) in LabVIEW is analogous to a pointer in C
A control reference, or pointer, represents an address
A control reference to a front panel object represents an address to the front panel control

14. Creating a Control Reference
Right click the front panel object
Select Create >> Reference

15. Explicitly Linked Property Nodes
Programming >> Application Control

16. Explicitly Linked Property Nodes
Link the property node to the front panel object using a control reference (refnum)
Right click the property node to set its attributes
refnum

17. Exercise 3
Modify the Exercise_1.vi so that is uses explicitly linked property nodes

18. Exercise 4 Open Exercise_4.vi
Use an explicitly linked reference node and an array of control references to set the min and max range of the three control knobs

19. Exercise 4

20. Boolean Control Property Node
When a Boolean control is set to a latching mechanical action, after you change the value of the Boolean control, it will change back to the default once the VI reads its value
Depending on when the value of the Boolean control is read by the VI or by the property node, you may miss a change in the value
For this reason, use a switched mechanical action if you wish to change a Boolean control using a property node

21. Event Programming Events are notification that something has occurred
Events originate from the user interface, external I/O, or other parts of the program
Clicking the mouse generates a mouse event
Event-Driven Programming is a method of programming where the program waits on an event to occur before executing one or more functions

22. Types of Events
Static events are only for interactions with the front pane
A static, notify event would be a “Value Change” for an OK button
Dynamic events are for interactions with any part of your block diagram
A rising edge on a signal connected to DAQ device

23. The Event Structure Examples of User Interface (Static) Events
Pressing a button on the mouse
Pressing a key on the keyboard
Changing the value of a numeric control

24. Parts of an Event Structure
Event Selector Label—Identifies event case viewed
Timeout—Value in ms to wait for events; default value is –1 (indefinite)

25. Parts of an Event Structure
Event Data Node—Identifies the data LabVIEW provides when the event occurs; similar to the Unbundle By Name function
Event Filter Node—Identifies the subset of data available in the Event Data node that the event case can modify

26. Using an Event Structure
Normally contained within a while loop
Event structure handles exactly one event per iteration
Sleeps when no events occur
Minimize the code appearing in the event structure

27. Exercise 5
Open Exercise_5.vi

28. Exercise 5
The Stop button uses latched mechanical action while the Refresh button uses switched mechanical action
The Loop Iteration indicator shows the number of times the Refresh button is read
The code polls the Refresh and Stop buttons every 10 ms
Polling is not efficient
While the processor is looping and reading controls, it is not executing other code

29. Exercise 5
Avoid polling the front panel controls by using the event structure on slide 30
Add events as shown on slide 31
The Stop Boolean control uses latched mechanical action and must be located inside the event case to work correctly
Verify that the Loop Iteration indicator increments only when the Refresh button is pressed

30. Exercise 5

31. Exercise 5

32. Val(Sgnl) Property Node
Suppose we change the value of the Refresh button in Exercise 5 using a property node
The properties Value and Val(sgnl) both change the value of the front panel object, but only the Val(sgnl) property generates a Valued Changed Event within an event structure

33. Exercise 6 Save Exercise_5.vi as Exercise_6.vi
Eliminate the Wait Until Next ms Multiple node
Not needed when the loop contains an event structure
Add a Boolean control Trigger Refresh that uses switched when released mechanical action
Add an event case that executes when the value of the Trigger Refresh control changes value
Within the new event case
Use a Value property node to set the Refresh control to true
Use a Value property node to set the Trigger Refresh control to false

34. Exercise 6

35. Exercise 6 Start the VI Press the Refresh Trigger control
Does the color of the Refresh Button change?
Does the One Button Dialog box appear?
Does the Loop Iterations indicator increment?
Use a Val(Sgnl) property node to set the Refresh control to true
Does pressing the Refresh Trigger control button now cause the One Button Dialog box to appear?

36. Exercise 6