Chapter 4
Human Machine Interface
4-24
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National Instruments Corporation
9.
Save the VI as
Monitor Product.vi
in the
BridgeVIEW\
Activity
directory.
10. Run the VI. The Engine launches, unless it is running already. The tank
level changes to reflect the changing values of the Product tag. When
the value goes over 800, the
in alarm
Boolean changes from OFF to
ON, indicating an alarm condition.
End of Activity 4-3.
How Do the Tags, and Alarms and Events VIs
Affect Startup/Shutdown?
When a user-defined VI runs and executes one of the Tags VIs or Alarms
and Events VIs, that VI checks the status of the BridgeVIEW Engine. If it
is not running, executing the VI automatically starts execution of the
BridgeVIEW Engine. The BridgeVIEW Engine loads and executes all
required device servers. When the VI returns, the Engine is running.
The BridgeVIEW Engine continues to run until you shut it down either
programmatically or through the Engine Manager. As the Engine shuts
down, first it checks to see if any application is running that requires its
services. If so, it waits until that application halts before shutting down.
Once it shuts down, it sends a shutdown message to the device servers.
You can monitor the status of the BridgeVIEW Engine with the shutdown
output of any of the Tags VIs, Alarms and Events VIs, or the Get Engine
Status VI.
Note
If you write applications that do not use the BridgeVIEW VIs that access the
Real-Time Database such as the VIs that retrieve historical data, those
applications can run without the BridgeVIEW Engine running. They do not
launch the BridgeVIEW Engine.
