National Instruments BridgeVIEW 用户手册
Chapter 11
Loops and Charts
© National Instruments Corporation
11-15
Wait Until Next ms Multiple function (Functions»Time & Dialog)
—This function ensures that each iteration of the loop occurs no faster
than the millisecond input. The input is 500 milliseconds for this
activity. If you pop up on the icon and choose Show»Label, the label
Wait Until Next ms Multiple appears.
activity. If you pop up on the icon and choose Show»Label, the label
Wait Until Next ms Multiple appears.
6.
Pop up on the input of the Wait Until Next ms Multiple function and
select Create Constant. A numeric constant appears and is
automatically wired to the function.
select Create Constant. A numeric constant appears and is
automatically wired to the function.
7.
Type
500
in the label. The numeric constant wired to the Wait Until
Next ms Multiple function specifies a wait of 500 milliseconds
(one half-second). Thus, the loop executes once every half-second.
(one half-second). Thus, the loop executes once every half-second.
Notice that the VI initializes the shift registers with a random number.
If you do not initialize the shift register terminal, it contains the default
value or the last value from the previous run and the first few averages
are meaningless.
If you do not initialize the shift register terminal, it contains the default
value or the last value from the previous run and the first few averages
are meaningless.
8.
Run the VI and observe the operation.
9.
Save this VI as
Random Average.vi
in the
BridgeVIEW\Activity
directory.
Note
Remember to initialize shift registers to avoid incorporating old or default data
into your current data measurements
into your current data measurements
End of Activity 11-5.
Using Uninitialized Shift Registers
You initialize a shift register by wiring a value from outside a While Loop
or For Loop to the left terminal of the shift register. Sometimes, however,
you want to execute a VI repeatedly with a loop and a shift register, so that
each time the VI executes, the initial output of the shift register is the last
value from the previous execution. To do that, you must leave the left shift
register terminal unwired from outside the loop. Leaving the input to the
left shift register terminal unwired preserves state information between
subsequent executions of a VI.
or For Loop to the left terminal of the shift register. Sometimes, however,
you want to execute a VI repeatedly with a loop and a shift register, so that
each time the VI executes, the initial output of the shift register is the last
value from the previous execution. To do that, you must leave the left shift
register terminal unwired from outside the loop. Leaving the input to the
left shift register terminal unwired preserves state information between
subsequent executions of a VI.
The following illustration shows an example of a subVI that calculates the
running average of four data points. The VI uses an uninitialized shift
register (with three additional elements) to store previous data points.
running average of four data points. The VI uses an uninitialized shift
register (with three additional elements) to store previous data points.