2. S.N.COLLEGE, GADHINGLAJ
Keyboard
• The
keyboard is a peripheral device which is directly
connected on to the motherboard (system board).
• The keyboard is basically a set of switches (much like a
typewriter)
• Switches are connected in the form of a matrix, surrounded by
electronic circuits
• This Circuits monitor the key matrix that continuously scans the
keys to recognize key action and generate a scan code.
3. S.N.COLLEGE, GADHINGLAJ
Key Boards Types
• Serial keyboard
• Parallel keyboard
• Serial Keyboard
–It is a keyboard which outputs the data in serial form, i.e. bit by bit.
–The computer converts serial data into parallel 8-bit data.
–They use only single line to transmit the data.
4. S.N.COLLEGE, GADHINGLAJ
Parallel Keyboard
• It is a keyboard which outputs all the 8-bits at a time in a
parallel form.
• All the bits are sent simultaneously on different lines.
• In this the transmission is faster, needs a thicker cable with more number
of wires.
• In PCs we always use serial keyboards.
6. S.N.COLLEGE, GADHINGLAJ
Functioning of the Keyboard
• The processor in the original PC keyboard was an Intel 8048
micro-controller chip
• Newer keyboards often use an 8049 version that has own RAM,
built in ROM or other micro-controller chips compatible with the
8048 and 8049.
• In an AT-type keyboard design, the keyboard serial interface is
connected to a special keyboard controller on the
motherboard(Intel 8042).
7. S.N.COLLEGE, GADHINGLAJ
Functioning of the Keyboard
• When a switch is pressed, a processor in the keyboard itself
identifies which key is pressed by identifying which grid
location in the matrix shows continuity.
• The keyboard processor also interprets how long the key is
pressed and can even handle multiple keystrokes at the same
time
8. S.N.COLLEGE, GADHINGLAJ
Functioning of the Keyboard
• When you press a key, in most cases the contact actually bounces
slightly .
• The processor in the keyboard is designed to filter this or
debouncethe keystroke.
• The keyboard processor must distinguish bounce from a
double key strike actually intended by the keyboard operator.
9. S.N.COLLEGE, GADHINGLAJ
Functioning of the Keyboard
• Universal Peripheral Interface (UPI) slave micro controller chipis
also available in the original AT design
• It has its own 2K of ROM and 128 bytes of RAM.
• Some systems may use the 8041 or 8741 chips, which differ only
in the amount of built-in ROM or RAM
• Other systems now have the keyboard controller built into the main
system chipset.
10. S.N.COLLEGE, GADHINGLAJ
Membrane switches
• Very similar in operation to rubber dome keyboards.
• A membrane keyboard does not have separate keys .
• In this keyboard, two rubber or plastic sheet are used as row
conductor sheet and column conductor sheet.
• When the key top is pressed, it forces the row conductor sheet
through the hole to touch the column conductor sheet.
11. S.N.COLLEGE, GADHINGLAJ
Membrane switches
• These row and column lines are made on the plastic or single
rubber sheet
• It uses silver or some other conductor ink for each row and column
of keys with bulges for each key on the keyboard.
• This keyboard can be made very thin, as a completely sealed unit.
12. S.N.COLLEGE, GADHINGLAJ
TypematicFunctions
• If a key on the keyboards is held down, it becomes typematic, which
means that the keyboard repeatedly sends the key press
code to the motherboard.
• In AT-style keyboards, typematic rate is adjustable by sending the
keyboard processor the appropriate commands.
13. S.N.COLLEGE, GADHINGLAJ
Keyboard Interface
• Interface between the keyboard cable and the system unit is the
keyboard interface which is a DIN (or mini-DIN if it's a PS/2) plug
that has five (or six, for mini-DIN) pins as shown.
• All together there are four lines (wires) used for interfacing the
keyboard with the system motherboard. They are
– Keyboard data (KBDATA)
– Keyboard clock (KBCLK)
– DC source (+5V VCC)
–DC ground (0V GND)
14. S.N.COLLEGE, GADHINGLAJ
Keyboard Interface
• Universal Serial Bus (USB) interface is becoming more popular for
keyboards, and some models may, in fact, require you to use a USB
port unless you have a USB-to-PS/2 adapter to make the USB
keyboard compatible with the PS/2 keyboard port.
• A USB connection is faster than the other, older I/O ports on your
computer, such as COM and parallel ports.
15. S.N.COLLEGE, GADHINGLAJ
Common Keyboards Types
• The most common keyboards are:
• 101-key Enhanced keyboard
• 104-key Windows keyboard
• 83-key PC and XT keyboard (Obsolete)
• 84-key AT keyboard (Obsolete)
16. S.N.COLLEGE, GADHINGLAJ
Enhanced 101-Key (or 102-Key) Keyboard
• In 1986, IBM introduced the "corporate" Enhanced 101-key
keyboard for the newer XT and AT models.
• IBM 101-key units originally came in versions with and without the
status-indicator LEDs, depending on whether the unit was sold with
an XT or AT system.
• With the replacement of the Baby-AT motherboard and its five-pin
DIN (an acronym for Deutsche Industries Norm) keyboard
connector by ATX motherboards, which use the six-pin mini-DIN
keyboard connector, virtually all keyboards on the market today
come with cables for the six-pin mini-DIN connector introduced
on the IBM PS/2s.
17. S.N.COLLEGE, GADHINGLAJ
Enhanced 101-Key (or 102-Key) Keyboard
• 101-key keyboard layout can be divided into the following four
sections:
–Typing area
–Numeric keypad
–Cursor and screen controls
–Function keys
18. S.N.COLLEGE, GADHINGLAJ
104-Key (Windows 9x/Me/2000) Keyboard
• When Microsoft released Windows 95, it also introduced the
Microsoft Natural Keyboard, which implemented a revised
keyboard specification that added three new Windows-specific
keys to the keyboard.
• 104-key layout includes left and right Windows keys and an
Application key which are used for operating system and
application-level keyboard combinations, similar to the
existing Ctrl and Alt combinations.
19. S.N.COLLEGE, GADHINGLAJ
104-Key (Windows 9x/Me/2000) Keyboard
• Windows keyboard layout calls for the Left and Right Windows
keys (called WIN keys) to flank the Alt keys on each side of the
spacebar, as well as an Application key on the right of the Right
Windows key.
• WIN keys open the Windows Start menu, which you can then
navigate with the cursor keys.
• Application key simulates the right mouse button which in most
applications, it brings up a context sensitive pop-up menu.
21. S.N.COLLEGE, GADHINGLAJ
Cordless Keyboards
• These are like regular keyboards, except instead of having a
keyboard cable that runs from the keyboard to the PC, they have no
cord.
• Wired interface between the keyboard and the motherboard is
replaced with a wireless one.
• They also typically cost more
than the corded models.
22. S.N.COLLEGE, GADHINGLAJ
Multimedia and Web-Enabled Keyboards
• Many keyboards feature fixed-purpose or programmable
hotkeys that can launch Web browsers, run the Microsoft Media
Player, adjust the volume on the speakers, change
tracks on the CD player, and so forth.
• Operating Systems such as Windows 98, Windows Me and
Windows2000 support these keyboards to use their hot keys.
24. S.N.COLLEGE, GADHINGLAJ
Ergonomic Keyboards
• A trend that began in the late 1990s is to change the shape of the
keyboard instead of altering the character layout which has
resulted in a number of so-called ergonomic designs.
• Most common of these designs splits the keyboard in the centre,
bending the sides outward.
