Download Updated Chapter 4 Input Output Devices

CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
Chapter 4- Input / Output Devices
Q.Write short note on Keyboard
Ans.One of the most basic system component is your keyboard. The keyboard is the
primary input device. It is used for entering commands and data into the system. The
keyboard consist of set of key switches there is one key for each letter , number , symbol
etc. Using a keyboard, a person can type a document, use keystroke shortcuts, access
menus, play games and perform a variety of other tasks.
When a key is pressed the electronics circuit inside the keyboard determine which
key is pressed and generate corresponding ASCII code this process is called encoding.
When the key is pressed microprocessor gets interrupted and jumps to ISR
program present in PCROM BIOS which gives respective scan code for keyboard pressed.
There are various types of keyboards depending on number of keys present on the
keyboard and various switches used in keyboard. There are four types of keyboards
depending upon number of keys.
83 key stroke keyboard(PC and XT keyboard)
84 key stroke keyboard(AT keyboard)
104 key stroke keyboard(Enhanced keyboard)
105 key stroke keyboard(Enhanced windows keyboard)
Most keyboards have between 80 and 110 keys, including Functional keys ,
control keys, Numeric keys , and Typing keyboards.
Q.List the different keys used in keyboard
Ans.Types keys used in keyboard are
1.Capacitive Key Switches
2.Hall Effect switches
3.Opto-electronics key switches.
4.Membrane Key Switches
5.Mechanical Key Switches
6.Foam Key Switches
7.Rubber Dome Key Switches
Q.Describe the constructional details of Capacitive and Hall effect key switches
Ans. Capactive Key Switches
A capacitive keyswitch has two small metal plates on the printed circuit board and
another metal plate on the bottom of a piece of foam. When the press the key, the
movable plate is comes closer to the fixed plate.
This changes the capacitance between the fixed
plates. Sense amplifier circuitry detects this
change in capacitance and produces a logic level
signal that indicates a key has been pressed. The
main advantage of a capacitive switch is that it
has no mechanical contacts to become oxidized or
dirty. And
disadvantage is the specialized
circuitry needed to detect the change in
capacitance. Capacitive key switches typically
have a rated lifetime of about 20 million
keystrokes.
Hall Effect Key Switches
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
This is another type of switch which has no mechanical contact. These switches
are based on the “Hall Effect” which specifies that when a magnetic field is applied to
any device perpendicular to the direction of the current flow inside the device the
resistance of the device increases and when this magnetic field is removed the resistance
decreases or the current starts to flow without any resistance.
This effect can be used as a switch to turn the current flow on and off. The current
flow can be turned off by applying a very strong magnetic field to completely block the
current flow through the deviceThis type of switch does not contain any mechanical contacts. It is more reliable
than ordinary mechanical switch. The switch will not deteriorate with the use also the
contamination of the contact will not have any effect on the working of the switch.
It takes advantage of the deflection of a moving charge by a magnetic field.
Hall effect keyboards are more expensive because of the more complex switch
mechanisms, but they are very dependable, and have typical rated lifetimes of 100
million or more keystrokes.
Opto Electronics switch
Switches are based on optical and electronic device. As shown in Fig. this type of
switches have a LED which generates light when proper electric power is applied.
Opposite to the LED, a photo-transistor is used. The property of photo-transistor is such
that it allow the current flow in the circuit, as long as light is applied to it. When the
light following to the photo-transistor is removed, it will no longer allow the current to
pass through it.
In this type of switch, when the key is not pressed, the light from LED falls onto
the photo-transistor. This makes the current to flow through the photo-transistor and
produces very low voltage at the output Vout
When the key is pressed, the light emitted from the LED is blocked. This will stop
the current flow through the phototransistor and the photo-transistor go in to a cut-off
condition. In this condition the current can not flow through the photo-transistor and a
different value will be produced at the output Vout
Logic 1 and Logic 0 is interpreted by the keyboard circuit as key being open and
key being close, respectively.
Membrane keyswitch
They consist of a three-layer
plastic or rubber sandwich as shown in
Fig. The top layer has a conductive line
of silver ink running under each row of
keys. The middle layer has a hole
under each key position. The bottom
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
layer has a conductive line of silver ink running under each column of keys. When you
press a key you push the top ink line through the hole to contact the bottom ink line.
The advantages of membrane keyboards is that they can be made as very thin,
sealed units. They are often used on cash registers in fast food restaurants, on medical
instruments, and in other messy applications. Lifetime of membrane keyboards varies
over a wide range.
Mechanical keyswitch
In mechanical switch keys, two pieces of metal are pushed when you press the
key. The actual switch elements are often made of a phosphor-bronze alloy with gold
plating on the contact areas: The keyswitch usually contains a spring to return the key
to the non pressed position. Mechanical switches are relatively inexpensive, but they
have several disadvantages.
First, they suffer from contact bounce. A pressed key may make and break contact
several times before it makes solid contact. Second, the contacts may become oxidized
or dirty with age so they no longer make a dependable connection. Higher quality
mechanical switches typically have a rated lifetime of about 1 million keyswitches.
Q.Explain the design of Keyboard using in PC
Ans.Keyboard is made up of 12 Rows x 8 Columns Capacitive matrix as shown in
figure. This matrix is scanned every 3 to 5 Milli-seconds. 12 pins of 8048 are connected
to matrix through port 1 and port 2.Port 2’s output pins 21, 22 and 23 give 3-bit binary
code as an input to keyboard sense decoder/amplifier. These 3 inputs are named as
select 0, select 1, select 2 as an input to amplifier where they are decoded to activate
particular sense output i.e. sense A through sense H that is connected to keyboard
Capacitive matrix as columns. The condition of decoded sense lines is returned to 8048
through SA(sense amplifier) closed pin.
Fig. Keyboard Design Using 8048 Micro-Controller
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
When the key is depressed, the condition is recognized by reading input lines MD00 –
MD11 and the scan lines sense A through sense H. Following steps are performed
when the key is pressed
1) Event timer program waits for few Milli-seconds and let the key bounce settle out
(de-bouncing)
2) Reads MD00-MD11 after de-bouncing.
3) Store 8-bit scan-code in the internal RAM.
4) If key is pressed for more than half a second, 8048 program causes same scancode to be generated 10 time each second.
5) The internal RAM enables 8048 to buffer up to 20 key scan-code and enable type
ahead for operator.
6) The release of key is also detected by 8048 and the 8th bit output of scan-code is
set to high i.e. it effectively produces a break code, which is 128 decimal value
added to scan-code.
7) The scanning is done in 3 to 5 Milli seconds and at least 20 to 30 Milli seconds
passes between the key entries. The matrix is scanned at least once each
keystroke and incorrect encoding is eliminated.
8) Each key action causes 8048 to generate unique scan-code of 11bit out of which 8
bit is data and 3 bit are framing and control bit..
9) 8048 output 8-bit scan-code on the serial data pin. In this code LSB is sent out
first.
Q.Describer the following terms 1.Keyboard connector
2.Make and Break code
3.Typematic action related to keyboard
The keyboard microprocessor transmits the scancode to the PC system over a serial interface. There are 1. Keyboard Clock
two bi-directional lines between keyboard and PC 2. KeyBoard Data
3
1
motherboard. One line is for serial data bit and other
for clock signal. The other signals between keyboard 3. Reset
5
4
and PC are RESET, GND and VCC. These signals are 4. Ground
2
carried by a shielded 4 wire cable, which is flexible
5. Vcc ( +5V)
coiled cable. In some keyboards, there are 5 wires, the
5th wire is for RESET signal from motherboard.
Figure shows the connector on motherboard side. In PC systems 5 pin DIN (Deutche
Industries Norm) connector is used.
Keyboard lock
The keyboard lock facility is provided by several clones but IBM provides this
facility only in PC-AT. By closing keyboard lock on front panel, a user can prevent entry
from keyboard. When keyboard lock is closed, IRQ1 flip/flop is permanently reset and
shift register is disabled. Hence, there is no effect of keyboard data received from
keyboard. The hardware simply ignores the data and software is not at all aware of this.
This feature is used in ALPS (Automatic ledger posting system) models of PCs used in
bank.
Make and Break codes
The PC keyboard follows a double scancode method for each key. When a key is
pressed ,the keyboard processor generated the scancode corresponding to that key.
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
When the keyboard processor detects the release of the key it generated a break scan
code corresponding to the same key. The make scan code and the break scan code for a
key differ only in one bit, D7.The D7 bit is ‘0’ for the make scancode and it is ‘1’ for the
break scan code .
Typematic action
The PC keyboard repeatedly sends the scan code of a key if it remains pressed for more
then 0.5sec. This action is termed as typematic by IBM. The typematic rate is 10 times
per sec .This action is performed for all the keys except for the SHIFT keys.
Input Device – Mouse
Introduction :The mouse was invented in 1964 by Douglas Englebart, who at the time
was working at the Stanford Research Institute (SRI), a think tank sponsored by
Stanford University. The mouse was officially called an X-Y Position Indicator for a
Display System. Xerox later applied the mouse to its revolutionary Alto computer
system in 1973.
Mouse come in many shapes and sizes from many different manufacturers. Some
have taken the standard mouse
design and turned it upside down,
creating the trackball.
In the
trackball devices, you move the
ball with your hand directly rather
than the unit itself.
IBM even produced a very
cool mouse/trackball convertible device called the trackpoint. The trackpoint could be
used as either a mouse (ball side down), or as a track ball (ball side up). In most cases,
the dedicated trackballs have a much larger ball than would be found on a standard
mouse. Other than the orientation and perhaps the size of the ball, a trackball is
identical to a mouse in -design, basic function, and electrical interface.
Q.List different classification of mouse
Ans. Another most common input device used in the computer field is mouse .In GUI or
operating system, using keyboard commands to carry out different work by computer is
difficult to remember and understand even for experienced users.
A computer user could move on pointing devices. A user can easily move select
different options on the screen using mouse.
Depending
Technology
Mechanical
optoMechanical
Optical
Classificaition of Mouse
on Depending
on Depending
Button
Interface
One
Serial
Two
USB
Three
Proprietary
Port
Scroll
Wireless
Bus
on
/Mouse
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
Q.Describe the Construction Details and Operation of Mouse
Ans.The standard mouse consists of several components:
 A housing that you hold in your hand and move around on your desktop
 A roller ball that signals movement to the system
 Buttons (usually two or three) for making selections
 A cable for connecting the mouse to the system
 An interface connector to attach the mouse to the system
1. The
housing
is
made of plastic and
consists of very few
moving parts.
2. On top ot keep
fingers there are
buttons.
There
may be any number
of buttons, but two
or
three
button
mouse are used.
3. In casing there is
small rubber ball
that rotates as we move the mouse across the tabletop.
4. The movements of this rubber ball are translated into electrical signals by the device
driver software and transmitted to the computer across the cable.
5. The cable can be any length, but is typically between four and six feet long.
6. The positional information gives position of the cursor on the screen and the status of
the button pressed.
7. The connector used with your mouse depends on the type of interface you are using.
8. These Drivers can be loaded separately or in built with the system software
9. Internally, a mouse is very simple as well. The ball usually rests against two rollers,
one for translating the X-axis movement, and the other for the Y-axis.
10. These rollers are usually connected to small disks with shutters that alternately
block and allow the passage of light.
11. Small optical sensors detect movement of the wheels by watching an internal
,infrared light blink on and off as the shutter wheel rotates and "chops" the light.
12. These blinks are translated into movement along the axes.
13. This type of setup is called an opto-mechanical mechanism and is by far the most
popular in use today
Q.Explain constructional details of Mechanical Mouse with neat sketch
Ans.
1. A mechanical mouse shown in the figure , works by the mechanical action of its
different parts.
2. When this mouse is rolled across a flat surface different rollers inside the mouse
move and generate electric signals.
3. These signals are given to the computer and computer converts them into proper
action on the screen.
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
Processing is as follows
1. As a mechanical mouse is
moved across a flat surface, a
ball (made of rubber or
rubber covering over a steel
ball) from the underside of
the mouse and touching the
surface, starts to rotate in the
direction of the movement.
2. As the ball rotates, it touches
and
turns
two
rollers
Fig : Mechanical Mouse
touching the ball inside the
mouse. These rollers are mounted at a 90-degree angle to each other.
3. One roller is used for the back-and-forth movements of the mouse, which correspond
to vertical movements of the cursor on screen.
4. The other roller is used for sideways movements, which correspond to horizontal
movement of the cursor on the screen.
5. Each roller is attached to an encoder, which is a wheel like structure. As the rollers
turns these encoders rotate with them.
6. Two pairs of contact bars touch the small metal contact points provided on the rims
of each of these encoders.
7. Each time a contact bar touches a point, an electrical signal is generated.
8. The number of these signals indicates how many points the contact bars have
touched. As the mouse is moved farther away, more signals are generated.
9. The direction in which the mouse is moving can be found out by finding the number
of signals from these two vertical and horizontal rollers.
10. The signal generated by these rollers are sent serially to the PC over the mouse's
tail like cable.
11. The mouse driver software converts these signals into the distance, direction, and
speed necessary to move the screen cursor.
12. Also pressing of any of the mouse buttons sends a signal to the PC. Based on
which button is pressed,
13. How many times you have pressed the button and the position of the cursor at the
time of the pressing the button, the mouse driver software performs the task you
want to accomplish.
Fig : Opto Mechanical Mouse
Q.Describe the construction and
operation of Opto Mechanical mouse
Ans.Only difference is in the sensing
circuit used to detect the movement of
the mouse.
In a mechanical mouse a decoder
is used to detect the distance mouse is
moved on the tabletop, whereas in a
opto-mechanical mouse a combination of
LED (Light Emitting Diode) and Photo
detectors is used to sense the distance
traveled by the mouse.
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
Operation of Opto-Mechanical mouse :
1. When the mouse is moved, the ball at the bottom of the mouse moves and it turns
two separate rollers fixed at 90 degree to each other.
2. One of these rollers is associated with back-and-forth movement of the mouse, i.e.,
vertical movement of the cursor on the screen.
3. The other roller is associated with the side-to-side movement of the mouse, i.e., the
horizontal movement of the cursor on the screen.
4. Each roller is further connected with a wheel. These wheels are rotated by the
corresponding movement of the rollers.
5. There are small openings on the rim of each wheel. As the wheel rotates, a pair of
LED and Photo detectors detect the number of openings passed between them.
6. Each opening on the wheel allow the light from the LED to fall on the Photo detector
and generate an electrical signal.
7. The number of signals generated indicates the number of openings in the rim of
wheel passed, i.e., the more is the number of signals, the farther has been the mouse
moved.
8. These signals are passed to the PC through the wire connecting mouse to the main
system.
9. The PC, in turn, passes them to the mouse driver software which then converts them
into distance, direction and speed required for the movement of the screen cursor.
10. Depressing any of the mouse button also produces a signal. Depending upon the
button being pressed, number of times the button being pressed and the present
location of the cursor on the screen, the software accomplishes the task desired by
the user.
Q.Describe Optical Mouse and its constructional details
Ans.
1. In this type of mouse, instead of the customary
ball and rollers, a light source and photodetector is used with a special mouse pad.
2. When the optical mouse is moved on this
special pad, light from the light source gets
reflected from the pad and special photodetectors inside the mouse detect the
horizontal and vertical movement based on the
reflected light received.
3. One of these photo-detector is used to detect
Fig : optical Mouse
the back-and-forth movement of the mouse,
i.e., vertical movement of the cursor on the screen and the other photo-detector is
used to detect the side-to-side movement of the mouse, i.e., the horizontal (left to
right and right to left) movement of the cursor on the screen.
4. The movement of the cursor on the screen depends on the number of the signals that
is passed to the PC through the wire connected with the mouse.
5. The PC in turn, passes them to the mouse driver software which then converts them
into distance, direction and speed required for the movement of the screen cursor.
6. Depressing any of the mouse button also produces a signal which is passed to the PC
and the PC passes it to the software.
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
7. Depending upon the button being pressed, number of times the button being pressed
and the present location of the cursor on the screen, the software accomplish the
task desired by the user.
Scanners -Introduction
1) Scanner are used for getting existing graphical images, photos, signature, logos of
companies, paintings, drawing into the computer
2) Once these images are scanned & brought in the computer these can be included in
the documents or we can edit them using image editing software like paint brush,
coral draw etc.
3) A scanner is useful for optical character recognition (OCR).
4) Scanner is also usefull when interfaced with fax card so that document can be faxed
after scanning.
5) Hand held scanner are cheapest and smaller in comparision with flat bed scanner.
Q.List the classification of Scanner
Ans.
Classification of Scanner
Based on the type of image that Based on scanner
a scanner can scan and store
working method
into the computer
Black and White
Hand Held
Gray Scale
Flat Bed
Color
Drum
Sheet Feed
Based on
Interface
Parallel
Serial
USB
B/W scanner
1) It scans image as the combination of black & white colour i.e there are no shades of
any other colour.
2) This type of scanner is used to scan line drawing, or sketches etc.
3) This type of scanned image take only one bit is needed to define Black/white color
Grey Scale Scanner:
1) Used to scan b/w photographs because gray shade can recognize Black and white
shade.
2) These scanners can scan & store different shades of grey colour available in
photograph.
3) This type scanned image uses 3 to 8 bit to define gray shade of the pixel
Colour Scanner:
1 Used to store colour diagrams or photographs in computer.
2 A scanner scans any image as a series of separate dots. Each of which can be b/w or
can have level of Grey as well as colour image
3 This type of scanned image take upto 24 bit to store intensity of each pel
4 Color scanner supports upto 16.7 million color (true color)
5 Scanner should be able to recognize minimum 300 dots per inch.
6 These number of dot per inch is called as resolution of scanner.
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
Q.Describe the oeration of FLAT bed scanner
Flat bed Scanner:
1) These scanners are more expensive than
hand held scanner.
2) These scanners look & work like photo
copier machine.
3) To scan an image one needs to simply put
the image on the flat bed scanner & then
issue command to scan the image.
Advantages:
1) Image or picture is stationary or the mechanism inside the flat bed scanner moves
over the image the picture tends to be less distorted as compared to picture available
from hand held scanner.
2) For high quality scan flat bed scanner is used.
Working of Flat Bed Scanner :
1. Paper is placed inside the scanner and scan command is given
2. A light source illuminates the paper placed
3. A motor moves the scan head (source of light)
4. As light source (head) moves it captures the light reflected from individual areas of
the page
5. This light is reflected through a mirror
6. A lens focus the beam of light on the light sensitive diodes ,which converts the light
to appropriate electrical signal.
7. More the reflected light more is the current hence voltage
8. The ADC coverts this analog voltage to digital bits (pixel ) representing picture area.
9. Which is stored in storage device and can reproduced through software as an when
needed
10. In grey scanner 8 bit ADC is used which translates image in 256 level of gray
shades
11. In Black and White scanner 1 bit is ADC translates image in 2 level i.e. black and
white color i.e. monochrome
12. In color scanner the light is passed through three filters RED , GREEN ,BLUE
before striking the image.
13. The reflected light is converted into digital information and stored as RGB values.
14. This digital information
then can be processed by
Graphics software or can
given to OCR software so that
it can be converted into text
information.
Q.Write Short Note on Hand
held Scanner
1 This type of scanner is less
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
expensive.
2 It is used to scan small images & store them in computer with this the user requires
some practice for moving the scanner head over the surface of document or image.
Hand held scanner can scan images usually of size 4 to 6” height depending on scan
resolution.
3 This scanner are usefull for scanning small images , small photographs , logos ,
signature etc because of its small size.
Working of Hand Held Scanner :
1. Paper is placed over which the scanner dragged by hand.
2. A LED light source illuminates the paper placed
3. Angled mirror above scanner window reflects this image on a lens in back of
window
4. The Lens focus the beam of light on the CCD (charge Coupled Device) ,
5. The CCD contain row of light detectors , which detects the light and convert into
voltage
6. Voltage generated by CCD is fed to ADC coverts this analog voltage to digital bits
(pixel ) representing picture area.
7. In Grey scanner 8 bit ADC is used which translates image in 256 level of gray shades
8. In Black and White scanner 1 bit is ADC translates image in 2 level i.e. black and
white color i.e. monochrome
9. AS the hand held scanner moves ,a rubber roller keeps the scanner path straight and
rotate the slotted disk.
10. As the disk turns a light shines through the slits and detected by the photo sensor
on the other side of the disk.
11. This sensed signal is send to ADC and then to PC
12. This digital information then can be processed by Graphics software or can given
to OCR software so that it can be converted into text information.
Q.List Scanner specifications
The following points should be considered for scanner. The normal scanner has the
following configurations
1.
Resolution
: 600 x 1200 dpi
2.
Maximum resolution
: 9600 x 9600 dpi
3.
Maximum scan area
: 8.5” x 11.7”
(W x H)
4.
Bit depth
: 36 bits/pixel with BET
5.
Interface
: SCSI, USB
6.
Transparency adapter : Inbuilt.
Q.Describe the
Block Diagram of
Scanner
Ans.
1. A light source
illuminates a
piece of
paper placed face down against a glass window above the scanning mechanism.
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
2. A motor moves the scan head beneath the page. As it moves, the scan head
captures light reflected from individual areas of the page.
3. The light from the page is reflected through a system of mirror A lens focuses the
beams of light onto light-sensitive diodes that translate the amount of light into
electrical current.
4. The more light that’s reflected, the greater the voltage of the current. White spaces
reflect more light than black or coloured letters or images.
5. An analogue to digital (A-D) converter converts each analog reading of voltage as
digital pixel representing the scanned area.
6. A-D converter on a monochrome scanner stores only 1 bit per pixel, either o or off,
representing black or white.
7. If the scanner is a colour scanner then the scan head makes three passes under
the images and the light on each pass is directed through a red, green or blue
filter before it strikes the original image.
8. The reflected signals from these three pass are converted into d information and
stored to represent red, green and blue colour value of the scanned area on the
page.
9. This digital information is sent to software in the PC, where the data is stored in a
format with which a graphics program or OCR work.
Modem – Modulator – Demodulator
Q.What is Modem and State its Neccessity
Ans.It is the circuit that converts digital data signal into analog data signal (carrier
based) during transmission and converts analog data into digital data during reception.
Necessity Two computers can communicate with each other through telephone lines,
but telephone lines are designed to handle voice whereas the digital signal has rise time
in nanoseconds, therefore requires a bandwidth much greater than that of telephone
lines. In order to enable the computer to use telephone line there should be mechanism
for converting digital signals into analog signals and viceversa. For this purpose a
modem is used.
Using the Modem
The digital information processed is translated or modulated into audib1e sounds that
are carried across lines to remote location computers. At these locations the sound
signals coming from the telephone lines are fed to the receiving modem. This modem
converts these signals back to digital signals by demodulating them.
The Voice/Fax/Data Modern connects the computer. to Internet, and all kinds of BBS,
and other Fax/Modems.
Modem are classified as
1.Internal Modem
2.External Modem,
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
Q.Describe operation of Internal Modem with respect to block diagram
Ans. The internal modem is a stand-alone board that plugs directly into an ISA or
PCI expansion slot.
Transmitter:The internal
modem contains its own
universal
synchronous
receiver
/
transmitter
(UART). The “UART” work
here as serial port.
When installing an internal
modem, care should be
taken that the IRQ number
and I/O address port
number do not conflict
with inbuilt motherboard
serial ports. (i.e COM1 and
COM2).
A modulator circuit converts the serial digital data from the computer into analog
signals to be transmitted over telephone line. The analog signal is then coupled to the
telephone line Sing a circuit very similar to that used by ordinary telephone to couple
voice signals. Then this analog signal passes to the telephone line through telephone
jack (RJ-11 connector socket).
Recevier : On receiver side, serial signals received from telephone line. The telephone
interface separate received signals and passes them to the demodulator. The
demodulator. converts analog signals into digital form and send this serial data to
UART. The UART convert serial bit data into parallel byte and placed-on the system’s
data bus.
When a remote modem dials-in, the telephone interface detects the incoming signals and
alerts the UART to begin to setup a connection. Finally the telephone interface drives a
small speaker, during final stages of modem operation.
The speaker is used to hear a dial tone, dialling signals and audio setup between the two
modems. Once a connection is established, the speaker is usually disabled.
A controller circuit manages the overall operation of the modem. Generally it is used to
manage modem between in control and data operating modes.The NVRAM is non volatile
RAM, it is used to store modem parameters.
Q.Describe
the
operation
of
External Modem with neat block
diagram
Ans
external modem is a stand alone
device connected to serial port
(COM1 or COM2) of system. Fig.
shows block diagram of external
modem.
Working of External modem is as
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CAM-Chapter-4 Input and Output Devices / Prof.Manoj kavedia / 9423088039 / 9860174297
same as internal modem. The difference is that it uses standard serial interface RS
232C, so that it uses motherboard UART for serial bit to parallel byte or parallel ‘byte to
serial bit conversion.
Advantages
 Installation modem is easy.
 Not required/needed to open the system.
 No worry about IRQ and I/O address conflicts.
 Easy to configure.
 Front panel LEDs of external modem shows status of communication.
Disadvantages
 Externally powered.
 Serial interfacing cable generates problem.
Q.Compare Internal and External Modem
14