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Input & Output Devices

Data input and output

There are many examples of input devices such as the scanner, keyboard and more - if data is send to the the system from a device this is a considered as an input device.

There are many examples of output devices such as speakers and storage devices - if data is sent out by the processor it is an output device

General and special purpose computers

General purpose computers are computers which are used in day to day life like your laptop

They can be used to do a variety of jobs such as watching videos and games

There are also computers which are the size of chips and are usually installed on large devices such as a car

These are known as embedded systems or special purpose computers which are able to perform a specific job or function

As usual a computer must have a processor and I/O system and a memory unit. Usually this is built in to one chip and is usually called a microcontroller.

Embedded systems are usually installed on devices to connect the devices to the internet and provide more functions

However as the embedded systems doesn't have an Os and only some simple code, it makes it easier to hack them

Main or primary memory

The computer has main memory which is divided to two types of main memory RAM and ROM


This is called random access memory

They are used to store programs and part of the Os currently in use

Data can be written to or read from many times

They are volatile (data is lost when the power is switched off) and so they store data temporarily

There are two types of RAM:

  • DRAM
  • Dynamic RAM

    This type of RAM stores data in capacitors and transistors where they are contantly charged to prevent data leaking out and losing their identity

    This usually has a greater capacity than SRAM

    They are much cheaper than SRAM but more slower to access

    Also DRAM uses less power

    DRAM has more memory cells per area than SRAM

  • SRAM
  • Static RAM

    Data are stored in flip flops

    Has a very fast access time and thus used for cache memory

    Stores less data than DRAM and is more expensive and uses more power than DRAM


    Read only memory

    Data can only be read from many times but can not be written

    It stores boot up instructions such as the BIOS

    It is non volatile memory and so it is permanent

    Usually ROM chips contains the instructions when the chips are manufactured but the other 3 are slightly different

    There are three type of ROM:

  • PROM
  • Programmable ROM

    The chips are given to the people who installed the code. Once installed it can not be changed and must be replaced

  • Erasable PROM

    This are ROM chips which data can be erased in the presence of UV light, however the chip must be removed from the circuit board

  • Electronically erasable PROM

    Data in the ROM can be removed using an electric signal and data can be written to it, but this is done very rarely


    A place made in the memory where data is temporarily stored when moving data from one location to another which functions as a queue

    When data is transmitted at a faster rate than it is processed or displayed ( Bit streaming ) then data must be temporarily stored as a queue

    Secondary memory

    There are many types of secondary memory, we will discuss them

    Magnetic strips and magnetic cards

    These store data on magnetic strips as there are varying magnetic field dots across the strip.

    Depending on the magnetic field the dots can either store 1 or 0

    So usually a read head is used to read the data and due to the varying magnetic fields it induces and a varying current

    Hard disk

    This also uses magnetism to store data

    Data is stored in many disks called platters which are spun at high speeds

    The disc are usually made out of aluminium plated with glass

    There is a read-write head on each side of the platter which either reads or writes data using the magnetic properties - this done by varying the electric current in the write head

    Data is stored in concentric tracks(circle) in each platter

    The tracks are divided into sectors and the sector contains the smallest unit of storage.

    Data in the hard disk are stored in blocks

    There is a confusion between a disk sector and a track sector

    When we talk about sectors, we are talking about the track sector which is a sector a single track and not the whole disk

    This shows a detail diagram of a hard disk which must be known according to the Cambridge computer science

    The above points are enought to score full marks

    For more information - similar data are stored on different disc of the same sectors

    Data are stored in sequence on the concentric tracks

    Optical media

    This uses a way different property and this is used by CDs and DVDs and more

    The CD - ROM and DVD - ROM have slightly different mechanisms which we will discuss

    • CD - ROM
    • Uses a red laser light to read data

      This is done by the read/head

      The disc spins at high speeds

      This stores data in spiral tracks

      The surface contains pits and lands

      The red laser is reflected by the surface. The pits reflect less light than the lands - this is then decoded in to binary code

      This shows a detail diagram of a optical media device which must be known according to the Cambridge computer science
    • DVD - ROM
    • Data is stored in concentric tracks so data can be read or written simultaneously

      Uses red laser however the wavelength is slighlty smaller

      DVD - ROM contains two discs so it can store more data

      The data is read using read heads

      The disc is also spin at high speeds

      This also stores data in pits and lands

    • CD - RW
    • A disc which is rewritable means that data can be burned/recorded and also erased

      The surface of the disc contains a special alloy

      When the red laser in focused on to the surface the alloy it forms a liquid which either cools down to a crystalline solid or a amorphous solid depending on the intensity of the laser.

      The crystalline solid reflects more light than the amorphous solid.

      This can be decode into binary data

      This process of storing data is usually known as burning

    • Blu-ray
    • This uses a blue laser light which has a shorter wavelength

      This means the light can be more focused and so the pits and lands are closely together and so higher storage

      Solid state media

      This is the technology used by flash sticks and pen drives

      This stores data in transistors called memory cells

      This controls the movement of the electrons through the ciruits using either NAND or NOT gates

      NAND Gate is used most frequently than NOT whereas NOT gates are very expensive

      Also data in the memory cells could be all deleted and written at once

      The solid state drive which is used instead of the hard disk uses the same solid state technology and there are many advantages over the Hard disk drive

      1. The main advantage of using SSD over HDD is that SSD has a faster access time so it is usually fast

      2. Also SSD does not have any moving parts and so it is more reliable and more long lasting

      3. Also SSD is more lightweight and portable as no moving parts, also it uses less power

      We will need to know some advantages of HDD over SSD

      1. The main advantage of HDD is that it usually has a higher capacity than SSD

      2. Also HDD is very durable under heavy use and thus used in gaming

      3. Cheaper than SSD


      Registers and cache memory has the smallest storage and they are highly expensive but has a very fast access time

      Secondary storage has a high storage and are cheap but has a slow access time

      Screen Displays

      There are 3 main types of displays

      • CRT
      • This controls the direction of the electron to display images on the screen

        The screen is coated with a substance which when an electron collides with it a light is produced

        Each dot on the screen is made when electrons collide with the fluorescent screen and glows

        The screen is full of dots which produces an image

        This is the old technology of screens and it is not used now

      • LCD/LED Screens
      • The screens is made out of pixels however each pixel is divided into sub pixels which have the color red green blue and sometimes yellow

        The Liquid crystal display is able to change the structure in order to polarise light differently by passing a voltage through it

        Usually the backlighting is from LED bulbs that is why the newer versions are called LED Tvs.

        The older technology is uses CCFL bulbs as backlightings and they were called LCD screens

        The white light is able to change color as it passes through the LCD display and so this is done by controlling the voltage passed in each pixel

        Finally there is a polariser(filter) which makes the image more clearer and crisp

      • OLED
      • Organic LED

        This does not require any backlighting and so the screens are very thin

        The screen contains and anode which emits there own light which causes an image on the screen

        These are made from polymers which are very flexible

        OLED screens are very expensive

      Virtual Reality Headset

      This contain both input and output components

      The displays and speakers in the VR headset are output devices which provide the user with 3D graphics created by the CAD software

      The sensors in the VR Headset are input devices which feed data about your position and all


      There are two main types of printers which we will discuss

    • Inkjet printers
    • These spray ink using small printer nozzles - this is done by either using thermal bubble technology or the piezoelectric technology

      The ink is sprayed line by line until the whole document is printed

      Usually inkjet printers are cheaper than laser however the ink costs more

      Also inkjet printers are relatively smaller than laser printers

    • Laser printer
    • This uses the idea of charge and printer toner to produce hard copies

      We will need to know each step in the right order

      1. Data is sent to the the printer driver to convert it to a form the printer understands

      2. Data is stored in the buffer

      3. Firstly the drum is given a charge and it starts rotating

      4. A red light is switched on and off on the drum as it rotates - when it switches on it discharges the charge on the drum

      5. The drum rotates step by step until a full electrostatic image is formed - the discharged areas are the content to be printed

      6. The printer toner which is also given a charge sticks to the discharged areas of the drum

      7. A charged paper is fed in and the toner sticks to the paper then the paper is passed through heaters which fuses the toner to the paper

      8. If you need to print color the paper must go through the printer several times for each color (CMYK)

      The advantage of laser printer is that it is very fast and also has a high quality

      However, every large and expensive compare to the inkjet printers

      The printing cost on the other hand is cheaper than inkjet

      Printing Colour schemes

      Usually images are displayed using the RGB format, however in t printed documents color is printed using the CMYK format

      C - CYAN . M - MAGENTA . Y - YELLOW . K - BLACK

      So different levels of cyan, magenta and yellow are used to produce color documents

      So to print a colored document it must go for each type of main color

      3D Printers

      These are used to create 3D objects which are created from of software called the CAD or the CAM(computer aided manufacture)

      The printer contain nozzles which sprays the powder or the material and nozzles to spray the glue

      The design of the 3D object is made from many layer designs and so the onject is created by layer by layer until the full object is made

      After this the object must be finished or cured - final touches


      This is an input device

      We will need to know the mechanism of this

      The keyboard contains wires which are aligned horizontally and vertically. Every key on the keyboard has its own key matrix which when clicked can be uniquely identified

      When a key is pressed this causes the wires to be pushed and cause the wires to contact with each other

      When the wires contact together it completes a circuit and the signal is sent to the microprocessor

      The processor determines the position of the contact and sents a signal to the keyboard ROM and accesses the corresponding character code and then send it to the computers processor

      Remember this processing is done by the keyboard itself

      The ROM contains the character codes whereas the microprocessor identifies the position of the contact


      This is a bit confusing but touchscreens are input devices this is because we usually talk about the input mechanism and not the display

      There are two main types of touchscreens but we will discuss the other ones too:

      • Resistive
      • Touchscree made out of two conducting layers where when touched causes the top flexible layer to contact with the lower layer and so it completes a circuit

        So once the circuit is complete the microprocessor detects the position or coordinates of the touch

        This is then decode to binary

        The main problem with this technology is that it doesn't support multitouch and also the screens have to be pressed quite hard

        However, this technology works with gloves and doesn't require hands

        It is also the cheapest type to manufacture

      • Capacitive
      • Touchscreen in which when the screen is touched it undergoes a capactive or electric change and this is then detected

        Moreover, the screen layer conducts electricity and so can change its electric state

        When we touch the screen the electric charge is flowed from the screen to the finger and so there is an electric change and this is detected by the sensors on the corners of the screen

        Then the position of the touch is calculated and decoded into binary data

        The good feature of this technology is that it can support multitouch if the screen is made out of an array of capacitors, which means the screen is divided in to cells which can undergo its own electric change

        The main problem of this technology is that it can not be used with gloves and it requires a finger or a capacitive pen

      Also the technology is relatively expensive to manufacture

      Other technologies

      Other screen technologies include infrared and sound waves

      When a finger is pressed the infrared signals or sound waves are blocked or weakened and detected by the sensors

      The position of the touch is calculated and decode to data by the processors


      In order to convert a hardcopy to a softcopy we need scanners

      Usually the paper or the document is fed in and a bright white light is shined over this document

      The reflected light from the document is focused using lenses and mirrors to a light sensitive screen known as the CCD - charge couple device

      The CCD contains smalls photoelectric sensors which converts the light into electric signal. Also the electric signal produced is proportional to the intensity of the light

      Each cell or sensor of the CCD represents a pixel when converting and storing the image as BMP or JPEG file

      Also the electric signals from the sensors must be converted to digital(binary data) using an ADC convertor

      Each cell or sensor of the CCD represents a pixel when converting and storing the image as BMP or JPEG file

      Microphones and Speakers

      The reason why I combined this is because they are basically the reversed process

      • Microphone
      • This is an input device

        You will need to know the components in a microphone

        1. Diaphragm

        2. Coil

        3. Magnet

        When the sound is entered due to vibrations in the air, it causes the diagraphm to vibrate and so the coil vibrates inside a magnet

        This induces an alternating current which is then converted to digital data using an ADC convertor

      • Speakers
      • This is an output device

        You will need to know the components in a speaker

        1. Diaphragm

        2. Coil

        3. Magnet

        They have the same components however most of the time they have extra components such as a filter and a amplifier

        This is basically the reversed process. Data or signals sent from the computer must be converted to analogue data using DAC convertor and so then the varying electric signals causes the coil and the digraphm to vibrate and cause sound

        Rarely asked devices

        We will discuss some rarely asked devices

        Graphic plotter

        This device is an output device which is used to display the vector graphics on to a paper

        This uses a pen which can move to draw the vector graphics and also the paper can move. This produces high quality vector graphics on to paper

        3D scanners

        This is an input device

        This uses the principle of tomography where the objects is broken down to small layers and stored in the computer

        Usually the layers can be viewed using the CAD software

        This technology is used in MRIs

        2D and 3D cutters

        This is an output device

        This uses either laser or high pressure water to cut objects and the information is given by the computer


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