CBSE Notes for Class 9 Foundation of Information Technology – Basic Components of a Computer System
A computer system has three basic components:
- Input Unit/Devices
- Central Processing Unit (CPU)
- Output Unit/Devices
The instructions and the data, on which a computer has to work, are fed to the computer by input unit/devices. There are many input devices like keyboard, mouse, joystick, light pen etc. We will discuss some of these here.
1. Keyboard: It is the most commonly used input device. It has various keys like numeric keys (0 – 9), alphabetic keys (A – Z), function keys (FI – F12), some special purpose keys like Delete, PageUp etc., direction keys represented by arrow and symbol (*, |, 😉 keys.
When a key is pressed, an electric signal exclusive for each key is generated which passes to the control unit. So, the input is converted into a computer understandable code by the keyboard itself. The keyboard we normally use has 101 keys and is called QWERTY as it is similar to the manual typewriter which has Q, W, E, R, T, Y keys on its very first row of alphabets.
2. Mouse: Another very popular input device is the mouse. It is about the size of a human palm and rolls on a small bearing and has one or more buttons on the top. When a user rolls the mouse on a flat surface, the cursor moves accordingly on the screen. Nowadays optical mouse is very much in demand. It uses the reflected light to sense the direction of movement.
3. Light Pen: It is a device for direct data entry. A special pen which has photo cells in its tube is attached to the monitor. When the pen is brought in contact with the screen, it detects the light emitted by the screen and this light activates the photocells. These photocells when activated generate electric pulses. Electric pulses send data to the computer and it can detect which menu option has been selected.
4. Magnetic Ink Character Reader (MICR): You must have seen cheques on which numbers are printed at the bottom. These numbers are written in a special font using a special magnetic ink. When these cheques have to be processed, they are put in a MICR, which reads the magnetic code written on the cheque. MICR can process 40 documents per second. A similar type of magnetic code can be seen on credit cards also. Whenever any transaction is made through a credit card, the magnetic code and details of the transaction and card are captured by the card reader.
5. Optical Mark Reader (OMR): This input device also senses data directly from the document. It is mostly used in the evaluation of multiple choice question papers. The user is required to mark one option out of a given number of options on a specially designed response sheet. The sheet is then exposed to a beam of light and the answers on the sheet are sensed
by OMR. A sample of OMR is shown in Fig. 2.5.
6. Optical Character Reader (OCR): This technique allows direct reading of any printed character. No special ink is required. The characters to be read by OCR are written in a specific font and the printed matter is scanned with a photoelectric device. The light is absorbed by the characters and thus recognized by the OCR.
7. Bar Code Reader: You must have noticed a pattern of bars on the items you buy from the market, similar to the one shown in Fig. 2.7(a). These are called bar codes. These bar codes are read by hand¬held optical readers or fixed bar code readers [Fig. 2.7(b)] and contain the information about the item like the price, code, weight, manufacturer details etc. When the bar code is scanned, reflected light patterns are converted into electrical pulses. They are then compared with the already fed patterns in the computer. If the pattern matches, the data is transmitted to the computer which displays the details of the item and in many cases also updates the inventory and sale record.
8. Graphics Tablet: A graphics tablet is a computer input device that allows us to hand draw image and graphics similar to the way we draw images with pencil and paper. It consists of a flat surface upon which the user may draw an image using an attached stylus, a pen-like drawing device. The image generally does not appear on the tablet itself but is displayed on the computer monitor. Graphics tablets are mainly used for Computer Aided Design (CAD) purposes.
9. Scanner: This is a very common input device which is used to transfer the image of any picture or text available on paper to the computer. Scanner converts the image into digitized map which can be stored in the computer and transferred to the screen.
10. Joystick: This input device is mostly used in computer games where the player needs to move the position of the cursor quickly across the screen. The function of joystick is similar to mouse.
11. Touch Pad: Touch pads are most commonly seen on laptops. Here the computer senses the position of touch of the finger on a pad and moves the cursor on the screen accordingly. The function is same as that of mouse. We have to move the mouse but here we have to move our finger on the pad to position the cursor on the screen.
12. Voice Recognition System: This is the most recent input method wherein the user has to only speak on a microphone and the computer recognizes the voice and takes it as input. This system is totally speaker dependent. Initially the user has to speak each word many times so that the computer stores all the possible pronunciations by the speaker in the memory. Later when the user speaks, his voice pattern is matched with the stored voice data and then stored in the memory.
13. Microphone: Microphone is a device which records voice or sound and puts it in the computer in form of a file. This file has an extension .wav.
Many laptops and some desktop monitors have inbuilt microphone in the form of a small hole in the front, otherwise a microphone is easily available in the market at a very low cost.
Many microphones come with a headset earphones too.
The user has to speak in front of the microphone but not too close as that would result in a mumbled sound. A microphone is generally connected to a computer using a plug which is put in the slot with MIC written on it.
Microphones are used for video conferencing, making calls
through a computer, recording sounds that may be used along with a PowerPoint presentation etc.
14. Digital Camera: A digital camera, more popularly known as digicam, is used to capture still photographs and videos.
It records the images and sound if required, digitally on a small memory device (a memory card). The images can be immediately displayed on the screen present in the camera itself. This way the user can immediately delete the unwanted images. Nowadays most of the mobile phones have inbuilt digicams in them. The images stored can be transferred to any other storage device or can be displayed on a computer and can even be used by many application softwares like MS-Word, MS-PowerPoint etc.
15. Web Camera: A web camera is also a digicam which is connected to a computer and gives online video input to the computer. These cameras are mainly used for video conferencing, security purposes, creating television programs where two or more people from different places can participate simultaneously.
The low cost, easy accessibility and ease to use are the major features for making web cameras popular.
CENTRAL PROCESSING UNIT (CPU)
CPU is known as the heart and soul of a computer system. CPU contains the following three units:
- Control Unit
- Arithmetic Logic Unit (ALU)
- Primary Storage Unit or Main Memory
1. Control Unit: Control unit keeps track of what is to be done by whom and when. If an input is to be received from the input device, the control unit sends control signal to input device to get ready to receive the input. In case data is to be sent to output device, the control unit sends control signal to it to get ready to receive the output.
2. Arithmetic Logic Unit (ALU): As the name suggests, ALU is the unit which performs all arithmetic operations and logical operations. Data and Instructions to be performed on the data are received from the input device and are stored temporarily in the main memory. Then one instruction is fetched at a time and the data required by it is also fetched from the memory under the supervision of the control unit. Data is processed on the basis of the instructions and the result is again sent to the main memory.
3. Primary Storage Unit or Main Memory: It is also called main memory and is the temporary rest house for the data. The data and instructions to be executed by the ALU must be present in the primary storage as ALU can fetch and send data from/to the primary memory only. Primary memory is volatile memory i.e., as soon as the power to the CPU is cut off, everything stored in the primary memory vanishes. The primary memory has the following basic areas:
(i) Input Storage Area: Data received by input device is stored here temporarily.
(ii) Program Storage Area: The instructions for the processing of data are stored here.
(iii) Working Storage Area: All the intermediate results of the calculations are stored
(iv) Output Storage Area: The output to be sent to the output device is stored here. The output device
1. Visual Display Unit (VDU): It is used to view the output generated by the CPU in text or pictorial form. A VDU is similar to a television screen and consists of a cathode ray tube (CRT).
Some characteristics of VDUs are given below:
(i) Display Colours: VDUs are available as monochromes (display is black and white) or coloured. Coloured VDUs have three electron guns which send a regular pattern of horizontal lines to scan dots to red, blue and green phosphor that coat the surface of the screen.
(ii) Size: VDUs come in different sizes but the most commonly used size for a desktop is 14″, 17″, 19″ or 21″. Nowadays flat screen VDUs are getting more popular as they occupy less space. The screen can display 80 columns and 25 rows of data.
(iii) Resolution: It is the distance between two points on the screen upto which they can be distinguished as two separate points by the human eye. The more close they are, the more the resolution is and the sharpness of picture increases. The graphical pictures on the screen are made using these dots called pixels. A display adapter card is required to support the display on the screen. These cards come with various capacities and capabilities. We need to have a graphics adapter card to display graphics on the screen. An EGA (Enhanced Graphics Adapter) card has high resolution (640 x 350 pixels). A VGA (Video Graphics Array) card is the most commonly used card. Its resolution is 640 x 480 pixels with 16 colours and 320 x 200 pixels with 256 colours.
2. Plotters: These are used to display graphical output. Flat Bed Plotters use a fixed paper and a pen-holding mechanism is allowed to move over it. Drum Plotters use a paper placed over a drum that rotates back and forth to produce an up-down motion. One or more pens containing same or different inks are mounted horizontally and their movement is controlled through a computer.
3. Printers: They are the most commonly used output devices which produce a permanent copy of the output on a paper called the hardcopy. The digital copy of the document stored in the memory is called the softcopy. Two broad categories of printers are Impact and Non-Impact Printers.
(i) Impact Printer: Printers in which the paper comes in direct contact with the printer ribbon and printer head are called impact printers, e.g., Dot matrix printer and Daisy wheel printer.
(a) Dot matrix printers are very commonly used as they are less expensive. The speed ranges from 30 to 600 characters per second. The printer-head contains a rectangle of pins. As the head moves across the paper, selected pins are made to strike against the inked-ribbon and a pattern of dots is printed on the paper.)
Daisy wheel printers print continuous characters unlike the dotted characters printed by Dot matrix printer. The printer head of Daisy wheel printer is like the daisy flower. There is a hub in the centre and the characters are on the spokes. The hub rotates and the character to be printed is hit by a hammer which leaves an impact of the character on the paper.
(iv) Non-Impact Printers: Printers where paper does not come in the direct contact with the printer-head are called non-impact printers. These printers use techniques like thermal, electrostatic, laser, chemical or inkjet technologies. Amongst these, laser and inkjet printers are very common.
(a) Laser printer: It is a noiseless printer and produces a high quality image. It can print 10-15 pages per minute. It uses a light beam controlled by the computer to create a copy of the image on a copier drum. The drum gets electrically charged at certain areas. This drum is then exposed to a laser beam which again charges these areas. The charged areas attract a toner ink which is fused on the paper using pressure and heat thereby creating the copy of the image on the paper.
(b) Inkjet printer: These are very cheap and fast printers which produce output as good as that of laser printers. In an inkjet printer, one or more nozzles in the print head emit a stream of ink drops which are guided to the paper by deflection plates which have electrostatic charges.
Printers can be divided into three categories on the basis of amount of data they
print such as:
• Character Printer: These printers print one character at a time on the paper, e.g., dot matrix printer.
• Line Printer: These printers print one line at a time, e.g., drum printer, chain printer. A drum printer consists of a rotating drum on which characters are embossed. A set of hammers strike desired characters on paper. Line printers are impact printers.
• Page Printer: These printers print a full page at a time, e.g., laser printer.
4. Speakers: They are also known as multimedia speakers and add life
to music, movies and games on personal computer by providing required sound effects. Speakers come in various sizes, output quality and price range. The speakers are equipped with low- power internal amplifiers and normally have a plug in connector lime green in colour. Many VDUs have built-in speakers with very Fi9- 2-23: Speakers basic configuration. Mostly the external speakers are two plastic boxes which have the volume and tone control buttons. The output of speakers in laptop is also very low due to space constraint but can be increased by connecting USB speakers which have a very high quality output.