Digital Technology and Computer FundamentalsChapter 6Computer
ObjectivesAt the end of this chapter, you should be able
to:describe the characteristics of different input and output
devices;explain why I/O interfaces are required in a computer
Objectives (Contd)describe the three different methods for
interfacing I/O; andexplain the advantage and disadvantages for
each of the I/O interface methods.
ReferencesM. Marris Mano, "Computer System Architecture," third
edition, Prentice Hall.G. Shelly, T. Cashman, G. Waggoner, W.
Waggoner, Discovering Computers 98, A link to the Future,
International Thomson, 1998.
Input / Output DevicesProvide communication for the CPU and the
outside worldKeyboardMousePointing DevicesTerminalVisual Display
KeyboardMost commonly used Keys are micro-switches.Alphabetic
keys.Numeric keypad.Arrow keys or cursor control keysFunction
MouseA pointing device.Controls the movement of an on-screen
symbol called the pointer.A ball or light reflection on the bottom
to sense its movement.Electronic circuits translate to electrical
signals that are sent to the computer to direct the pointer.
TrackballSimilar to a mouseThe ball is on top of the device.
Touch padA flat surface usually attached in a notebook
computer.Controls the movement of the pointer by sensing the motion
of a finger on its exterior.
PenPen input devices accept input with hand written
characters.Usually a special software must be required for this
type of input.Can also be used as a mouse
Touch screenAllows users to touch areas of the screen to enter
data.The action of screen-touching is equivalent to the movement
and clicking the mouse.Two types of touch screens:combined with the
monitorattached in front of the monitor
Image scannerElectronically captures an entire page and converts
the document into digital format.Special software required to
capture images.To convert the image to text, Optical Character
Recognition (OCR) software is needed.
Optical recognition devicesUsing a light source to read optical
codes.For example, a bar code reader reads the standard Universal
Product Code (UPC) and converts them into digital format.
MicrophoneAccepts voice inputVoice recognition software is
required to convert into digital format.For voice to text
conversion, familiarization is need for the conversion software to
recognize the characteristics of the voice of the user.
TerminalConsists of a keyboard and a screen.Dumb terminals have
no independent processing capability.Intelligent terminals have
built-in processing capabilities.Special purpose terminals uniquely
designed for use in a particular industry.
Visual Display Unit (VDU)A visual output device.A monitor looks
like a television and consists of a cathode ray tube (CRT) display
screen.An LCD display is a thin display screen, and most often used
in notebook computers.
PrintersUsed to produce hard copies.Two categories: impact and
non-impact printers.Impact printers: striking a ribbon to deposit
ink on the paper.Non-impact printers: use some non-mechanical
technique for printing.
Dot-matrix printersConsists of a printer head with a vertical
column of 9 or 24 needles.The needles are selectively energized and
forced against the ribbon in proper combinations, to form the
desired character image on the paper.Major advantage: capability to
print multiple copies simultaneously.
Laser printersBased on the principle of photocopying machines.A
laser beam produces mirrored images on a photo-sensitize selenium
drum.Toner gets stuck to the drum by an electrostatic process, then
transferred to the paper.
Laser printers (Contd)Finally, The inked image on the paper is
fused by heat.Provide very good quality printing at a moderately
Ink-jet printersUsing the dot-matrix approach with droplets shot
at the paper to form the desired character.Quiet in operation with
good print quality.
PlottersUsed to produce high-quality line drawings.Usually, the
outputs of plotters are large size posters, graphics, etc.
Data projectorsIt takes the image that appears on a computer
screen and project it.It is a very effective method for
I/O InterfaceUsually, I/O devices are not connected directly to
the system bus in the computer system.An interface module for each
device is needed.
Need of interface moduleI/O Operations are different from CPU
and memory operations .Operations of different peripheral devices
are different.Data format and word lengths are different.Conversion
of signal values for each device may be required.
Need of interface module (Contd)The data transfer rate of these
devices are much slower than that of the memory or CPU.Direct
connection to the system bus is impractical.
SolutionsSpecial hardware components are required to supervise
and synchronize the input and output data transfer.These components
between the CPU and the peripheral devices are called
I/O Bus and Interface ModulesEach peripheral device connects to
a separate interface module All interfaces are attached to the I/O
Interface ModuleEach interface decodes the address and control
signals received from I/O bus;interprets and provides signals for
the peripheral controller;synchronizes the data flow and supervises
Accessing I/OCPU places an address on the address bus.The
addressed interface detects it and activates the path. Other
devices are disabled.CPU provides an I/O command in the control
lines.The interface selected responds and proceeds to execute
I/O Bus and Memory BusCPU communicates with both the I/O and the
memory.Memory bus contains data, address, and read/write control
lines.Need to separate them.
Distinguish between I/O bus and Memory busThree different
ways:Two separate buses are used, one for memory and the other for
I/O.Common data bus and common address bus with separate control
buses for memory and I/O.All buses are common for memory and
Separate buses for memory and I/OTwo independent sets of buses,
one for memory, other for I/O.A separate I/O processor (IOP) is
provided in addition to the CPU.Memory communicates with both CPU
and IOP through memory bus.
Separate buses for memory and I/O (Contd)IOP also communicates
with the I/O devices through a separate I/O bus with its own
address, data and control lines.The purpose of the IOP is to
provide an independent pathway for the transfer of information
between external devices and internal memory.
Isolated I/OOne common bus to transfer data and address
information between memory or I/O and the CPU.Separate read and
write lines are used to distinguish the memory and I/O
Isolated I/O (Contd)CPU places the address information on the
address bus.It then enables one of the two possible read/write
lines (memory R/W or I/O R/W) to specify the operation.Specified
device will be activated.
Isolated I/O (Contd)Distinct input and output instructions are
available.Each is associated with the address of an interface
because of the small number of I/O devices.Thus, the program size
reduced and decoding speed increased.
Isolated I/O (Contd)Extra cost for the separate sets of
Memory-mapped I/OOnly one set of read and write signals and
common address bus for both memory and I/OThe CPU treats an
interface as being part of the memory.
Memory-mapped I/O (Contd)Each interface is assigned an address
same as the one in the physical memory space. The physical memory
space cannot be used.Therefore, the total number of memory address
range is reduced.
Memory-mapped I/O (Contd)Usually, a segment of memory address
space is reserved for interface.References to these addresses will
be ignored by the memory and responded by the specific I/O
Memory-mapped I/O (Contd)No specific input or output
instructionsAll access to the I/O can be made used of the large
amount of memory-reference instructions.Increases the flexibility
for the programmer to design programs.