It's all about how computer takes input or give output. Covered Topics : Bus > Serial Port, Parallel Port, USB Port Input Units: > Keyboard, MICR, OMR, OCR, OUput Unit: > Printer (Types of Printer and work operation), CRT

1. Computar Fundamental and
Organization

2. Unit 3: Input Output Devices

3. Bus
• It is set of parallel lines used to connect two or more devices of a computer.
• All buses are used to transfer data in the form of bits from one component to
another component.
• Types of Bus
– Internal bus
– External bus
• Internal bus
– A bus or set of wires which connects the various components inside a
computer is known as internal bus.
– It is used for internal communication purpose. It connects various components
inside the cabinet, like CPU, memory and motherboard. It is also known as
system bus.
• External bus
– A bus or set of wires which is used connect outer components to computer is
known as external bus.
– It allows different external devices to be connected to computer.
– It is slower than internal or system bus. It is also known as expansion bus.

4. Serial Port
• It is serial communication interface through which information transfers in or out
one bit at a time.
• Throughout most of the history if the personal computers, data was transferred
through serial ports to devices such as a modems, terminals and various
peripherals.
• It contains 9 pin.
• Pin 1 and pin 5 on computer’s port connect respectively, to pin 8 and pin 7 on
modem port. Pin 1 and 8 are share common ground connection. Pin 5 and 7 let
the PC detect a phone-line signal.
• Pin 6 sends a signal that data is ready to be send.
• Pin 4 on PC connect to pin 20 on the modem. It signals that the PC is working
properly to receive data.
• Pin 7 on PC connect to pin 4 on modem and modem create request that modem is
ready to receive data.
• pin 8 on the PC sends a signal to pin 5 on the modem when PC is ready to receive
data from modem

5. Cont…
• pin 2 se on PC sends a data to pin 3 on the modem only one bit either 0 or 1 bit
can travel along a wire at a time. The fact that data is sent serially gives the
connection its name.
• Pin 3 on PC receives data from pin 2 on the modem. Again the bit can move
through the wire, only one bit at a time.
• Pin 9 on the PC connects to pin 22 on the modem to detect a telephone ring.

7. Parallel Port
• A connector for a device that sends or receive several bit of data
simultaneously by using more than one wire.
• It is also known as a printer port.
• It contains 25 lines.
• A signal to the PC on line 13 tells the computer that the printer is ready to
receive data.
• Data is loaded on line 2 through 9 in form of “high” voltage signal
– About five voltage signal signify a 1
– Nearly zero voltage signal signify a 0
• After the voltages have been set on all lines, line 1 sends a strobe signal to
the printer for one microsecond to let the printer know that it should read
the voltage on the data lines.
• A signal from the printer on line 11 tells the PC when the printer is too
busy so PC should refrain from sending the next byte until the signal is
cleared. A busy signal might be generated because the printer is printing
the last character, the buffer is full, there is paper jam, or any other
condition that prevents the printer from using any further data.

8. Cont...
• A signal on line 10 acknowledges about receiving the data set on line 2
through 9 and tells the PC that the printer is ready to receive another
character.
• Line 12 sends a signal from the printer to the PC if the printer runs out of
the paper.
• The printer uses line 15 to tell the PC some error condition exist, such as a
jammed print head or an open panel. It doesn’t specify the nature of error.
• A signal from the PC on line 16 causes the printer to reset itself to its
original state-the same as if the printer were turned off and on.
• A low voltage or zero voltage signal from the PC on line 14 tells the printer
to advance the paper line when it receives a carriage return code . A high
voltage signal tell the printer to advance the paper line only when it
receives a line advance code from the printer.
• A signal from PC on line 17 tells the printer not to accept data. This line is
used only with some printers, which are designed to be switched on and
off by the PC. Line 18 through 25 are simply grounded lines.

10. USB port
• Universal Serial Bus (USB) controller – a set of specialized chips and
connections- act as an interface between software and hardware.
Operating system, and device drivers – which provides details about how
particular hardware devices work – sends commands and data to the USB
host hub, located on the controller.
• Leading from the host hub are special USB connectors, or ports. Matching,
four wire cables plug into the ports.
• The cable can attach to another hub, the only purpose of which is to
provide more ports to which USB devices are attached – sort of a digital
extension card.
• or a cable can lead directly to the USB device, such as webcam. It support
connections for every type of external peripherals like keyboard, mouse,
modem, external hard drive, microphone, scanner and printer. Two of the
four wires in the USB cable are used to supply a limited amount of
electrical power to peripherals, sometimes eliminating bulky power
supplies.

11. Cont...
• Other two lines called D+ and D- are used for sending data and
commands. A high voltage on D+ but not on D- is a 1 bit. A high voltage on
D- but not on D+ is a 0 bit.
• Any USB device can also include a hub, like a monitor. Suppose monitor
contain USB port for multimedia speakers, microphone. These devices can
provide ports for further USB hardware.
• when new USB device is plugged into a port, it automatically causes a
voltage change on one of the two data wires. If the voltage is applied to
D+, the peripheral is saying that it’s a high speed device, capable of
sending 12 megabits per second, used for monitors, scanners, printers,
and other devices that send a high volume of data. D- indicates it can get
by with a slow transfer speed of 1.5MBPS, for keyboard or mouse.
• It allows automatic configuration of internal PC components, the USB host
controller tells the new device to identify itself, finds out what it requires
for sending and receiving data, and assign the device an identification
number.

12. Cont...
• Now the new device is official device of the bus, it takes place in device to
issue command, to ask whether the device is ready to send or receive
data. It sends queries or command to all device on USB. All messages are
begins with a token that identifies to which device it’s addressed. Message
goes to all device on bus, but don’t match the token address simply ignore
it. Device send data to host only after host gives them permission.
• USB can work with three types of data transfer with their priorities.
– Highest priority Isochronous, or real time where there can be no
interruption in the flow of data, such as video or sound.
– Second highest priority interrupt transfer, which occur only when a
device, such as a keyboard or joystick, generates as occasional
interrupt signal to get the processor’s attention.
– When time permits priority bulk transfer of data for printers,
scanners, and digital cameras, in which there’s a lot of data to send
but no particular hurry to get it there.

14. Input Units: Keyboard
• The most common input unit is a keyboard used for manual data entry.
• It is called QWERTY keyboard because these are the first six letters in third
row from top.
• The arrangement of letters was standardized for mechanical typewriters in
the last century but has continued as most typist are used to it and there
is unwillingness to change.
• Keyboard consist of the following major categories of keys:
– Letter keys: these are the 26 letters of English alphabet arranged as in
a typewriter.
– Digit keys: there are two sets of digit keys; one on the second row
from the top of the keyboard just as typewriter, and the other is
numeric keypad at the bottom right which allows quick entry of
numbers with the fingers of one hand.
– Special character keys: these are characters such as
<,>,?,/,{,},[,],(,),.,”,.!,@,#,$,%,^,&,*,-,+,=,_. Most of this printed when
shift key on CPU is pressed down and the key on which it is written is
pressed.

15. Cont...
– Non-printable control keys: this are used for backspacing, going to
next line, tabulation, moving the cursor up or down, insert, delete
characters etc. There is also a spacebar at bottom for leaving space.
– Function keys: these are labelled F1, F2, upto F15 and when pressed
invoke programs stored in the computer.
• Functions of non-printable control keys are listed below:
– Backspace key: this key backs the cursor to the previous character and
deletes the current character. If it is pressed 3 times the 3 previous
characters are deleted.
– Enter key: at the end of a line of typing, this key is pressed to go to
next line.
– Arrow keys: moves the cursor up, down, back and forward.
– Tab key: moves the cursor to the next tab stop.
– Shift key: when kept pressed and a letter key is pressed, the upper
case letter is printed.

17. MICR
• Magnetic Ink Character Recognition (MICR)
• In this method, human readable characters are printed on documents
using special magnetic ink. A magnetic ink character reader can recognize
such characters.
• In a cheque, the cheque number and branch code are pre-printed at the
bottom using magnetic ink. The amount written in the cheque is later
entered by a clerk using a machine ink which prints the amount with
magnetic ink.
• The cheque itself can now read using a special input unit which can
recognize magnetic ink characters.
• This method eliminates the need to manually enter data from cheques.
• Besides saving time, this method ensures accuracy of data entry.

19. OMR
• Optical Mark Reading and Recognition (OMR).
• In this method special pre printed forms are designed with boxes which
can be mark with a dark pensile or ink.
• Each box is annotated distinctly so that the user clearly understand his
response when he darkens a box. Such a document read by document
reader transcribes(type) the marks into electronic pulses which are
transmitted to the computer.
• These documents are applicable in the areas where responses are one out
of a small number of alternatives and the volume of data to be processed
in large. They are used for:
– Objective type answer papers in examination in which large number of
candidates appear.
– Market surveys, population surveys, etc where responses can be
restricted to one or more out of a few possibilities.
– Order forms containing few choice of items.
– Time sheets of factory employees in which start and stop times may
be marked.

20. Cont...
• The advantage of this method is that data is entered in its source and no
further manual transcription(written or printed version) is required.
• This minimizes data entry error.
• The main disadvantage is the need for accurate alignment of printing on
forms and the need for good quality expensive paper.
• The form cannot be redesigned frequently because any change will
require reprinting of the form which is expensive.

22. OCR
• Optical Character Recognition(OCR)
• An optical scanner is used to read an image, convert it into set of 0s and 1s
and store this in computer’s memory.
• The image may be hand written document, a typed or a printed document
or a picture.
• Example:
– The image is drawn on graph sheet. If the image is drawn over an
intersection of a horizontal and a vertical line of the graph, the image
is represented by 0. if not it is represented by a 1.
– The representation of drawing as 0s and 1s is called the bit map of the
image. Each bit in the representation of the image is called pixel.
• A CRT converts an image to an equivalent bitmap representation. There
are two main type of scanner.
– Hand- held scanner
– Flat bed scanner

23. Cont...
• Hand held scanner
– It is around 13cm long and 15cm wide with a handle to enable it to be
held conveniently in hand.
– Material is placed over material to be scanned and slowly drag from
the top to the bottom.
– The light is not reflected by black lines but it is reflected by white
areas. The material is converted and stored as bitmap.
– If the scanner is moved slowly the image is converted into 400 bits per
inch and stored.
– The hand held scanner is expensive device costing around Rs. 1000/-.
– The scanner has to be dragged very steadily and carefully over the
material. If it is not dragged carefully, the conversion of the material
into bitmap will not be correct.
– It is thus is used in cases where high accuracy is not needed.
– Typical application are to store pictures and reproduce them in
publication.

24. Cont...
• Flat bed scanner
– It is consist of box with a glass plate on top and cover which covers the
glass plate.
– The document to be scanned is placed above the glass plate. The light
beam is situated below the glass plate and is moved from left to right
horizontally. After scanning one line the beam moves up a little and
scan the next line.
– It takes about 20 seconds to scan an entire page of size 21cm*28cm.
– The content of the page stored as a bitmap of 400 dots per inch. Each
dot may be encoded as a 1 bit.
– If 256 shades from black to gray to white are to be encoded, then 8
bits are used to represent each dot. It also scan and store colour
images so 24 bit representation for each dot is used to represent a
maximum of 2^24=4 million colours.
– If the scanner is connected to computer than image is stored in it.
– Compression and decompression
– Two popular format: gif, jpeg.

25. Output Unit
• Three principle devices are used to output data from a computer. They
are: printer, video monitor and computer output microfilm. Of this three,
printing is the most common method.
• This units are called hard copy device and another set of output device
called soft copy device like CDROM, pen drive.
• Printers
• There are two type of printers:
– Line Printers
– Serial Char
• Printers are available in almost all scripts like japanese, arabic, russian,
hindi, etc.
• Nowadays both printing method used, both are similar but their speed
and versatility differ.
• Line Printers
– Line printer prints complete line at a time.
– Printing speed is from 200 lines to 2000 lines per minute with 75 to
300 characters.

26. Cont...
– Early line printers were of two type called drum printers and chain
printers.
– Drum printer consist of cylindrical drum on which character to be
printed were typed.
– Chain printer had a steel band on which the character sets were typed.
– Due to inflexibility to change fonts and print graphics this printer have
been known as line matrix printer.
– Both types of printers use print head consisting of pins which are
moved electromechanically to strike a ribbon placed between the
print head and paper. So the black dot is printed when one pin strikes
the ribbon.
– Features of this printer:
• Any one of the several fonts can be selected.
• Any one of the several language besides English can be printed.
• Graphics can be printed.
• Bar code can be printed.
• By using carbon paper between sheets of paper, can make sets of
copies. 6 copies can be printed.

27. Cont...
• Ribbons can be easily changed and reused.
• Printer speed is from 200 to 2000 lines per minute depending on
type of font, language, etc.
– Papers used in line printer
• Papers used in line printer are fan-fold form rather than single
sheets. This paper are called continuous stationary and has holes
on both sides.
• Printer using sprockets which use the holes on the sides of paper.
• Paper with several widths are available depending upon the
application.
• Some of them are pre printed like cheques and examination result
sheets.
– Application of line printers
• This printers are used primarily when large volumes of data to be
printed.
• Example: electoral rolls, pay roll, mark sheets in examination of
thousands of students.
• In this example pre printed continuous stationery is used and
several copies are required.

29. Cont...
• Serial Printer
– Serial printer print one character at a time. They are similar to
typewriters.
– Serial printers are normally slow(30 to 300 character per second).
– A popular serial printer called dot matrix printer. In printer print head
consist of an array of 24 pins arranged vertically. The head is moved
horizontally in small increments to print character.
– A dot density of 360*180 dots per inch is now standard.
– Rather than character these printers can also print graphics. Multi-
colour is feasible by using multi-colour ribbon but it is not popular
because colour quality is not good.
– Character to be printed are sent one character at a time from the
memory to the printer. The character is decoded by the printer
electronics and activates the appropriate pins in the print head.
– These pins are moved forward to form a character and they hit the
carbon ribbon in front of the paper so character is print on paper.
– Many dot matrix printers are bidirectional, that is they print form left
to right as well as right to left.

30. Cont...
– Advantage of dot matrix printer is that they can be converted to print
alphabet other than English. It is possible to adopt them to print
Devnagari script, tamil script, etc. They are also inexpensive because
their running cost is low. Multiple copies can be taken by placing
carbon paper between sheets of paper.
– Character printers are less expensive than line printers but they
cannot be used continuously for more than an hour so they are called
“light duty” printers.
– Line printer and dot matrix printers are called impact printer because
hammer strikes a carbon coated ribbon to print. In impact printers it is
easy to obtain multiple copies by placing carbon sheets between
paper sheets.
– As the cost is low and print and graphics quality are adequate for most
office like small business and government office.
– Most primary disadvantage is that they are very noisy .
– Another variety of printers called non-impact printer. They don't have
hammers but traces a characters to be printed using ink or graphite
particles.

32. Cont...
• Inkjet Printers
– As a character produced by dot matrix printer is made up of a finite
number of dots, the appearance of the printed output is not very
good. For better appearance characters are represented by sharp
continuous lines, this type of printers are known as an inkjet printers.
– It consist of printing head which has number of small holes or nozzles.
Individual holes can be heated very rapidly by an IC resister. When the
resister heats up, the ink near it vaporizes and is ejected through a
nozzle and make a dot on paper near the head.
– High resolution inkjet printer has around 50 nozzles within a height of
0.276 inch and can print with a resolution of 300 dots per inch.
– Electronic system selects the holes to be heated based on the
character to be printed.
– The printer has enough memory to print an entire page with different
fonts.
– Latest inkjet printer has one more head that is colour head which
allows colour printing.
– The printing speed of inkjet printer is around 120 characters per
second.

33. Cont...
– Major disadvantage is that the cost of ink cartridge is high thus cost
per page of printing is quite high about 10 times than dot matrix
printing.
– Multiple copies cannot be taken unlike in dot matrix printer which
uses carbon printer.

35. Cont...
• Laser Printer
– The basic limitation of serial dot matrix printer and inkjet printer is the
need of a head to print characters.
– This mechanical movement is relatively slow due to the high inertia of
mechanical elements.
– Computer manufacturer’s work is to research and development to
eliminate mechanical motion in printers. This lead to development of
laser printers.
– Electrically controlled laser beam traces out the desired character to
be printed on photoconductive drum. Drum attracts an ink toner on
exposed areas. This image is transferred to the paper which comes in
contact with the drum.
– Low speed laser printers which print 4 to 8 pages per minute are now
very popular and cost around Rs. 15,000.
– Very fast printers are also available which prints over 10,000 lines per
minute. At present thy are very expensive. These printer gives
excellent output and can print variety of fonts.

36. Cont...
• Colour laser printers are also available, but they are very expensive.
• Major disadvantage is the high cost of toners required for printing. The
cost is much higher for colour toners.
• The cost per page of printing is highest for inkjet, medium for laser and
lowest for dot matrix printers. As these printers do not have type head
striking on a ribbon, they are known as non-impact printers.

37. CRT
• Cathode Ray Tube (CRT).
• The technology is used in most televisions and computer display screens.
• Digital signal from the operating environment or application software go
to the Super Video Graphics Array(SVGA) adapter. The adapter runs signal
through a circuit called a Digital-to-Analog Converter(DAC). DAC circuit is
contained within one specialized chip that contain the DACs- one for each
primary colour used in a display: red, blue, green.
• DAC compares the digital values sent by the PC to a look-up table that
contains the machine voltage level for the three primary colours needed
to create the color of a single pixel. In normal VGA adapter, the table
contains values for 262. Super VGA adapter have enough memory to store
16bits of information for each pixel (65,536 colors called high color) 24 bits
a pixel (16,777,216 shades- true color).
• The adapter sends signals to three electron guns located at the back of the
monitor’s CRT. Through vacuum inside the CRT