Fixed disk drive (FDD) is equipped with a permanent disk or disk pack. Exchangeable disk drives provide for easy interchange of disk or packs. Disks made from syntheric plastic like material are flexible and therefore called floppy disk or floppy diskettes. These are cheaper and more rugged and are designed so that they can be portable. Each floppy is packed inside a protective envelope which gives it rigidity and cleans as it rotates. Small cut-outs in the envelope allow the disk to be sensed by the control circuiting and the read/write head to approach close to the tracks.
Disk come in varying sizes:-
Rigid disks are made in four standard diameters. Large mainframes and mini computers normally use 14 inches (356mm) rigid in packs of upto 12. Smaller computers use 8", 5.25" or diameter fixed disks also knows as Winchester Disks.
T3he floppy disk gets its name from the fact that it is made of plastic material. This plastic base is coated with an iron-oxide recording substance. Data is recorded as tiny invisible magnetic stops on this coating. This is covered with plastic jacket for safety purposes. The functioning of the floppy is similar to record player. If you take actual media out of plastic jacket you will find it similar to audio record with number of tracks on it. You can get data randomly. Each floppy has one index. You can see a hole in the floppy. This is the index where address of data is stored. A floppy is divided into tracks and sectors and each sector can hold 512 bytes of information.
Floppies are available in 3 sizes-8" , 5.25" and 3.5". But 8" floppy is absolute now.
Technique for encoding digital data into light intensities achieve high precision when laser is used as a source of light. Disks based on light rather than magnetism are widely used in consumer electronics for recording both audio and video signals with high quality reproduction. Compact Disks used for music are an example of these. The main advantage of these disks is that data is stored at very high densities, access is relatively fast and disks are rugged unlike magnetic disks and tapes. Each disk can store 550 MB of data which is roughly equivalent to 15 magnetic tapes.
Laser beam technology is used for recording/reading of data on the disk. Because laser technology has been used in these disks hence known as laser disk or compact disk.
Worm Disk:-
The word WORM stands for Write once read many. The disk allows a user to create his own CD-ROM disk by using CD-Recordable (CD-R) drive which can be connected to computer like any other peripheral device. These WORM disk look the CD-ROM disks can be purchased from market and can be encoded by CD-R drive. One must keep in mind that on this disk data can only be written once and can be read as many times as required but this data cannot be changed. In WORM disk laser technology is being used for recording/reading data..
INPUT AND OUTPUT DEVICE:-
1. Input device:- The electronic device through which you give information to the computer are called input device.
For example:- keyboard, mouse.
2. Output device:- The electronic device through you recieve information are called output device.
For example:- Monitor, printer, scanner.
Various input devices as as:-
1. Punched card:- These card is a rectegual paper card. Which is devided jnto 18 colums 12 rows. Each colums is devided into 12 positions at which hols can be punched. A perticular combination of holes in a column represents a specific character. There is a special punching machine which punches the information on the cards. Punched cards were very much in use. Now a day they have become outdated because of the advancement of the sophisticated technology. A number of source of feeding data have been developed.
2. Keyboard:- The keyboard of a computer is almost similar to a typewriter with a few differences. In computers a cursor is always flashing on the monitor. When a key is pressed on the keyboard, the same character is displayed on the monitor. This keyboard has all the alphabets and (0-9) numeric digits on upper line with some control characters like !, @, #, $.% etc. White above the numeric digits. On the right side of the keyboard is a numeric key pad with all numeric digits and calculation symbols.
Different keys of the keyboard:-
1. Functions key:- These are ten keys numbered from F1 to F10 on top most line to perform special functions.
2. Alphabet keys:- These keys are 26 in number and contain alphabets from A to Z in upper and lower case.
3. Numerical key:- These are 10 key numbered from (0-9). These keys are presents in second row from top of keyboard and also on numeric keypad.
4. Logical sign keys:- These keys on which logical sign are inscribed such as <,>, ?, !, *, = etc.
5. Arithmatic and mathematical keys:- These are operational keys for example-+, _, &, @ etc.
6. Special keys:- These keys are used for special purpose. Various special keys are described below:-
(a). Cursor movement keys:- These are four keys used for cursor movement to right, left, up and down direction. There are four arrow as ⬆⬇➡⬅.
(b). Escape key:- This is an important key used to deactivate the wrongly written command.
(c). Enter key:- This is an important key on the keyboard. It sends the data into the computer. It is also used to move the cursor to the next line.
(d). Numeric keypad:- This keypad is located on right side of the keyboard. It has numeric digits from 0-9 and mathematical operators +, -, *, /, = etc. It functions when Num. Lock key ON.
(e). Alt and ctrlkl key:- These keys are used alongwith the combination of alphabetical keys. These keys have different uses in each software.
(f). Insert key:- This key is used to insert a latter in a word. A missing alphabet can easily be inserted a misspelt word.
(g). Caps Lock key:- This key is used to get letters in upper or lower case.
(h). Space bar:- This key creates space between words.
(i). Delete key:- This key is used to delete written word.
(j). Back space key:- This key is used when a letter or word is needed to move backward.
(k). Pause key:- It is used to temporarily stop the activity on the screen.
(L). Print key:- This key is used to print the information on screen and printer.
3. Mouse:- It is a hand held operating device rolled across a flat surface. Its purpose is to provide easy and accurate cursor movement has buttons on it to facilitate other functions such as data entry.
4. Optical Readers:- Optical readers include various devices that read the characters with the help of light beams. These are:-
A. Optical mark reader(OMR):- It is a device which can detect the presence or absence of mark on a paper. Light is thrown on the paper and the reflected light is detected. When a mark is present the amount of light reflected is less. This device is used for reading answers to questions in a test or survey. Questions are designed with a choice of multiple answer of which one is selected by plotting a mark in a box. Spurious marks and folds have to be avoided as the detector is sensitive to these too.
B. Optical character recognition (OCR):- Optical readers not only detect a mask but also recognise its shape and identify characters. The amount of light reflected differs depending on the shape and identify character and the OCR can detect these minor differences. However, ducument have to be filled very carefully and character shapes have to be fairly precise. This is often done in machines to print characters. The presence of any other mark can confise the device. These readers are popularrly used to process turn around documents i.e. those documents that are produced as output and reused as an input by the computer. For example in electricity bill, the tear off section could be used to reinput into the computer after the bill is paid.
C. Magnetic ink character recognition (MICR):- It uses special ink to print characters. They can be read and decoded by special magnetic devices. This system is used by banks in the metropolitan cities to process cheques. The cheque number, the bank and branch code and the account number are printed with ink containing magnetiseable particles of iron oxide. At the time of processing, the amount of transaction is also written out in magnetic ink by the bank. The position for each data items is fixed and only the decimal digits and few special characters may be used. Roughly handled cheques can also be read with greater accuracy.
5. Point of scale terminals:- The terminals are used in retail or wholesale organisations as the customer transection occurs (a crude from is what is being used in some of the conteens). The keyboard may consist of a numeric cluster and some specially labelled keys. These are used to indicate particular types of input such as method of pahment quantity purchased etc. The terminal may also control one or more of the folloeing specialised devices.
A. Bar code Reader:- Bar codes are marked on wrappers of products which you can see on all items including books. These are small and thick lines and now without it wedo not find any item. These codes are ready by a 'wand' or a pen like instrument called as Bar Code Reader which transmits a laser beam and receives the reflection from the label. The thickness of the bars and gheir relative spacing provides a meaningful code
B. Credit Card Reader:- Plastic credit cards with embossed characters are also becoming popular to-day as a means of identification. These cards have a strip of magnetic tape on them containing coded information. Inserting the card into a slot allows the details to be read. The use of these cards is often combined with an extra numeric input for greater srcurity. Other uses include Clocking-on, banking terminals and security access.
C. Kimball Tag:- The kimball tag is in effect a miniature punched card containing details of goods to which it is attached. The tag is removed upon sale of goods and is read by the point of sale terminal. The method removes any possibility of transcription errors and speeds up operation at the point of sale.
D. Sensors:- For industrial computer applications such s process control, the computer is required to directly interact with the plant. Since the computer can only communicate with the outside world by using electronic signals. It must conform to certain standards. The hardware problem is therefore to convert the various physical variable like temperature, pressure, voltage, switch position (ON/OFF) etc. To electronic signals and vice versa. The system requirements are to be able to make appropriate action depending on the values read-in and to do this rapidly enough to attain effective control as would be required say in a nuclear power point.
6. Graphic Terminals:- There are basic types of graphic terminals:-
A. Low resolutions.
B. High resolution.
Low resolutions graphics can be used in business applications to display graphics or simple diagrams. High resolutions graphics are neccessary for applications such as computer-aided-design. The devices used to enter graphical information are:-
A. Light Pen:- It is a stylus connected by a cable to the terminal. It can be touched to the CRT screen where it can detect the flying spot or raster on the screen as it passes. It can therefore be used to point to symbols on the screen to draw lines or figures.
As the tip of light pen makes contact with the screen, it sends a signal back to the computer containing X-Y coordinates of the pixels at that point.
B. Tracking Ball:- The original tracking balls were built into video keyboard and could be moved by running a hand over it. This device is like a joystick meant for children. When a ball is moved which is on its base then it moves the cursor or object on the screen.
7. Credit Cards:- These cards are also called as cash dispensers which are very familiar in advanced countries. It is a plastic embossed card with information coded on a magnetic strip is inserted into a slot and a security number input on a small numeric keyboard together with the amount reauired to be withdrawn. The whole transaction is prompted by instructions shown on a small display screen. Details of transactions are recorded and used either immediately or later for updating master files.
8. Joy Stick:- The concept of joy stick is used in video games. It is a device which works on electricity. With this a picture on the screen can be moved. Its upper part is like a handle which is moved by a socket. There is also a button called trigger very near to it and when it is pressed it bombard on the screen. Below this stick is a printed circuit which controls its movement. On its movement the picture on the screen moves.
9. Sound Synthesizer (Talking to a computer):- Speech is a natural form of communication which computers find it difficult to understand. Physically speech consists of sound waves and each word spoken has a characteristic sound pattern. The voice recognition system stores the voice pattern by repeating the same word or words till the computer has analysed a consistent pattern. Comparison at a later stage, would enable the computer to select the correct word. Such voice recognition systems have obvious limitations. The aim of subsequent development has been to create speech understanding with enhansed vocabularies. Techniques are being developed which enable 'concensus' voice patterns to be stored. Techniques of direct voice input may well bypass keyboard input in many activities. Voice outputs are being used generally to alert people to possible error or danger situations and in devices like talking clock and voice programs.
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