The previous chapter covered many of LabVIEWs basic functions. The functions
give a programmer the ability to produce a wide range of applications in a relatively
short time. While the previously discussed functions provide enough of a basis to
build an application, there are a number of LabVIEW features that can make an
application more flexible and easier to use, and can give your application a professional
appearance. Some of these features will be discussed in this chapter.
2.1 GLOBAL AND LOCAL VARIABLES
Global variables are used when a data value needs to be manipulated in several VIs.
The advantage of using a global variable is that you only have to define that data
type once. It can then be read from or written to in multiple VIs. The use of global
variables is considered poor programming practice; they hide the data flow of your
application and create more overhead. National Instruments suggests that you structure
your application to transfer data using a different approach when possible.
However, there are instances when global variables are necessary and are the best
approach for an application. One example would be updating a display from data
being generated in a subVI. The application could have two While loops running in
parallel. Data could be generated in a subVI in the top loop while the bottom loop
reads the data from the global and writes the information to the user interface. There
is no other method for obtaining data from a subVI while it is still running.
The global variable must be created and its data types defined before it can be
used. To create a global, first drag the icon from the Structures palette and drop it
onto a block diagram. Figure 2.1 shows the global as it appears on the diagram. The
question mark and black border indicate that it cannot be used programmatically.
The global has a front panel to which you can add controls, identical to a VI. Globals
do not have a block diagram associated with them. To open the front panel of the
global variable, simply double-click on the icon. The front panel of the global is
shown in the bottom window of Figure 2.1.
Two controls have been created on the global front panel. A global variable can
contain multiple controls on the front panel. Try to logically group related controls
and tie them to a single global variable. Once the data types have been defined, save
the global as a regular VI. The global can then be accessed in any VI by using the
same method you normally follow to place a subVI on the code diagram. If you
have more than one control associated with the global variable, pop up on the icon
once you have dropped it onto a block diagram and use the Select Item submenu to
select the appropriate one.