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.
There are 4 major types of keys.
1.Function Keys(F1-F2)
2.Numeric keys
2.Numeric keys
3.Cursor central
4.Alpha numeric keys
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.
Typical applications include;
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.
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
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.
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 |