General and Special Purpose  Computer Input Devices

There are different types of computer input devices and different types of output devices used by computers for different operations. Primarily the data has to be first converted to a form which can be read by an input unit of a computer. This form is known as machine readable form. The data in machine readable form is read by an input unit, transformed to appropriate internal code and stored in the computer’s memory.

Computer input devices are classified into two types. They are as follows:

1. General purpose devices

2. Special purpose devices

General purpose devices: These units can be used by any computer. The most commonly used devices of this type is a keyboard of a desktop computer and video terminal units.

Special purpose devices: These units are used in special cases. Special purpose units are as follows:

Magnetic Ink Character Readers [MICR]

Optical Mark Readers[OMR]

• Bar Code Readers.



The most widely used input devices or unit is keyboard and is used for manual data entry. The keyboard has been used in all types of computers such as a Personal Computer, a Work Station (or) a Notebook computer. It is also called the QWERTY keyboard as these are the first six letters in the third row from top.

The following are the major key categories in a typical keyboard.

Letter Keys: There are 26 letters of English alphabet arranged as in a typewriter and these are called Letter keys.

Digit Keys: There are two sets of digit keys. One on the second row from the top of the keypad and the other is a numeric keypad at the bottom right which allows quick entry of numbers with the fingers of one hand.

Special Character Keys:  The special characters are <, >, ?, /, {, }, [, ], (, ), , ., “,\, !, @, #, $, %, ^, &, *, _, +, =, -. These can be printed only when the shift key is pressed down and the key on which it is written is pressed.

Non-printable control keys: The non-printable control key are backspace, going to next line, moving the cursor up and down [directions], insert, delete, tabulation and a space bar at the bottom of the keyboard.

Function keys:  The function keys are labelled as from F1, F2…..F15 and when pressed will invoke programs stored in the computer.

There are non-printable control keys which have its own functions and are listed below:

 Backspace key: This key is used to delete the previous character that is the cursor moves for right to left deleting the current character.

Enter key:  This key is pressed to go to the next line.

Arrow keys: This key moves the cursor in four directions that is up, down, back (or) forward.

Tab key: This key moves the cursor to the next tab stop.

Shift key:  To display the upper case [i.e., Capital letters] letters press down the shift key and press the desired character. To display $ symbol, keep pressing the shift key and then press the digit key 4.


A Video Display Device consists of a screen and a keyboard. When a key is pressed from the keyboard the corresponding key [character] will be displayed on the screen.

A dimension and limit of a normal display has 80 characters per horizontal line and 24 such lines on the screen. The cathode ray television tube is scanned by an electron beam to create a raster of horizontal lines. When a key is pressed on the keyboard the intensity of the electron beam will be increased at certain moments creating bright spots on the face of the tube. The display contains a matrix of 5 dots along horizontal direction and 7 dots along vertical direction. Few terminals have extra computing power and storage allowing desired editing of information before sending it to the computer.


Magnetic Ink Character Recognition [MICR]:

In this method a special magnetic ink is used to print the characters on the documents. A magnetic ink character reader can recognize such characters. For example a cheque book; in a cheque book the cheque number and the corresponding branch code will already be printed at the bottom of the cheque using magnetic ink. Now the cheque can be read by a special input unit which can recognize magnetic ink characters. This method saves lot of time and the data entry would be accurate enough.

Optical Mark Reading and Recognition [OMR]:

In this method unique pre-printed forms are designed with boxes which can be marked with a dark pencil. Each box is annotated differently so that the user clearly recognizes what kind of response to mark. This type of document or form can be read by document reader which transcribes the marks into electrical pulses which are transmitted to the computer. These documents are used for the purpose of objective type answer papers in examinations, population surveys, market surveys, time sheets of employees, order forms which contains multiple choices of items.

In this method no further manual transcription is required. This minimizes the data entry errors. Once the form is filled it cannot be redesigned because any manipulation of the data requires reprinting of the form which is very expensive.

Bar Coding:

In this method small bars of varying thickness and spacing are printed on items with packages, price tags etc. These are read by optical readers and converted to electrical pulses. The pattern of bars is unique for their corresponding product.  In some countries each product will be given a unique 10 digit code and this is represented in bar code form in every container of the product. To view the bar code a scanner is required.