UNIT 2 – PERIPHERALS
INPUT DEVICES
Input devices supply external information to the computer. These fall into several categories according to the type of
information that they supply. These categories are Text Input Devices, Graphical Input Devices, Data Input Devices and
Control Input Devices. These devices connect using Bus Ports, Serial Ports, Parallel Ports, SCSI, or USB ports.
Text Input Devices
 The Keyboard & Numeric Keypads provide character entry and limited cursor control.
 Optical Character Recognition (OCR) can identify printed characters on a tablet or scanned paper copy.
 Optical Mark Readers (OMR) used marks on a sheet for limited character input. (Scantron)
 Bar Code Readers read data encoded numerical as a series of vertical bars arranged in a pattern.
 Magnetic Strip Card Readers allow limited character input.
 Smart Card Readers. These cards contain circuitry that can communicate directly. (EasyPass)
 Voice Pattern Recognition identified a digital oscilloscopic sound pattern and compares it to a database of stored
sound patterns equivalent to known words. Dictation into a text document is then possible.
Graphical Input Devices
 Scanners allow the scanning of existing images into digital data for storage or recognition.
 Digital Cameras allow still or motion images of both 2D and 3D objects.
 Graphics Tablets use a coordinate grid and stylus to allow drawing or tracing images as digitized data.
 Light Pens use a coordinate system synchronized with the screen scan to allow drawing directly on the display
screen.
Data Input Devices
 These include probes and sensors used for remote sensing within almost every device in use. These detect motion,
heat, light, x-rays and other radiation and electro-magnetic fields.
 Voice Recognition uses the comparison of a user’s voice patterns with a database of voice patterns.
Control Input Devices
These allow cursor control and option selection. These fall into two operational categories.
 Relative Positioning where cursor movement is calculated from the last known position. The mouse and trackball fall
into this category.
 Absolute Positioning where the cursor is directed to a new position regardless of the former position. Touch Pads,
Tablets, Light Pens and Touch-sensitive screens fall into this category
 Game Controllers and Joysticks
STORAGE DEVICES
Storage Devices allow permanent or semi-permanent storage of digital information. Currently, these fall into the following
operational categories: Magnetic, Magneto-Optical, Optical and Solid State.
Magnetic Storage Devices
All magnetic storage devices have some things in common. These use a read-write head, basically an electro-magnetic,
that rearranges molecules in the magnetic media in response to digital signals. Magnetic media is constructed from a
Mylar (plastic) film cut in the form of tape or a disk or hard surfaces like aluminum or glass. These surfaces are covered
with an magnetic iron oxide coating that is easily magnetized. Devices differ in the method by which they format the
information and the mechanics which make up the particular device.
Tape Drives Tape
Tape drives today are mostly configured to act as media backup devices instead of primary storage devices. These
drives are available in various capacities and operating speeds. Prices may range from under $100 to over $4000. Tape
can be in any number of widths and write/read formats:
 9 Track Reel to Reel or Cartridge
 QIC 80 Cartridge
 Helical Scan Cartridge
 DLT Digital Cartridge
 SLR Cartridge
Disk Drives
 All disk drives make use of some form of soft or hard platter coated with a magnetic coating. A magnetic read-write
head is moved over the surface of each platter by a head mechanism driven by a stepper motor or voice coil motor in
fixed increments. This movement aligns the heads with tracks, sometimes called cylinders. These tracks are further
divided into short arcs called sectors. The disk drive can locate data by keeping tract of the track and sectors where
the data is placed. Data is usually written in fixed amounts called clusters. Disks are always formatted in a standard
way prior to storing data for the first time.
 Floppy Disks employ a soft Mylar disk in a hard plastic case. When the disk is inserted in the drive, a little slide opens
revealing the disk surface. Heads contact both surfaces of the disk. When data needs to be read or written, a motor
turns the disk and a stepper motor moves the heads to read the file table (FAT) to find the location. Then moves the
heads to the location where the data is read or written by the appropriate head. Then the disk stops turning. A red
LED indicator lights on the front of the drive during this process. Ordinary floppy disks may format data in 720 KB,
1.44 MB or 2.88 MB formats. A newer LS-120 floppy can achieve densities of 120 Mb by more precise head tracking
using a small laser to track minute optical guidance marks on the surface of special disks. These drives also read and
write standard floppy formats. Zip drives use the floating head technology of hard drives to achieve densities up to
250 MB. Since there is no actual head contact, the soft disk can be spun at high speed and tracks can be closer
together. Zip drives use special disks and they are not compatible with floppies. Both the LS-120 and Zip drives
connect to the 16-bit IDE port instead of the slowed 8-bit FDD port.
 Hard Drives make use of aluminum or glass platters coated with a magnet surface. A set of heads, mounted on head
arms, float over the surface of each platter. These platters continuously spin at high speeds. Since the platters are
not removable, they are manufactured to higher tolerances. They can contain thousands of tracks divided into many
sectors. Today, high densities are quite common. Since these drive use very close tolerances, they must be sealed
in airtight chambers to eliminate dust on the disk and head surfaces. Hard drives can be made removable by
mounting them in a special rack that permits the entire drive to be removed from the front of the computer, even when
it’s running. This process is called “Hot Swapping”. Some hard drives, like the JAZ drive, make use of platters in
removable cartridges. However, densities are limited since tracks must be made larger and further apart. This is
because a cartridge may not align as precisely each time it is inserted and heads must ride at greater distances from
the surface of the platter. This is to allow for dust, since the cartridge is not a sealed clean environment.
Optical Storage Devices
 CD-ROMS, CD-R, CD-RW and DVD drives all use optical media. These make use of disks made of a high-grade
plastic that is pressed with an etched series of pits representing the recorded image. A reflective aluminized
surface is added. Laser light will be diverted by the pits and allowed to reflect by the clear areas, allowing the “0”
or “1” binary information to be read. The drive contains a read head consisting of a laser LED and an optically
sensitive transistor. As the disk spins and the head aligns with the track, the laser light is reflected off the foil
surface to the transistor. The transistor detects the varying strength of the reflected light that it translates into
electrical impulses.
 CD-R/’DVD-R disks use a photo sensitive acetate layer over the aluminized layer. This photo-sensitive layer is
sensitive the spectrum of the burning laser in the drive. Slight color differences developing in this layer allow
passage or restriction of the read head’s laser beam creating the same effect as the pits on the CD-ROM.
 CD-RW/DVD-RW disks make use of a phase-shifting layer, similar the LCD process. When the record laser light
strikes this layer, molecules rearrange creating differing refractive patterns. During erasure or re-recording these
molecules are rearranged again.
 Holographic Recording: The latest optical technology is holographic video storage. Inphase Technologies has a
product called Tapestry that promises to store 100 GB of video at transfer rates of 20 MBps on a small disk. This
will be a major breakthrough.
All of these drives are now made to interface with EIDE or SCSI with external models available for Parallel Port, USB, and
PCMCIA ports. The following table lists some facts about these devices.
Solid-state Storage Devices
 These devices, usually in the form of a PCMCIA card, contain memory chips that hold data in a number of ways.
 CMOS, a special RAM with low power requirements that is kept energized by a small battery.
 EPROM, an electrically programmable ROM chip that holds its data until reprogrammed.
 Bubble Memory that holds its programmed state until polarity is reversed in each cell.
 Flash Memory that behaves similar to Bubble Memory but is constructed differently. The most popular are Compact
Memory Cards, Smart Media Cards, SD Memory Cards and Memory Sticks. These devices, spearheaded by the
digital camera and MPEG player market, have become affordable.
OUTPUT DEVICES
Output devices receive data from the computer for external use by human or machine. These fall into several categories.
 Displays: CRT, LCD, Plasma or Video Projector displays
 Printers: Impact, Ink-Jet, Laser, Thermal Dye-Transfer printers or Plotters
 Voice: Voice synthesized devices
 Electro-mechanical: Electro-mechanical servomotors used in aeronautical, space, robotics or industrial applications.
Displays
 Cathode Ray Tube (CRT) displays use the same basic technology used in television picture tubes but produce higher
resolutions and scan speeds. These large vacuum tubes employ a cathode-powered electron gun that produces a
stream of electrons directed through an aperture grill at a specific combination of color phosphor dots on a screen at
the face of the tube. Using a set of electro-magnetic coils, the electron beam is moved horizontally and vertically in a
regular scan pattern, changing intensity in response to the video signal. The phosphor screen glows in response to
the strength of the beam, producing the image.
 Liquid Crystal Displays (LCD) use a back lighted panel and a liquid that has refractive properties, when energized.
This allows a dot of polarized light to be twisted to pass through a polar screen and color filter to appear as a dot. A
tiny transistor controls each dot on the screen. LCD screens are flat and take little space. However, they are
expensive and limited in size and viewing angle.
 Plasma Displays make use of flat glass panels within which is trapped inert gas designed to glow when energized by
intersecting charged conductors. U.V. Radiation from these ionized gas strikes phosphor stripes that glow in primary
colors. These screens are expensive, heavy and fragile but produce bright displays with little thickness.
 Video Projectors use one of two methods. 1. Three very small and bright CRT displays, one for each primary color,
the images are aligned optically and projected through lenses to a large screen. 2. LCD powered projectors are
basically slide projectors with small LCD panels in place of the slide. Usually, three LCD panels are used, one for
each primary color. Images are aligned optically to be projected through a single lens to a large screen. Older
projectors used three high-powered CRT units, one for each primary color.
 DLP (Digital Light Processing) Uses a reflective solid state chip to “valve” light used for projection. A separate chip for
each color is used to reflect light beams of additive primary colors. Since these chips are either on or off, intensity is
controlled by modulating the pixel at a specific frequency.
Printers
Printers operate according to different principles and vary greatly in performance and purpose. The basic types are:
 Impact Printers make use of mechanical pressure against an inked ribbon to produce characters on paper. Impact
printers are of two types: Those that use preformed character elements and work much like a traditional typewriter.
These employ a changeable font element in the form of a ball, wheel, cylinder or band. An electro-magneticallycontrolled hammer strikes the appropriate character against an inked ribbon. This produces the printed character.
Dot-matrix printers, on the other hand, use an electro-magnetic print head containing vertical columns of pins that
extend to press the inked ribbon against the paper. As the head sweeps across the paper, the dot pattern formed by
the extending pins forms the characters. Since the characters are not preformed, soft scaleable fonts and even limited
graphics can be printed. While impact printers are rapidly being replaced by other technologies, they alone can
produce multiple impression copies in a single run.
 Ink-Jet Printers use precise nozzles to spray colored ink on paper to form characters. Ink-Jet printers provide an
economical answer to color printing. These employ a print element consisting of vertical columns of tiny spray
nozzles that spray ink of the appropriate colors directly on the paper. These also use a dot-matrix pattern to produce
characters and graphics. Primary colors (red, blue & yellow) in conjunction with black ink can produce true color
renditions. Since this ink is transparent, the paper texture and type play an important part in the overall result. The
major drawback of ink-jet printers is slow speed, especially in the full color mode.
 Laser Printers Laser printers use copy machine technology to print a full page at a time. These employ a
photosensitive drum and toner hopper for each primary color. A laser scans the rotating drum(s). The laser is
modulated according to print data to form an electrostatic image on the drum that causes the finely powdered toner to
be attracted to the drum, forming the image. A high voltage corona wire charges paper to the opposite charge. As
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paper passes under the rotating drums, the toner is attracted to the paper forming the image on the paper. Paper
then passes between two hot rollers called a fuser. This causes the wax-like powdered image to melt into the surface
of the paper, forming a permanent image. While laser printers are fast, their color renditions tend to be limited in
quality. They should be avoided where large fields of color are required.
Thermal Dye Transfer Printers use heat to transfer primary color wax-like dye to paper to form an image. Thermal Dye
Transfer Printers are simple in concept and produce excellent color renditions. These employ a roll of Mylar (plastic)
wide enough to fit the width of the paper. The Mylar sheet has bands of a wax-like substance in primary colors. Each
band is the size of the length of the paper. There is a clear band, red band, blue band, yellow band and black band.
A row of electrically heated pins above the band melts dots of colored toner to the surface of the paper. The Mylar
must cycle back and forth for each appropriate color. Once the color is transferred, a new section of Mylar is used.
Hence, this type of printer in only economical where large colored areas must be printed. These printers should be
avoided where large amounts of white space exist. This type of job is best handled by the laser or ink-jet printer.
Plotters draw linear images using marker-style pens. They draw characters just as we would print them by hand.
Plotters are not really printers. They literally draw lines on paper using pens, much as we do. These have a very
narrow application. They are used where very large format printouts are needed as in drafting and architecture. They
are expensive and slow and give poor color renditions.
Voice Synthesizers requires a sound circuit card capable of producing digital and analog sound. Wave files are stored and
played back in response to some action or voice input. When OCR (Optical Character Recognition) and the proper
synthesizer software are combined, printed text can be read into voice.
Electro-Mechanical Servos
These and stepper motors are employed in constantly expanding areas of our life. Robotics, prosthetics, industrial,
automotive and aerospace applications are the most common. Digital data from a computer is passed to a controller that
switches more powerful electrical currents that move these motor-like devices to produce mechanical motion and/or
torque in response to the computer commands. Most large and military aircraft flying today, as well as ships and
spacecraft use fly-by-wire systems, employing this technology.