LabVIEW Skills Series (I) - Event Structure

    The event structure is the essence of the LV programming structure. It is similar to the interrupt function of a single-chip microcomputer, and its advantage is that it does not occupy CPU resources. For example, when a key is pressed, the serial port sends data, and keyboard shortcuts are set, etc., the event structure saves a lot of system resources compared to the query structure. Below I will introduce the key points of using the event structure. For the basics, please read the help yourself.

1. Regarding the timeout connection of the event structure: The event structure has an input for the timeout event. When it is not connected, it means that it will never time out. You should pay attention to this because sometimes your event will be placed in a while loop with other structures, such as the following figure:

Our ideal situation is that other service programs run normally in a loop (the small light flashes), and when the mouse is pressed, the event structure is processed. However, if it is written like this, since the event structure never times out, the while loop will only run once, and the second loop will only start when the mouse button is pressed. The result of this program is that the small light changes once the mouse is pressed.

    There are two ways to solve this problem: one is to add a delay time to the delay input of the event structure, such as 5ms, so that the loop will run once every 5ms and will not affect the normal operation of other service programs. The other is to put other service programs in another while loop and the event structure in a separate loop. I personally recommend the second method.

 

2. Regarding the soft trigger of the event structure: There is a button on the front panel. It is very simple and commonly used to trigger after pressing it. However, sometimes it involves the issue of soft triggering, which is to use the program (rather than the controls on the front panel) to trigger an event. Let's take a look at the following example:

The "service program" software changes the value of the "soft trigger". Our ideal goal is that each time the service program value is changed to true, the small sign light will change once. However, the actual program running result is that no matter how many times it is pressed, the small sign light does not change. This is because direct assignment can only change the value of the variable, but cannot cause the event trigger of the secondary variable.

 

The solution is: to trigger an event by software, you must right-click the variable -> Create -> Property Node -> Value (Signal) and assign a value to it, so that the event structure can be triggered. For example, the figure below.

(When setting the event structure, uncheck the lock front panel.) Program running results: When the "service program" pops up, the small light does not change, but the event will be triggered all the time. You can see it in highlight mode. If you assign F to the "soft trigger" twice in a row, the event will also be triggered. In other words, as long as the value is assigned, the event will be triggered, not when the value changes. So a conditional structure is added to the event structure. When the "service program" is pressed, the "soft trigger" is continuously assigned T, the event structure keeps running, and the small light flashes.

 

Event structure is the essence of LV