Types Of Keyboards

The Keyboard is the most friendly input peripheral both data and program can be keyed in through it.In addition certain command software can be given from keyboard.The keyboard consist of set of key switches.There is one key switch for each letter,number,symbol etc..When a keyboard has an electronic circuit to determine which key has been pressed.Then the standard 8bit code is generated and send to the computer to detecting which key is pressed.A serial keyboard sense the data bit by bit in serial pattern.The wireless keyboard is a recent development.Which doesn't need a physical cable,allows conversant location of the keyboard.



There are 4 major types of keys.

1.Function Keys(F1-F2)
2.Numeric keys

3.Cursor central
4.Alpha numeric keys

Membrane Keyboard 

       It is a computer keyboard whose "keys" are not separate as with majority of other keyboards.Membrane keyboards,which work by electrical contact between the keyboard surface and underlying circuits when key-top areas pressed.Modern Pc keyboards are essentially a membrane keyboard mechanism covered with an array of dome switches which give positive tactile feedback.

Working:-

      As can be seen from the diagram below, the membrane keyboard basically consists of three layers; two of these are membrane layers containing conductive traces. The center layer is a "spacer" containing holes wherever a "key" exists. It keeps the other two layers apart.

Cross-section diagram of a typical membrane keyboard.
The thickness of the bottom three layers has been exaggerated for clarity; in reality, they are not 
much thicker than pieces of paper or thin cardstock.

      Under normal conditions, the switch (key) is open, because current cannot cross the 
non-conductive gap between the traces on the bottom layer. However, when the top layer is 
pressed down (with a finger), it makes contact with the bottom layer. The conductive traces 
on the underside of the top layer can then bridge the gap, allowing current to flow. The switch 
is now "closed", and the parent device registers a key press.

Typical applications include;

• Industrial controls
• Access control systems
• Medical equipment
• Telecommunications apparatus
• Telephone systems
• Household appliances
• Security systems

Mechanical Keyboard  

They are more indicate and higher quality than either of the types of membrane keyboard.Each has it's own independent key switch mechanism that will register when a key is pressed.  

Working:-

       In contrast to other keyboards, themechanical keyboard uses a physical switch lying below the key. Thuswhen you press the key, you actually press the switch down. Thissends the signal to the PC that a user has pressed a particular key.Most of the keyboards used nowadays have three layers of plasticmembrane below the key. Thus every time the top and bottom layersconnect; an electric circuit gets closed so that the information issent to the PC. The middle membrane possesses holes where differentkeys are placed. On pressing the key, a dome-shaped piece of siliconeor rubber is pushed through the hole to connect both the top andbottom membranes. The functioning of keyboard is quite similar tokeypad on an oven. Many console gaming systems also use gamepads withrubber-dome system.
      The rubber-dome keyboards are muchcheaper than mechanical keyboards and are great at resisting theliquid spills, yet they do not provide an effective feedback whenevera button is pressed. On the other hand, mechanical keyboards givegreat feedback.
     One of the greatest features of a goodmechanical keyboard is N-Key Rollover. All the keyboards possessingthese features are capable of detecting each key press. This wouldensure that all the letters are being recorded on a single press of key.

Dome-switch Keyboard

       Dome-switch keyboards are a hybrid of flat-panel membrane and mechanical keyboards. They bring two circuit board traces together under a rubber or silicone keypad using either metal "dome" switches or polyester formed domes. The metal dome switches are formed pieces of stainless steel that, when compressed, give the user a crisp, positive tactile feedback. These metal types of dome switches are very common, are usually reliable to over 5 million cycles, and can be plated in either nickel, silver or gold. The rubber dome switches, most commonly referred to as polydomes, are formed polyester domes where the inside bubble is coated in graphite. While polydomes are typically cheaper than metal domes, they lack the crisp snap of the metal domes, and usually have a lower life specification. Polydomes are considered very quiet, but purists tend to find them "mushy" because the collapsing dome does not provide as much positive response as metal domes. For either metal or polydomes, when a key is pressed, it collapses the dome, which connects the two circuit traces and completes the connection to enter the character. The pattern on the PC board is often gold-plated.
Both are common switch technologies used in mass market keyboards today. This type of switch technology happens to be most commonly used in handheld controllers, mobile phones, automotive, consumer electronics and medical devices. Dome-switch keyboards are also called direct-switch keyboards.

How a dome-switch keyboard works: Finger depresses the dome to complete the circuit

Dome switches mesh with keys (keyboard is upside down in this image)

 

When a key is pressed, the foil tightly clings to the surface of the PC board, forming a daisy chain of two capacitors between contact pads and itself separated with thin soldermask, and thus "shorting" the contact pads with an easily detectable drop of capacitive reactance between them. Usually this permits a pulse or pulse train to be sensed. The keys do not need to be fully pressed to be fired on, which enables some typists to work faster.

Debouncing

    When striking a keyboard key, the key oscillates against its contacts several times before settling. When released, it bounces again until it reverts to its rest state. Although it happens on such a small scale as to be invisible to the naked eye, it's sufficient for the computer to register multiple key strokes inadvertently.
    To resolve this problem, the processor in a keyboard "debounces" the keystrokes, by aggregating them across time to produce one "confirmed" keystroke that (usually) corresponds to what is typically a solid contact. Early membrane keyboards limited typing speed because they had to do significant debouncing.

Keyboard switch matrix

     The keyboard switch matrix is often drawn with horizontal wires and vertical wires in a grid which is called a matrix circuit. It has a switch at some or all intersections, much like a multiplexed display. Almost all keyboards have only the switch at each intersection, which causes "ghost keys" and "key jamming" when multiple keys are pressed. Certain, often more expensive keyboards have a diode between each intersection, allowing the keyboard microcontroller to accurately sense any number of simultaneous keys being pressed, without generating erroneous ghost keys.

Keyboard Connectors

1.AT keyboard connector (DIN5)

       It is the 5-pin DIN connector used on earlier PC keyboards. It was subsequently replaced with the smaller 6-pin MiniDIN connector, which is known as a "PS/2 connector" and which was first used on laptops.

Connector Pin #	Purpose
Pin 1	KBDCLK (clock)
Pin 2	KBDAT (data)
Pin 3	KBRST (reset, not used)
Pin 4	GND
Pin 5	VCC (+5V)

2.PS/2 keyboard connector (MINI-DIN6)

     The PS/2 connector is a 6-pin Mini-DIN connector used for connecting some keyboards and mice to a PC compatible computer system. Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987. The PS/2 mouse connector generally replaced the older DE-9 RS-232 "serial mouse" connector, while the PS/2 keyboard connector replaced the larger 5-pin/180° DIN connector used in the IBM PC/AT design.

Connector Pin #	Purpose
Pin 1	KBDAT (data)
Pin 2	not used
Pin 3	GND
Pin 4	VCC (+5V)
Pin 5	KBDCLK (clock)
Pin 6	not used