How a Computer Works (2015)

16. Keyboard

Introduction

The keyboard is the main input device we use to communicate with the computer. The keyboards purpose is to pass numbers, letters and special function keys to the computer. These are passed when a specific key is pressed. Using a combination of keys enables certain keys to have dual functions.

For instance when the Shift key is down letter keys become CAPITAL LETTERS instead of lower case letters. This is useful as it cuts down on the number of keys required. Imagine the size of keyboard required if this was not the case.

Keyboard

The keys on the keyboard are individual switches. When a key is pressed a special code is sent to the computer. The computers BIOS chip recognises the code and interprets this into a number or letter. The keyboard layout has not really changed since the days of the original typewriter.

Keyboards are known as QWERTY keyboards as they represent the first six letters. The keyboard above is a 104-key Windows version.

Keys

This keyboard has keys, which offer functions, which are used in conjunction with the Windows operating system. For instance the key on the bottom row two in from the left opens the Start menu. The top row consists of twelve function keys labelled F1 to F12. These keys are used by certain software packages to carry out tasks.

A good example is Word, which uses the F7 key to start spell checking. Another piece of software may use F7 to carry out another task so the function of the key changes with the software you are using. You may find the key isn't used at all in a program.

Numeric Keypad

On the right hand side are a group of keys known as the Numeric keypad. These keys are useful as their layout enables quicker data entry of numbers than would be possible using the number row above the letter keys. The bottom left hand key is the Control key labelled Ctrl.

This key when used with other keys enables for example menu options to be used without opening them with the mouse.

Key Functions

Holding down the Control key and f key in Word opens the Find and Replace dialog window. Other keys, which have non-writing functions are, the Page Up and Page Down keys, which in Word scroll the screen up or down a page. The arrow keys enable the cursor to move up through lines in Word. The Return key is used to start a new line in Word.

The Delete key deletes the last letter/number typed in. The Tab key creates fixed spaces between letters/numbers which is useful for creating presentations.

Inside a Key

In a typical keyboard the key switch sits above a special plastic circuit board. When the key is pressed it pushes onto a rubber dome which in turn compresses three circuit connections together. The bottom circuit contains low voltage electricity. This passes through to the other two circuits, but only when the key is pressed down.

Keyboard Matrix

Inside the keyboard a matrix of conducting circuits interconnect each of the keys to a decoder chip. Once a key is pressed the rubber dome represented by a circle here presses the two matrix connections together.

The decoder chip senses which key has been pressed and sends a hexadecimal code to the computer.

I/O Controller

When the decoder chip produces the Hexadecimal code it is sent along the keyboard cable to the connector on the back of the computer. Once inside the computer the code goes to the I/O controller chip.

ROM BIOS

When the code leaves the I/O controller it eventually ends up at the CPU.

Here a lookup table of codes is fetched from the ROM BIOS stored in memory. An ASCII key code is found for the key that was pressed.

The code is presented to the program which is running. The program then decides what to do with it. The letter or number will be displayed, if it is not assigned to an application.

Displaying a character

The character data is sent to the graphics card. The graphics card processes the information and updates the monitor.