Parallelism

Parallelism •

Parallelism is a way to execute multiple tasks at the same time, such as creating and displaying two sine waves at different frequencies by using two different loops.

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A challenge in programming parallel tasks is passing data among multiple loops without creating a data dependency.

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For example, if you pass the data using a wire, the loops are no longer parallel.

Sharing Data: •

In the multiple sine wave example, if you want to share a single stop button between the loops.

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The Boolean control is a data input to both loops, therefore the Loop Control terminal is read only once, before either While Loop begins executing.

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If False is passed to the loops, the While Loops run indefinitely. Turning off the switch does not stop the VI because the switch is not read during the iteration of either loop. (Incorrect Method)

Sharing Data: • • •

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Move the Loop Control terminal inside Loop 1 so that it is read in each iteration of Loop 1. Although Loop 1 terminates properly, Loop 2 can not execute until it receives all its data inputs. Loop 1 does not pass data out of the loop until the loop stops, so Loop 2 must wait for the final value of the Loop Control, available only after Loop 1 finishes. Therefore, the loops do not execute in parallel. Also, Loop 2 executes for only one iteration (Incorrect Method).

Sharing Data: Solution 1 •

If you could read the stop button from a file, you would no longer have a dataflow dependency between the loops, as each loop can independently access the file.

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However, reading and writing to files can be time consuming, at least in processor time.

Variables: Solution 2 • Variables enable you to perform normal dataflow by passing data from one place to another without connecting the two places with a wire. • Therefore, you can create block diagrams that have simultaneous operations. • To make the two loops run in parallel, remove the wire, pass data between the subVIs using a technique called variable.

Variables • In LabVIEW, variables are block diagram elements that allow you to access or store data in another location. • Variables can be of the following types. – Local: store data in front panel controls and indicator – Global: Global variable and process shared variables store data in special repositories than you can access from multiple VIs – Functional Global: stores data in while loop shift registers.

Variables • Using Variables in a Single VI • Local variables transfer data within a single VI.

Creating Local Variables •

Right-click an existing front panel object or block diagram terminal and select Create>>Local Variable from the shortcut menu to create a local variable.

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A local variable icon for the object appears on the block diagram.

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You also can select a local variable from the Functions palette and place it on the block diagram.

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To associate this local variable with a control or indicator, right-click the local variable node and select >> Select Item from the shortcut menu. The expanded shortcut menu lists all the front panel objects that have owned labels.

Reading and Writing to Variables • After you create a local or global variable, you can read data from a variable or write data to it. • By default, a new variable receives (write) data. • Right-click the variable and select Change To Read from the shortcut menu to configure the variable to behave as a control. • To change the variable to receive data from the block diagram rather than provide data, right-click the variable and select Change To Write from the shortcut menu.

Global Variable •

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You also can use variables to access and pass data among several VIs that run simultaneously. For example, suppose you have two VIs running simultaneously. Each VI contains a While Loop . The first VI contains a Boolean control to terminate both VIs.

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Use a global variable to terminate both loops with a single Boolean control.

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Creating Global Variables •

For every global variable, LabVIEW creates a special global VI, which has a front panel but no block diagram.

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Add controls and indicators to this front panel to define the data types of the global variables it contains. This front panel is a container, which can be accessed by several VIs. 1. Select a global variable from the Functions palette and place it on the block diagram. 2. Double-click the global variable node to display the front panel of the global VI.

Creating Global Variables 3. Place controls and indicators on this front panel the same way you do on a standard front panel. 4. Save it an name.vi and return to the block diagram of the original VI. 5. Select the object in the global VI that you want to access. Right-click the global variable node and select a front panel object from the Select Item shortcut menu. The shortcut menu lists all the front panel objects in the global VI that have owned labels.

Creating Global Variables If you want to use this global variable in other VIs. • Select Select a VI from the Functions palette. • Select the global VI you saved before and put it in the diagram panel. • Right-click the global variable to associate the global variable with the data from another front panel object.

Initializing Variables • Verify that the local and global variables contain known data values before the VI runs. Otherwise, the variables might contain data that cause the VI to behave incorrectly. • If you do not initialize the variable before the VI reads the variable for the first time, the variable contains the default value of the associated front panel object.

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