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Using Synchronous Display for LabVIEW Controls and Indicators



Primary Software: LabVIEW Toolkits>>Test Executive
Primary Software Version: 5.1
Primary Software Fixed Version: N/A
Secondary Software: N/A

Problem:
How does the synchronous display option work for LabVIEW controls and indicators? Does it improve the execution efficiency of LabVIEW?

Solution:
The synchronous display option is related to multi-threading.  Since the user interface (UI) has been separated from the execution of the block diagram, it is no longer mandatory that every data value gets displayed on the front panel. When data is sent to a terminal on the block diagram, an event is put on the user interface queue to update that indicator when the UI thread runs. Since the block diagram keeps executing throughout this, it may generate a different data value for that same indicator, before the first value change has been updated on the block diagram.

If synchronous display is enabled, the block diagram will not overwrite the data for an indicator with a new value before the first has been displayed. The diagram will stop executing until the User Interface has updated the original value.

With synchronous display disabled (the default) not every data point will be displayed. Old data may be overwritten by new data while it is in the User Interface Queue, waiting to run. Therefore, we see an increase in execution speed with synchronous display turned off.  

For high speed charts, it might not be critical to see every data point, if they are updating faster than the eye can see. Turning synchronous display on is not the same as turning multi-threading off, but it does more closely simulate the way updates were made to the user interface in LabVIEW 4.x and previous versions.
 
In certain Data Acquisition (DAQ) applications under Windows NT, it is necessary to turn synchronous display on in order to see any reasonable response to mouse events, such as clicking the stop button. It seems that the UI thread is not getting scheduled correctly, so we can force it to run by setting synchronous display for some indicator.

Related Links:
LabVIEW Help:  VI Execution Speed
LabVIEW Help:  Configuring Controls and Indicators
Knowledge Base DIRECT-3S8R4F: Why Doesn't My Front Panel Draw all My Data?

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Report Date: 03/25/1998
Last Updated: 11/04/2013
Document ID: 17OC8QWV

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