Programming Designs

TOPICS

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TOPICS

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Programming Designs

A. Event-Driven Programming

B. Sequential Programming

C. State Machine Design Pattern

D. Multiple Loop Design Patterns

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A. Event-Driven Programming

Events and Event Structure

Parts of an Event Structure

Configuring the Event Structure

Recommendations

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Events

Events originate from the user interface, external

I/O, or other parts of the program.

Examples of events:

changing value of a control, pressing a key, clicking the
mouse, resizing the window, closing the window.

Event

—An asynchronous notification that

something has occurred

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Using Event Structures for Event-Driven
Programming

Event Structure

— LabVIEW’s programmatic tool for

handling events.
By using it, execution of code can be made dependent
on whether or not an event has occurred.
Use an Event structure
to handle user-interface
events such as:

• Pressing a button

on the mouse.

• Pressing a key on

the keyboard.

• Changing the value

of a numeric control.

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Parts of an Event Structure

•

Event Selector Label

—Identifies the event case viewed.

•

Timeout

—Specifies time in ms to wait for events.

Default value is

–

1 (indefinite).

Timeou
t

Event Selector Label

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Parts of an Event Structure
(continued)

• Event Data Node—Identifies the data LabVIEW provides

when the event occurs; similar to Unbundle By Name
function.

• Event Filter Node—Identifies the subset of data available in

the Event Data node that the event case can modify.

Event

Data

Node

Event
Filter
Node

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Using an Event Structure

•

In general, place Event structures inside While
Loops.

•

Event structures
handle exactly
one event per

iteration of

the While Loop.

•

Event structures
sleep when no
events occur.

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Configuring the Event Structure

Use a dialog box to configure
each event by right-clicking the
Event structure border and
selecting Edit Events
Handled by This Case from
the shortcut menu.

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Edit Events Dialog Box

Events

Event
Sources

Configured
Events

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Notify and Filter Events

Notify Events (

green

arrow)

• User action has already occurred

and LabVIEW has processed the
event.

Filter Events (

red

arrow)

• User action has already occurred

and LabVIEW has NOT
processed the event.

• Filter events allow you to override

default behavior for event.

DEMONSTRATION

Configure and Use Events

Demonstrate configuring and using an Event
structure.

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Recommendations

•If no timeout is configured and an event never
occurs

– event loop is stuck waiting indefinitely.

Therefore, wire the timeout terminal with a value
other than -1 (at least while developing).

•“Heavy” code within the Event Structure causes
the front panel to lock up, often confusing the user
– try to keep code in an event case to a minimum.

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Recommendations (Continued)

•If your event-driven application has a STOP
button, add a Value change event for the button

–

otherwise stopping the VI might be delayed.

•Event structure may be used in a state machine
design pattern. One state is waiting for events and
depending on which of them occurs, the next state
to execute is chosen.

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B. Sequential Programming

Using Sequential Programming

Flow-Through Parameters

Sequence Structures

Flat Sequence Structure

Stacked Sequence Structure

Error Case Structures

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Using Sequential Programming

•

Many of the VIs you write accomplish sequential tasks.

•

By default, LabVIEW does not force sequential
programming.

•

Example: Nothing forces the execution order of these tasks.
Any one of these tasks could happen first

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Flow-Through Parameters Force
Execution Order

Use error clusters and refnums to force order of
execution.

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Sequence Structures Force
Execution Order

•

Sequence structures are a structure with frames,
where each frame executes in order.

•

The second frame cannot begin execution until
everything in the first frame completes execution.

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Flat Sequence Structure



Executes each frame beginning with the left-most
frame and ending with the right-most frame.



The previous frame must complete before the next
frame executes.



Data can be passed out of or between frames using
tunnels.



Once the sequence
begins, it cannot be
stopped.

1st

2nd

3rd

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Stacked Sequence Structure



Stacks each frame so you see only one frame at a time.



Executes frame 0, then frame 1, etc. until the last frame
executes.



Returns data only after the last frame executes.



To transfer data from frame to frame, a Sequence Local

must be created (right-

click » Add

Sequence Local).



Once the sequence begins,
it cannot be stopped.

Sequence Local

DEMONSTRATION

Flat Sequence and Stacked
Sequence Structures

• passing data between frames
• tunnel outputs
• shifting between Flat and Stacked Sequence

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Avoid Overuse of Sequence
Structures

… because you cannot stop the execution in the
middle of a sequence.

Single frame sequence structures are better than
multi-sequence frames.

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Better to Use Error Case Structures

Instead, write this VI to enclose the dialog boxes in
Case structures, wiring the error cluster to the case
selectors.

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C. State Machine Design
Pattern

State Programming

State Transition Diagrams

State Machines

Infrastructure

Transitions

Event-Based State Machine

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State Programming

State programming helps you solve the following
issues that sequential programming or flow-
through parameters do not:

•

What if you need to change the order of the sequence?

•

What if you need to repeat one item in the sequence more
often than other items?

•

What if some items in the sequence execute only when
certain conditions are met?

•

What if you need to stop the program immediately, rather
than waiting until the end of the sequence?

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State Transition Diagram

A state transition diagram is a type of flowchart
that indicates the states of a program and
transitions between states.

Transition

— Condition, action, or event that

causes the program to move to the next state

State

— Part of a program that satisfies

a condition, performs an action or waits for
an event

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State Transition Diagram Example:
Coffee Vending Machine

initialize:

money=0

€

wait for

event

money=money+coin

dispense money;

money=0

€

dispense coffee;

money=money

–0.50€

coin

inserted

money

≥0.50€

money<0.50

€

money>0

€

money=0

€

cancel button

pressed

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State Machines

•

The state machine design pattern implements
a state diagram or flow chart.

•

It can programmatically determine which state to
execute next.

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State Machines

– Infrastructure

•

State machine is a set of states and transitions between them.

•

Each state can lead to one or multiple states or end the process
flow.

•

State Functionality Code = which action would you like to
perform in this state?

•

Transition Code = which state is next?

While Loop

Case Structure

Shift Register

Type-Defined

Enum

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State Machines

– Transition Code

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Event-Based State Machine

•

Combines event programming with a State
Machine design.

•

Includes a “Wait on Event” case to processes user-
interface events.

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D. Multiple Loop Design
Patterns

Parallelism

Master/Slave Design Pattern

Notifiers

Producer/Consumer Design Pattern

Queues

Notifiers vs. Queues Summary

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Parallelism

• Some tasks require multiple actions to be run in

parallel on several processor cores.

• LabVIEW has built-in methods for communicating

between parallel loops that execute these actions.

Parallelism

—executing multiple

tasks at the same time

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Multiple Loop Design Pattern

Parallel Loops

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Multiple Loop Design Pattern

Producer/Consumer

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Master/Slave Framework

Master/Slave: a type of Producer/Consumer

• Master loop
tells the slave
loop(s) when
to execute.

• Notifiers
allow loops to
run at different
rates.

DEMONSTRATION

Notifiers in Action

Build a simple VI following the Master/Slave
design pattern.

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Notifiers

Pros:

• Both loops are synchronized to the master loop—the slave loop

only executes when the master loop sends a notification.

• You can use notifiers to create globally available data, since it is

possible to send data with a notification.

• Using notifiers creates efficient code—there is no need to poll to

determine when data is available from the master loop.

Cons:

• Notifiers do not buffer data.
• If the master loop sends another piece of data before the first piece

of data has been read by the slave loops, that data is overwritten
and lost.

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Queues

• Queues are similar to notifiers, except that a queue can

store multiple pieces of data.

• By default, queues work in a FIFO (first in, first out)

manner.

• Use a queue

when you want
to process all data
placed in the queue.

• Use a notifier if you only want to process current data.
• Use the Queue Operations functions to create a queue

for communicating data between sections of a block
diagram or between VIs.

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Producer/Consumer Framework (Data)

Data type here
determines the
data type that
can be placed
in the queue

DEMONSTRATION

Queues in Action

Build a simple VI following the Producer/Consumer
design pattern.

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Producer/Consumer Framework
(Events)

• Efficiently responds
asynchronously to the
user interface

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Notifiers vs. Queues

– Summary

Tool

Function

Use when

…

Notifier

–

sends alert to helps
control timing of parallel
loops

–

can transfer data
between loops

–

data NOT buffered
(lossy)

–

you have parallel loops that
are running at different
rates and 1+ loop is
dependent on another

–

you want to transfer data
from one loop to another

Queue

–

helps control timing of
parallel loops

–

transfers data between
loops

–

buffers data to be
transferred (FIFO)

–

you have parallel loops that
are running at different
rates and 1+ loop is
dependent on another

–

you want to transfer ALL
data from one loop to
another

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Create a VI using events. Its front panel should
contain:

• Numeric

Control (I32)

• Numeric

Indicator (I32)

• 2 x String

Indicator

• Stop Button

Homework: Events

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One string indicator should display the coordinates
of each mouse click.

You could use two numeric indicators as well,
but I recommend playing with string functions (Concatenate,
Number to String) for practice.

Homework: Events

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When you enter a value in the numeric control, the
remaining indicators should display the difference between
old and new value (positive, negative or 0) and tell if the
number has grown, decreased or remained unchanged.

Homework: Events