Input and Output Devices
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Before a computer can
process your data, you need some method to input the
data into the machine. The device you use will depend on what form
this data takes (be it text, sound, artwork, etc.).
Similarly, after the computer has processed your data, you often need
to produce output of the results. This output
could be a display on the computer screen, hardcopy on
printed pages, or even the audio playback of music you composed on the
The terms “input” and “output” are
used both as verbs to describe
the process of entering or displaying the data, and as nouns referring
to the data itself entered into or displayed by the computer.
Below we discuss the variety of peripheral devices used for computer
input and output.
Flat Panel Display
Ink Jet Printer
The computer keyboard is used
to enter text information into the computer, as when you type the contents
of a report. The keyboard can also be used to type commands directing
the computer to perform certain actions. Commands are typically chosen
from an on-screen menu using a mouse, but
there are often keyboard shortcuts for giving
these same commands.
In addition to the keys of the main keyboard (used for
typing text), keyboards usually also have a numeric keypad (for entering
numerical data efficiently), a bank of editing keys (used in text editing
operations), and a row of function keys along the top (to easily invoke
certain program functions). Laptop computers, which don’t have room for
large keyboards, often include a “fn” key so that other keys can perform
double duty (such as having a numeric keypad function embedded within
the main keyboard keys).
Improper use or positioning of a keyboard can lead to repetitive-stress
injuries. Some ergonomic keyboards are designed
with angled arrangements of keys and with built-in wrist rests that can
minimize your risk of RSIs.
Most keyboards attach to the PC via a PS/2 connector or
USB port (newer). Older Macintosh computers used an ABD connector,
but for several years now all Mac keyboards have connected using USB.
The graphical user interfaces (GUIs) in use today require
some kind of device for positioning the on-screen cursor. Typical pointing
devices are: mouse, trackball, touch pad, trackpoint, graphics tablet,
joystick, and touch screen.
Pointing devices, such as a mouse, connected to the PC
via a serial ports (old), PS/2 mouse port (newer), or USB port (newest).
Older Macs used ADB to connect their mice, but all recent Macs use USB (usually to a USB port right on the USB keyboard).
PC Keyboard (you have one in front of you that you can see for a closer look)
The mouse pointing device sits
on your work surface and is moved with your hand. In older mice, a ball
in the bottom of the mouse rolls on the surface as you move the mouse,
and internal rollers sense the ball movement and transmit the information
to the computer via the cord of the mouse.
The newer optical mouse does
not use a rolling ball, but instead uses a light and a small optical
sensor to detect the motion of the mouse by tracking a tiny image of
the desk surface. Optical mice avoid the problem of a dirty mouse ball,
which causes regular mice to roll unsmoothly if the mouse ball and internal
rollers are not cleaned frequently.
A cordless or wireless
mouse communicates with the computer via radio waves (often
using BlueTooth hardware and protocol) so that a cord is not needed
(but such mice need internal batteries).
A mouse also includes one or more buttons
(and possibly a scroll wheel) to allow users to interact with the GUI.
The traditional PC mouse has two buttons, while the traditional Macintosh
mouse has one button. On either type of computer you can also use mice
with three or more buttons and a small scroll wheel (which can also usually
be clicked like a button).
Two-button mouse with scroll wheel
Wireless Macintosh mouse
Most laptop computers today have a touch
device. You move the on-screen cursor by sliding your finger along the
surface of the touch pad. The buttons are located below the pad, but
most touch pads allow you to perform “mouse clicks” by
tapping on the pad itself.
Touch pads have the advantage over mice that they take
up much less room to use. They have the advantage over trackballs (which
were used on early laptops) that there are no moving parts to get dirty
and result in jumpy cursor control.
Touch pad of a PC laptop
Some sub-notebook computers (such as the
IBM ThinkPad), which lack room for even a touch pad, incorporate a trackpoint,
a small rubber projection embedded between the keys of the keyboard.
The trackpoint acts like a little joystick that can be used to control
the position of the on-screen cursor.
The trackball is
sort of like an upside-down mouse, with the ball located on top. You
use your fingers to roll the trackball, and internal rollers (similar
to what’s inside a mouse) sense the motion which is transmitted
to the computer. Trackballs have the advantage over mice in that the
body of the trackball remains stationary on your desk, so you don’t
need as much room to use the trackball. Early laptop computers often
used trackballs (before superior touch pads came along).
Trackballs have traditionally had the same problem as mice:
dirty rollers can make their cursor control jumpy and unsmooth. But there
are modern optical trackballs that don’t have this problem because their
designs eliminate the rollers.
Joysticks and other game controllers can
also be connected to a computer as pointing devices. They are generally
used for playing games, and not for controlling the on-screen cursor
in productivity software.
Some computers, especially small hand-held PDAs, have touch
sensitive display screens. The user can make choices and press button
images on the screen. You often use a stylus, which you hold like a pen,
to “write” on the surface of a small touch screen.
A graphics tablet consists of an electronic
writing area and a special “pen” that works with it. Graphics
tablets allows artists to create graphical images with motions and actions
similar to using more traditional drawing tools. The pen of the graphics
tablet is pressure sensitive, so pressing harder or softer can result
in brush strokes of different width (in an appropriate graphics program).
A scanner is a device that images
a printed page or graphic by digitizing it, producing an image made of
tiny pixels of different brightness and color values which are represented
numerically and sent to the computer. Scanners scan graphics, but they
can also scan pages of text which are then run through OCR (Optical Character
Recognition) software that identifies the individual letter shapes and
creates a text file of the page’s contents.
A microphone can be attached
to a computer to record sound (usually through a sound card input or
circuitry built into the motherboard). The sound is digitized—turned
into numbers that represent the original analog sound waves—and stored
in the computer to later processing and playback.
Instrument Digital Interface) is a system designed to transmit
information between electronic musical instruments. A MIDI musical
keyboard can be attached to a computer and allow a performer to play
music that is captured by the computer system as a sequence of notes
with the associated timing (instead of recording digitized sound waves).
The traditional output device of a personal computer has been
the CRT (Cathode Ray Tube) monitor. Just like a
television set (an older one, anyway) the CRT monitor contains a large cathode
ray tube that uses an electron beam of varying strength to “paint” a
picture onto the color phosphorescent dots on the inside of the screen. CRT
monitors are heavy and use more electrical power than flat panel displays,
but they are preferred by some graphic artists for their accurate color rendition,
and preferred by some gamers for faster response to rapidly changing graphics.
Monitor screen size is
measured diagonally across the screen, in inches. Not all of the screen
area may be usable for image display, so the viewable area is also specified.
resolution of the monitor is the maximum number
of pixels it can display horizontally and vertically (such as 800 x 600,
or 1024 x 768, or 1600 x 1200). Most monitors can display several resolutions
below its maximum setting. Pixels (short for
picture elements) are the small dots that make of the image displayed on
the screen. The spacing of the screen’s tiny phosphor dots is called
dot pitch (dp), typically
.28 or .26 (measured in millimeters). A screen with a smaller dot pitch produces
Your computer must produce a video signal that a monitor
can display. This may be handled by circuitry on the motherboard, but is
usually handled by a video
one of the computer’s expansion slots; often the slot is a special one
dedicated to video use, such as an AGP slot (Accelerated
Graphics Port). Video cards are also called video
and graphics cards. Many video cards contain separate
processors and dedicated video memory for generating complex graphics quickly
without burdening the CPU. These accelerated graphics
cards are loved by
A flat panel display usually
uses an LCD (Liquid
screen to display output from the computer. The LCD consists of several thin
layers that polarize the light passing through them. The polarization
of one layer, containing long thin molecules called liquid crystals,
can be controlled electronically at each pixel, blocking varying amounts
of the light to make a pixel lighter or darker. Other types of flat
panel technology exist (such as plasma displays)
but LCDs are most commonly used in computers, especially laptops.
Older LCDs had slow response times and low contrast, but
active matrix LCD screens have a transparent
thin film transistor (TFT)
controlling each pixel, so response, contrast, and viewing angle are
Flat panel displays are much lighter and less bulky than
CRT monitors, and they consume much less power. They have been more expensive
than CRTs in the past, but the price gap is narrowing. You will see many
more flat panels in the future.
As with CRTs, the display size of a flat panel is expressed
in inches, and the resolution is the number of pixels horizontally and vertically
on the display.
Flat panel display (LCD)
For hardcopy (printed) output, you need
some kind of printer attached to your computer (or available over a
network). The most common type of printer for home systems is the color ink
jet printer. These printers form the image
on the page by spraying tiny droplets of ink from the print head. The
printer needs several colors of ink (cyan, yellow, magenta, and black)
to make color images. Some photo-quality ink jet printers have more colors
Ink jet printers are inexpensive, but the cost of consumables
(ink cartridges and special paper) make them costly to operate in the
long run for many purposes.
A laser printer produces
good quality images by the same technology that photocopiers use. A drum
coated with photosensitive material is charged, then an image is written
onto it by a laser (or LEDs) which makes those areas lose the charge.
The drum then rolls through toner (tiny plastic
particles of pigment) that are attracted to the charged areas of the
drum. The toner is then deposited onto the paper, and then fused into
the paper with heat.
Most laser printers are monochrome (one color only, usually
black), but more expensive laser printers with multiple color toner cartridges
can produce color output.
Laser printers are faster than ink jet printers. Their
speed is rated in pages per minute (ppm).
Laser printers are more expensive than ink jets, but they are cheaper
to run in the long term if you just need good quality black & white pages.
Multi-function printers are available that not only operate
as a computer printer, but also include the hardware needed to be a scanner,
photocopier, and FAX machine as well.
Dot matrix printers use small
electromagnetically activated pins in the print head, and an inked ribbon,
to produce images by impact. These printers are slow and noisy, and are
not commonly used for personal computers anymore (but they can print
multi-layer forms, which neither ink jet or laser printers can).
Computers also produce sound output, ranging
from simple beeps alerting the user, to impressive game sound effects,
to concert quality music. The circuitry to produce sound may be included
on the motherboard, but high quality audio output from a PC usually requires
a sound card in one of the expansion slots,
connected to a set of good quality external speakers or headphones.
Multimedia is a term describing
computer output that includes sound, text, graphics, movies, and animation.
A sound card is an example of a multimedia output device (as is a monitor
that can display graphics).