Apr 132013

A Glance at Computer Keyboard

Keyboards are one of the most important input devices which are used in feeding input to computers. They are designed for the input of texts and characters, and also to control the operation of the computers. The most common and widely used arrangements of keys in many countries are based on the QWERTY layout. The number of keys on a keyboard generally varies from the standard 101 keys and 104-key Windows keyboards, to as many as 130 keys, with many programmable keys.

A computer keyboard is effectively an array of different switches and buttons, each of which sends the computer a unique type of signal when pressed. The switches are generally kept in a keyboard by the means of either mechanical manner or a rubber membrane.

The keys kept in mechanical manner are simply spring-loaded. They are of ‘push-to-make’ types. This means that when they are pressed down, they complete the circuit and then break it again when they are released. Likewise, the keys kept on a rubber membrane are composed of three different sheets. The first sheet has conductive tracks printed on it, the second is a separator with holes in it and the third is a

conductive layer with bumps on it. A rubber mat over this gives a springy feeling. When a key is pressed, it pushes the two conductive layers together to complete the circuit. On the top is a plastic housing which includes sliders to keep the keys aligned.

A dome-switch keyboard is commonly used variation of the membrane-type keyboard. With these, a key depression pushes down on a rubber dome sitting beneath the key. A conductive contact on the underside of the dome touches and hence connects a pair of conductive lines on the circuit below. This bridges the gap between them and allows current to flow by changing the signal strength. A scanning signal is emitted by the chip along the pairs of lines to all the keys. When the signal in one pair becomes different, the chip generates a ‘make code’ corresponding to the key connected to that pair of lines. The code generated is sent to the computer either through a keyboard cable (using on-off electrical pulses to represent bits) or over a wireless connection. A chip inside the computer receives the signal bits and decodes them into the appropriate key press. The computer then decides what to do on the basis of key pressed.

The keys are connected up with each other like a matrix, and their row and column signals feed into the keyboard’s own micro-controller chip. This is mounted on a circuit board inside the keyboard, and interprets the signals with its built-in firmware program. A particular key press might signal as row 3, column B, so the controller might decode this as an A and send the appropriate code for A back to the computer. These ‘scan codes’ are defined as standards in the computer’s BIOS (Basic Input Output System), though the row and column definitions are specific only to that particular keyboard.