Input and Output
Standard Methods of Input
Input device à enables you to input information and commands into the computer.
The Keyboard
One of the first peripherals to be used with a computer and is still the primary input
device for text and numbers.
How a computer accepts input from the keyboard.
1. Key is pressed on the keyboard.
2. Keyboard controller sends scan code for the key to the keyboard buffer.
3. Keyboard buffer sends interrupt request to the system software.
4. System software responds to the interrupt by reading the scan code from the
keyboard buffer.
5. System software passes the scan code to the CPU.
The Mouse
An input device that rolls around on a flat surface and controls the pointer. The
pointer is an on screen object (usually an arrow) that is used to select text, access
menus and interact with programs, files or data that appears on the screen.
The mouse was first packaged with the Apple Mac in 1984. Instead of forcing you to
type or issue commands from the keyboard the mouse and mouse based Operating
Systems let you choose commands from east to use menus and dialog boxes.
Variants of the mouse
1. The trackball
Pointing device that works like an upside down mouse. Rest your thumb on
the exposed ball and to move the pointer around the screen you move the
ball with your thumb.
2. The trackpad (or touchpad)
A stationary pointing device that some people find less tiring than a mouse
or trackball. Movement of a finger across a small touch surface is translated
into pointer movement on the computer.
3. Pointers in the keyboard
Small joystick positioned near the centre of the keyboard – typically between
the g and the h keys. Joystick is controlled with either forefinger.
Non standard methods of Input
Pens
Use a pen (or stylus ) for data input. Hold the pen in your hand and write on a special
pad or directly to the screen. Also can be used as a pointing device like a mouse. Pen
based systems are not generally used to enter large amounts of text because
handwriting recognition is so complex. They are frequently used for note taking and
creating short messages. More commonly used for data collection where the touch of
a pen may select a check box. Another common use is inputting signatures or
messages that are stored or transmitted.
Touchscreens
Accept input by allowing the user to place a fingertip directly on the computer screen
to make a selection. Most touchscreens use sensors in or near the computer screen to
detect the touch of a finger. Useful in an environment where dirt or weather would
render keyboards or pointing devices useless. Well suited to simple applications such
as ATMs or public information kiosks. Have become popular in fast food restaurants,
department stores and supermarkets.
Game Controllers
Can be considered an input device because a computer game is a program. It accepts
input from the user, processes data and produces output in the form of graphics and
sound.
Bar code readers
Most widely used input device after the keyboard and mouse. Most common type of
barcode reader is the flatbed model commonly found in supermarkets. Devices
convert a barcode – a pattern of printed bars – into a code that the computer can
understand. The barcode reader emits a beam of light that is reflected by the barcode
image. A light sensitive detector identifies the bar code image by recognising special
bars at both ends of the image. Both these bars are different so the reader can tell if
the bar code is being read the right way up. After identification the barcode is
converted into numeric digits that are fed into the computer as if a number had been
typed on the keyboard.
Image scanners and optical character recognition
Scanners convert any printed image into electronic form by shining a light onto the
image and sensing the intensity of the reflection at every point. Colour scanners use
filters to separate the components of colour into the primary colours at every point. If
you have scanned a text document you might want to use Optical Character
Recognition (OCR) software to translate the image into text that you can edit.
Microphones and speech recognition
Now that sound capabilities are standard in computers microphones are becoming
increasingly important as input devices to record speech. For sound input you need a
microphone and a sound card that translates the analogue signal (sound waves) from
the microphone into digital codes that the computer can store and process. Speech (or
voice) recognition, use it to dictate to the computer instead of typing. Newer
generation speech recognition programs are much more reliable. Some packages
accurately recognise 80% of spoken words. The user may need to “train” the
software to recognise speech patterns. Speech recognition packages usually require
the use of a noise-cancelling microphone.
Video input
PC video cameras digitise images by breaking them into individual pixels. Each
pixels colour and other characteristics are stored as a digital code. This code is then
compressed so that it can be stored on disk or transmitted. Many PC video cameras
attach to the top of the screen enabling the user to capture images of themselves while
working at the computer. Using a video capture card the user can also connect other
video devices enabling transfer of images from video equipment to the computer.
Digital cameras
Work similar to PC cameras except that they are handheld devices that capture still
images. The digital camera digitises the image, compresses it and stores it on a
special disk or memory card. The information can then be copied, printed or used in a
document.
Output Devices
Monitors
The most commonly used output device.
2 basic types of monitors used with pc’s
• CRT Cathode Ray Tube – works in the same way as a tv screen using a large
vacuum tube.
• Flat Panel Display – primarily used with portable computers and are
becoming more popular with desktops.
How a typical CRT works
1. Electron gun shoots streams of electrons towards the screen.
2. Magnetic coil guides the stream of electrons across and down the screen.
3. Phosphor dots on the back of the screen glow when the electron beam hits
them.
Flat Panel Monitors
2 major disadvantages associated with CRT monitors,
• Big, difficult to move. Flat panel monitors are comparatively lightweight.
• CRT’s require a lot of power. Not practical for notebook computers which
have a built in battery.
Most common type of flat panel monitor is the liquid crystal display (LCD). Creates
an image with a special kind of liquid crystal that is normally transparent but becomes
opaque when charged with electricity.
Two main categories of LCD displays,
1. Active matrix - assigns a transistor to each pixel and each pixel is turned on or
off individually.
2. Passive matrix - transistors for each row and each column of pixels creating a
grid that defines the location of each pixel.
PC Projectors
More common now to use software to create presentations directly to the screen. A
pc projector plugs into one of the computer’s ports and projects the video output onto
an external surface. Most pc projectors use LCD technology to create images. Room
needs to be darkened and display is blurry. Newer technology à Digital light
processing (DLP) displays brighter crisper images. DLP devices use a special
microchip called a digital micro mirror device that uses mirrors to control the image
display.
Sound Systems
Speakers and their associated technology are now important output devices. Speakers
attached to a pc system are similar to those on a stereo only on a smaller scale and
they contain their own amplifiers. More complicated part à the sound card.
The sound card translates digital sound to electric current to be sent to the speakers
1. Electric current is sent to the speakers.
2. Electromagnet receives the signal.
3. Magnet vibrates due to the signal.
4. Wave created à Sound.
Printers
2 categories:
1. Impact
2. Non-impact
Impact
Creates an image by pressing an inked ribbon against paper using pins or
hammers to shape the image e.g. typewriter. Most common type of impact
printer à Dot matrix.
Non-impact
Use other means to create an image. Most popular types à Inkjet printers,
Laser printers.
Dot Matrix Printer - Commonly used in workplaces where physical impact with the
paper is important, such as when the user is printing to carbon copy or pressure
sensitive forms. Can produce sheets of plain text very quickly. Used to print very
wide sheets.
Line Printers - Works like a dot matrix printer but prints an entire line at a time. Not
very high resolution but very quick – approx 3000 lines of text per min.
Band Printers - Features a rotating band that is embossed with alphanumeric
characters. Very fast and robust – approx 2000 lines of text per min.
Daisy wheel printers - Almost obsolete. Spinning wheel with characters embossed
around its edge. Creates clean text but no graphics and is very slow.
Inkjet Printers - Creates an image directly onto the paper by spraying ink through
tiny nozzles. Most inkjets use separate cartridges for colour and black and white
printing. This saves money by reserving coloured ink only for coloured printing.
Laser Printers - More expensive than inkjet printers but offer a higher print quality
and are faster. A separate CPU and memory are built into the printer to interpret the
data received from the computer and to control the laser. Technology is similar to
that of a photocopier. Laser can aim at any point on a drum creating an electrical
charge. Toner, which is composed of tiny particles of oppositely charged ink sticks
to the drum in the places that the laser has charged. Pressure and heat transfer the
toner from the drum to the paper.
Snapshot Printers - Small format printers that use special glossy paper to create
medium resolution prints of 150 – 300 dpi. Best snapshot printers can create images
that look nearly as good as a photo.
Thermal wax printers - Used primarily for presentation graphics and handouts.
Create bold colours and have a low percentage cost. Produces vivid colours because
the inks do not bleed into each other or soak the specially coated paper. Ribbon
coated with panels of coloured wax that melts and adheres to plain paper as coloured
dots when passed over a focused heat source.
Dye – sub printers - Dye sublimation printers have ribbons containing panels of
colours which are moved across a focused heat source capable of subtle temperature
variations. Heated dyes evaporate from the ribbon and diffuse on specially coated
paper creating extremely sharp images but are very slow and costly.
Fiery printers - Special purpose computer – fiery print server – that transmits
documents to a digital colour copier where they are printed. These printers are used
in print shops as an alternative to press printing.
IRIS printers - Used by print shops to produce high resolution presentation graphics
and colour proofs resembling images. Individual sheets of paper mounted onto a
drum, nozzles on the printing head pass from one end of the spinning drum to the
other spraying minute drops of coloured ink to form an image.
Plotters - Special kind of output device, like a printer because it produces images on
paper but typically used to print large format images.
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Table plotters (or flatbed) use 2 robotic arms. Complex, large and slow.
Roller plotters (or drum plotters) uses 1 drawing arm but moves the paper
instead of holding it flat and stationary.
Mechanical plotters have been displaced by thermal, electrostatic and inkjet
plotters as well as large format dye – sub printers which are all faster and
cheaper.
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