Chapter 4
Human Machine Interface
©
National Instruments Corporation
4-21
period is exceeded, whichever occurs first, then reads the database and
returns the current tag information. By default, timeout is 0 seconds.
If you wire nothing into the timeout input of your diagram, the VI reads the
database and returns immediately. How you use the timeout input depends
on whether you want to implement event-driven or polled programming
techniques in your HMI.
All VIs that read information form the BridgeVIEW database have a
changed? output that is TRUE if the returned information is new or
updated. If the VI returns and changed? is FALSE, the VI might have timed
out, or the information in the database did not change since the last time you
read it. You can use this output to make your program more efficient by
using a case statement to update the user interface only if the information
has changed.
Some of the more advanced Tags VIs and Alarms and Events VIs also
return an initialize headers or config changed output that tells your
program whether your HMI object needs to be initialized with new
information. In most cases, this corresponds to the first time the VI is
called, and you only need to update that part of your user interface once.
For more information about the Tags VI Library, refer to Appendix A,
.
Activity 4-3. Read a Tag
Your objective is to monitor a single tag value using the Read Tag VI.
In this activity, the Read Tag VI returns when a new value for the tag is
acquired from the Tanks Server, and updated in the RTDB, or a timeout
of 1 second is exceeded, whichever occurs first. This loop continues
executing until the Engine shuts down. You will use
mytanks.scf
in
the
BridgeVIEW\Activity
directory, which you edited in Activity 3-1,
Configure a Tag, and View the Tag Configuration Parameters and Tag
Values
.
Front Panel
1.
Open a new VI (File»New) and place a tank on the front panel
(Vessels»Tanks). Label the tank
Product
. Edit the tank scale to range
from 0 to 1000.
