B&B Electronics VFG3000 - Manual - Manual (Page 162)

R ETURNING V ALUES V LINX F IELDBUS G ATEWAY M ANAGER U SER M ANUAL
P AGE 146
R
ETURNING
V
ALUES
As mentioned above, programs can return values. Such programs can be invoked by other
programs or by expressions anywhere in the database. For example, if you want to perform a
particularly complex decode on a number of conditions relating to a motor and return a value
to indicate the current state, you could create a program that returns an integer like this…
if( MotorRunning )
return 1;
else {
if( MotorTooHot )
return 2;
if( MotorTooCold )
return 3;
return 0;
}
You could then configure a multi-state formula to invoke this program, and use that tag’s
format tab to define the names of the various states. The invocation would be performed by
setting the tag’s Value property to
Name() , where Name is the name of the program in
question. The parentheses are used to indicate a function call, and cannot be omitted.
H
ERE BE
D
RAGONS
!
Note that you have to exercise a degree of caution when using programs to return values. In
particular, you should avoid looping for long periods of time, or performing actions that make
no sense in the context in which the function will be invoked. For example, if the code
fragment above called the
GotoPage function to change the page, the display would change
every time the program was invoked. Imagine what would happen if you, say, tried to log
data from the associated tag, and you’ll realize that this would not be a good thing! Therefore,
keep programs that return values simple, and always consider the context in which they will
be run. If in doubt, avoid doing anything other than simple math and
if statements.
P
ASSING
A
RGUMENTS
As also mentioned above, program can accept arguments. As an example, suppose you want
to write a program called
FindMean to take the average of two values. The program could be
configured to accept two integer arguments, a and b , as shown in the example given when
defining the purpose of the Arguments property. The program would also be configured so as
to return a integer value. The code within the program would then be defined as…
return (a+b)/2;
Once this program has been created and translated, you will be able to enter an expression
such as
FindMean(Tag1, Tag2) to invoke it with the appropriate arguments. In this case, the
expression will be equal to the average of Tag1 and Tag2 .