User-Interface Design

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User-Interface Design Real-time Systems Design

• Can the user easily do what he wants to do?

An unresponsive real-time system may be worse
than no system at all
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Lecture Objectives

• To understand the important aspects of user
  interface design
• To discuss the guidelines for producing better
  user interface
• To understand the major issues in designing
  real-time systems
• To illustrate the design considerations for
  real-time systems

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User Interface

• The interface is the system for most users
• Communication between the user and the system
• When interface is not easy, user may reject
  system
• Important to have more user-friendly interface to
  improve usability
• User training may help improve users
  understanding and use of system

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Types of Interface

• Natural Language Interface

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Types of Interface

• Question and Answer Interface

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Types of Interface
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Types of Interface

• Menus
• Input/ Output Forms
• Command-Language Interface

User interface has two main components
Presentation language Action language
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Human Factors

• It is important to understand the human factors
• Better understanding of skills, behaviour and
  limitations can produce better and friendlier
  interface
• The factors
• Human Perception
• Human Skills
• Type of Task

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Human Perception

• Visual - size, shape, color, orientation,
  movement, etc
• When we look at an item, we are receiving info
  from all parts
• The brain can efficiently process 7 chunks
  (items) at a time
• So when designing a screen, these factors should
  be considered

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Human Skills

• Understand the target user and the skill level of
  the user
• Education level, e.g. clerk, manager, engineer or
  programmer
• Computer skills - novice, average, expert
• For unskilled users, user interface have to be
  more robust and more help should be available

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Types of Tasks

• Communication tasks - information transferred
  from producer to consumer
• Dialogue tasks - user direct and control
  interaction with the system
• Cognitive tasks - performed once information is
  obtained associated with system function
• Control tasks - allow user to control info and
  cognition and order the process through which
  other generic tasks occur

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General Interaction Guidelines

• Be consistent
• Offer meaningful feedback
• Ask for verification of any non-trivial
  destructive action
• Permit easy reversal of most actions
• Reduce the amount of information to memorise
  between actions
• Seek efficiency in dialog, motion and thought

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General Interaction Guidelines (Continued)

• Forgive mistakes
• Categorize activities by function and organise
  screen geography accordingly
• Provide help facilities that are context
  sensitive
• Use simple action verbs or short verb phrases to
  name commands

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Information Display Guidelines

• Display only information that is relevant to the
  current context
• Dont bury the user with data, use a presentation
  format that enables rapid assimilation of
  information
• Use consistent labels, standard abbreviations,
  and predictable colours
• Allow user to maintain visual context
• Produce meaningful error messages

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Information Display Guidelines (Continued)

• Use upper and lower case, indentation, and text
  grouping to aid in understanding
• Use windows to compartmentalize different types
  of information
• Use analog displays to represent information
  that is more easily assimilated with this form of
  representation
• Consider available geography of the display
  screen and use it efficiently

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Data Input Guidelines

• Minimize the number of input actions required of
  the user
• Maintain consistency between information display
  and data input
• Allow user to customize input
• Interaction should be flexible but also tuned to
  the users preferred mode of input

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Data Input Guidelines (Continued)

• Deactivate commands that are inappropriate in the
  context of current actions
• Let user control the interactive flow
• Provide help to assist with all input actions
• Elliminate mickey mouse input

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Real-time Systems

• Systems that are integrated with real-world
  devices
• Generate action in response to external events
• High speed data acquisition and control under
  severe time and reliability constraints
• System performance is most important

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Real-time Systems Interfaces
Sensors
Control device
environmental data
control instructions
Real-time System
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Examples of Real-time Systems

• Aircraft avionics
• Manufacturing process control
• Industrial instrumentation
• Intelligent buildings
• Anti-lock Braking System (ABS)

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Differences in Real-time System Development

• Design of real-time system is resource
  constrained
• Real-time systems are compact, yet complex
• Real-time systems often work without the presence
  of a human user

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Demands of Real-time System

• Information flow that is gathered or produced on
  a time-continuous basis
• Control information passed throughout the system
  and associated control processing
• Multiple instances of same transformation,
  sometimes encountered in multitasking situations
• System states and the mechanism that cause
  transitions between states

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Time-continuous Data Flow
Monitored temperature
Input continuously
Output continuously
Monitor and adjust temperature level
Corrected value
Temperature set-point
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Extensions to Data Flow Notation
Data object that is input or output from a
process on continuous basis
quasi-continuous data flow
Control process
Transformer of control or events accepts
control and input and produces control as output
Control item or event takes on a Boolean or
discrete value
control item
Repository of control items that are to be stored
for use by one or more processes
control store
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Example of Data and Control Flow
Status of each fixture
Movement alarm
Robot initiation control
parts status buffer
Start/stop flag
Bit string
Monitor fixture operator interface
Process activate
Operator settings
Position commands
Process robot commands
Robot movement record
Operator commands
robot command file
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Dynamic Attributes

• Interrupt handling and context switching
• Response time
• Data transfer rate and throughput
• Resource allocation and priority handling
• Task synchronization and intertask communication

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Interrupt Handling and Context Switching

• Interrupts - external stimuli
• System must respond to interrupts
• Usually multiple interrupts - need to prioritize
  and service most important ones
• Many real-time operating systems make dynamic
  calculations to monitor servicing of interrupts,
  within given time constraints
• Context switching - switch among tasks
• Interrupt latency - time lag before interrupt

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Interrupt Handling Routine

• Save state of interrupted program
• Determine nature of the interrupt
• Service interrupt
• Restore state of interrupted program
• Return to interrupted program

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Response Time

• Time within which a system must detect an
  internal or external event and respond with an
  action
• Usually event detection and response generation
  are simple
• Determining appropriate response may involve
  complex, time-consuming algorithms
• Context switching and interrupt latency affects
  response time

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Data Transfer Rate and Throughput

• Data Transfer Rate - how fast data move into or
  out of the system
• Throughput - rate of output compared to rate of
  input (system efficiency)
• Timing and capacity is usually given by vendors
• However, overall system performance need to
  consider other factors e.g. device performance,
  buffer size, I/O device, etc.

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Resource Allocation and Priority Handling

• Distributed databases are preferred in real-time
  systems because of multitasking
• Data often processed in parallel
• Concurrency control required - locking and time
  stamping to ensure data integrity
• Real-time operating systems also manage resources
  such as memory and processors

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Task Synchronisation and Communication

• Real-time multitasking systems must have
  mechanism to synchronize and communicate between
  tasks
• Queuing semaphores - synchronization and
  signaling items that manage traffic
• Mailboxes and Messaging systems - alternative to
  sending information between tasks

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Conclusion

• Real-time system emphasized on performance and
  reliability
• Fault tolerance and system restart is most
  critical
• Often redundancy and backup is built into system
  design
• Design requires additional consideration for
  size, efficiency and reliability

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References

• Software Engineering A Practitioners Approach
  5th Ed. by Roger S. Pressman, Mc-Graw-Hill, 2001
• Software Engineering by Ian Sommerville,
  Addison-Wesley, 2001