Peripheral Devices

Peripherals: What is a peripheral device?

A peripheral device is any input, output or backing storage device, e.g. a keyboard, a printer a USB stick.

Peripherals

Input Devices

Input Devices

Input devices allow users to enter text or graphics into a computer system

They are part of the Human Computer Interface, allowing entry of commands

Common input devices include keyboards, mouse ,digital camera, scanner and webcam

An interface connects devices & CPU

Types of Input Devices

Keyboard for text and commands

Pointing devices for Graphical User Interface (GUI) & graphics are; Mouse Track ball Trackpad Touch-screen

Scanner and graphics tabletfor digitising input devices for graphics

Devices for machine readable data OCR

Which Input Device?

Selecting a device for a task means the device type and performance…

Matches User priorities Depends on the type of task Task may be general or specialised

Other user preferences

Matches System requirements Compatible with the type of computer ie desktop, portable ,networked system

Input device characteristics

Devices vary in terms of… Function

Is data entry text or graphics? Speed

How much data is there and how fast must data be input? Accuracy or quality What precision is needed?

Input Performance

Performance depends on the device technology and specification level.

Each technology has its own strengths.

Performance levels range from

low specification and basic features (entry level / lower cost) to

high specification and powerful features (high end / expensive).

Input Devices - Keyboard

A keyboard allows alpha-numeric text data to be entered.

Keyboard helps the user to enter commands (HCI).

How the Keyboard Works

User presses a key on the keyboard.

Keyboard senses the key-press, identifies the character, generates the ASCII bit pattern used to represent the character.

Character is transmitted to the CPU via the keyboard interface.

Keyboard Speed

Speed & accuracy is restricted by operator typing skills and the design limitations of the QWERTY layout.

A good typist can achieve speeds of around 90 words per minute (wpm).

Numeric keypads allow skilled users fast entry of numeric data (commands)

Keyboards on palmtop computers are slower because the keys are smaller.

What are Pointing Devices

Input devices control a screen pointer:

As a key part of the user interface

To allow simple graphics to be entered.

Several pointing technologies exist.

Mouse: for standard desktops

Touchpad: popular on laptops

Trackball: for laptop and desktops

Touch screen: for kiosks and children

Types of Mouse

Standard A roller-ball on the underside of mouse A button on top (at least one)

Optical No moving parts, no cleaning, very accurate Small changes in reflected light generate tracking

signal. No moving parts, no cleaning, very accurate

Wireless (infra red IR) No cord, no tangle, remote operation

Digital Cameras

A digital camera looks like a conventional camera, but the images are stored on a memory card.

Digital Camera Features

Special Effects: Black and white Rotation of image Sepia Redeye flash removal

Zoom (optical and digital): Digital and optical zoom, when making comparisons the optical

zoom factor is more important than the digital zoom. Photo Imaging Software:

Download/preview software Direct Printing:

Straight to printer without using a computer.

Webcams

A webcam consists of a digital camera attached to a computer, these are connected through a USB port

You can compare a webcam with the accuracy, speed of transfer and cost

What is a Scanner

Photographs or artwork can be input.

Scan may be manipulated with image editing software, eg in publishing.

Scanning text in order to recognise the characters requires OCR software.

Scanner types

flat-bed: looks like a photo copier

hand-held scanners are rolled over the document/picture.

Drum scanner: professional standard

Optical Character Reader

An OCR reads printed characters.

Some OCR systems use specialised readers, others use a standard scanner with OCR software.

Some OCR can read specially formed hand-written characters.

OCR Software

Light is reflected off text on a page.

The reflected pattern is compared with patterns stored in computer memory in order to recognise the shape of each character.

Once recognised, the system stores the ASCII code of the character.

Microphone

The purpose of a microphone is to allow sound to be inputted in to the computer.

Naturally sound is analogue and this must be converted into digital form for the computer to make sense of it.

Output Devices

Output Devices

Output devices allow users to observe the results of processing, e.g. what they have typed in.

Common output devices : Monitors Printers Loudspeakers

Monitors

The Monitor is the main output device of the computer A monitor displays data entered into a computer,

and the results produced.

Monitors help the user to enter commands and allow the system to keep the user informed (HCI).

There are two types of monitors Cathode Ray Tube (CRT) Liquid Crystal Display (LCD)

Cathode Ray Tube

CRT monitors are heavy and bulky.

CRT monitors are found on desktop computers and network workstations.

Liquid Crystal Display

An LCD screen display is a flat panel made up of a few layers. There are special crystals in one of the layers. Electric signals can change the crystals from transparent to coloured.

LCD monitors are lightweight, compact and can run on the low power supplied by batteries.

They are ideal for portable computer systems such as laptops and palmtops.

Printers

There are 2 main types of monitor

Laser

Ink Jet

Laser Printer

An laser printer works by using a laser beam to trace the image of the page layout onto a photosensitive drum. This image then attracts toner by means of an electrostatic charge. The toner if fused to the paper by heat and pressure.

Ink Jet Printer

An Ink Jet printer works by spraying a fine jet of ink, which is both heated and under pressure, onto the paper.

Most have a black cartridge.

Comparing Printers

When comparing printers you should take several features into consideration: Accuracy Quality Speed Cost

Printer Performance

Speed pages per minute, time for 1st page out Performance “hit” on user’s computer

Quality Sharpness /detail of text, resolution Colour realism, bit depth

Cost Cost to purchase the printer

Running cost Paper, photo paper, ink, toner, pens

Printer Quality

Printers & monitors make images from dots called pixels, formed in a grid.

A pixel can be black/white or coloured.

Images made from many small pixels close together look sharp and detailed, this is high resolution. There are many dots per inch (dpi) across and up/down.

Images made from fewer but larger pixels spaced apart look chunky, this is called low resolution. The dpi is low.

Written Task

Exercise 5

Backing Storage

What is Backing Storage

Backing storage includes devices such as disc and tape drives.

Such devices are needed to store user data and software.

CPU memory is limited and RAM is erased when the power is off.

Backing store devices offer large amounts of storage.

Features of Backing Storage

Access method

Random or Serial How the transfer of data to and from storage is organised.

Medium

Magnetic or Optical The data storage technology.

Performance

Capacity and Speed How much and how fast.

Random Access

A disk is a random access medium.

Random access means the system can go directly to the data it requires.

This makes a disk drive a faster method of data storage and data access than a tape drive

Serial Access

Items in a file are read one at a time from the physical start of a file in the order in which they are stored. A tape (video) is a serial access medium.

Magnetic Storage

In magnetic storage systems, the surfaces of the disks or tapes are coated in a substance that can retain data as magnetic patterns. These magnetic patterns are written to – and read from – the surface using an electromagnetic read/write head.

Magnetic storage systems fall into two categories: disk or tape.

Optical Storage

Optical storage devices use laser technology to store and retrieve data. We shall consider both CD-ROM and DVD-ROM.

Types of Access

Magnetic Storage Device

Random (direct)

Serial

Floppy Drive √

Hard Drive √

Zip Drive √

Magnetic Tape √

Optical Storage Device

Random (direct)

Serial

CD-ROM √

CD-R √

CD-RW √

DVD-ROM √

Disc Summary

Interface

An interface is a hardware device that is needed to allow the processor to communicate with an external or internal device such as a printer, modem or hard drive.

Sometimes the interface is a board in the computer and sometimes it is a connection to a port, e.g USB port

Why is an Interface Needed

An interface is required because there are differences in characteristics between the peripheral device and the processor.

These characteristics include Data Conversion

Converting analogue to digital and vice versa Speed of operation

Compensates for differences in speed Temporary Storage Data

Types of Conversion

The most common example of data conversion is when the peripheral accepts an analogue signal that must be converted into digital for the processor to comprehend it.

Computer Task

Peripherals Exercise Sheet P99