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Lesson 3Local and Global Variables

You Will Learn:

A. An overview on data managementB. How to use local variablesC. How to use global variablesD. Advice about using local and global

variablesE. How to use DataSocket

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Data Management Basics

Dataflow is the most efficient way to transfer data Nodes execute when data is available to all inputs Nodes supply data to all outputs when finished executing Data passes immediately from source to destination

Most applications use wires to transfer data Cannot use wires to transfer data between parallel

tasks

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Creating Local Variables

Access panel objects from several locations onthe diagram

Two Ways to create alocal variable:

Right-click on anobjects terminal andselect Create >> LocalVariable

Select a local variable

from the Structurespalette

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Creating Local Variables

Right-click on the local variable node andchose Select Item to select the desired object

Owned label becomes variable name Select whether you want to read or write to the

local

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Local Variable Example

Consider an application where you need to stoptwo data-independent (or parallel ) While Loops atthe same time

Each While Loop plots a sine wave on a chart

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Solutions to Local Variable Example

Method 1(Incorrect)

Method 2(Incorrect)

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Solution to Local Variable Example

Method 3 (Correct)

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Local Variable Reminders

If a front panel object will have a local variableassociated with it, it must have an owned label

When you write to a local variable, you update

its corresponding front panel object When you read from a local variable, you read

the current value of its corresponding frontpanel object

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Exercise 3-1 on Page 3-10

Students build Login VI

*You will use this VI in the final application

Approximate time to complete: 15 min.

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Exercise 3-2 on Page 3-12 (Optional)

Students finish building Select and Plot Waveform.vi


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Global Variables

Special kind of VI Front panel and front panel objects only No block diagram

Panel objects are storage places to write and read data Used to pass data between VIs that are executing or

between VIs that cannot be connected by a wire Read and write global variables

Write global used to write a value to the global Read global used to read the current value of the global Right-click on the node to toggle between read and write

Write Global Read Global

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Passing Data Between VIs

VI Number 1 VI Number 2

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Creating Global Variables

Create the controls neededas global variables

Each control must have owned label

Right-click usingthe Operating Tool

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Global Variable Reminders

Give each global variable an owned label

Write to (initialize) a global variable beforereading from it

If you do not initialize a global variable, thedefault value for that object is returned

You write to the global variable at a separatelocation from where you read it, usually in adifferent VI

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Students build Data to Global VIand My Globals VI


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Students build Display Global Data VI


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Important Tips about Locals and Globals Make sure to initialize local and global

variables before reading them You must carefully avoid race conditions

Use locals and globals only when necessary Each Read local or global generates a copy of data Accessing globals and locals can increase memory

requirements and reduce execution speed

LabVIEW Code Sequential Code x = x * 5

x = x + 2 OR

x + 2

x = x * 5

No clear datadependency exists, sothe order of executionis not precisely known

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LV Basics II 54

DataSocket

An Internet programming technology basedupon TCP/IP that simplifies data exchangebetween computers and applications

Data is sent over the Internet without the

complexity of low-level TCP programming Data can be sent between VIs on same

computer or over network Data can be sent between LabVIEW and any

other programming language Platform independent

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Components of DataSocketDataSocket API

Converts data into streamof bytes for transfer

Converts data back tooriginal form at

destination LabVIEW DataSocket API

is in the Communication>> DataSocket palette

DataSocket Server Stand-alone application

that programs can use tosend data over the internet

Automatically manages

connections to one or more clients

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DataSocket Functions

DataSocket Writesends any type of datato a specified URL

DataSocket Read

receives the specifiedtype of data from theURL

Uniform ResourceLocator (URL)addresses are used for DataSocket data dstp://

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Automatic Publish and Subscribe Right-click on any panel object and select Data

Operations >> DataSocket Connection to create anautomatic connection

When data is attached to a URL, a small rectangleindicates the DataSocket connection status

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Students build DS Generate Data andDS Read Data VIs

Approximate time to complete: 20-25 min.

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Summary Local variables access front panel objects of the VI in which

they are created When you write or read a local variable, you update or read the

current state of its corresponding panel object Global variables pass data between simultaneously running

VIs Write a value to a global variable before reading from it DO NOT OVERUSE Local and Global Variables!!

DataSocket is an Internet-based method of transferring datathat is platform-independent, programming language-independent, and protocol-independentDataSocket uses URLs to specify the specific data connectionDataSocket consists of two parts the API and the Server Any panel object can connect to the DataSocket Server usingthe Data Operations>>DataSocket Connection window

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Additional Exercises on Page 3-40

3-6 Students modify In Range VITime to complete: 15 min.

3-7 Students add handshaking to the Data toGlobal and the Display Global Data VIs

Time to complete: 30 min.

3-8 Students modify Select and Plot Waveform VITime to complete: 20-25 min.

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