Input Device
The Keyboard is the most common input device
The most common keyboards are:
•101-key Enhanced keyboard
•104-key Windows keyboard
•82-key Apple standard keyboard
•108-key Apple Extended keyboard
kinds of switches
• To understand a keyboard, one must first understand the kinds of
switches that are used.
Types of keyboard switches
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Mainly four technologies are used to make keyboard-switches
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Mechanical switches
Membrane key-switches
Capacitate key-switches
Hall effect key-switches:
Mechanical switches
In mechanical switch keys two pieces of metals are pushed together when one presses
the key. The metal switch elements are often made of a phosphor-bronze alloy with
gold plating on contact areas. They contain a spring to return the key to the non-pressed
positioned a form to damp bouncing.
Mechanical switches are inexpensive but they suffer from contact bounce. A pressed
key may make & break contact several times before it makes solid contact. In addition,
the contacts may become oxidized or dirty with age. Results in an improper contact.
They have a lifetime of 1 million keystrokes.
Mechanical switches
Membrane key-switches
These are special types of mechanical switches. This consists of 3 layers of plastic or
rubber sandwiched as shown below. When we press a key we push the top ink line
through the hole to contact the bottom ink line.
The advantage is that these types of key-switches can be made very thin and sealed
units. Lifetime of these key-switches vary over a wide range.
Capacitate key-switches
• This type has two small metal plates on the PCB and another metal
plate on the bottom of a piece of foam. When we press a key the
movable plate is pushed closer to the fixed plate & this changes the
capacitance between the fixed plates. The sense amplifier detects this
change in capacitance and produces a logic level signal indicating a
key-press.
The advantage is the absence of mechanical contacts to be oxidized or dirty and the
only disadvantage is that a special circuitry is required to detect the change in
capacitance. The lifetime is about 20 million keystrokes.
Hall effect key-switches
This type also has no mechanical contacts. It uses the principle of deflection of a
moving change by a magnetic field. A reference current is passed through a
semiconductor crystal between two opposite faces. When a key is pressed, the crystal is
moved into a magnetic field, this has its flux lines perpendicular to the direction of
current flow in the crystal.
This causes a small voltage to be developed between two of other opposing faces of the
crystal. This indicated a key-press. It is more expensive but they are very dependable
and have a lifetime of 100 million or more Keystrokes.
Keyboard Matrix
The architecture of most of the keyboards looks like a matrix. Key
switches are connected in a matrix of columns & rows.
The rows of the matrix are connected to four output port lines and the
column lines of the matrix are connected to four input lines.
When no key is pressed, the column lines are held high by the pull-up
resistors to +5v. The main principle here is that pressing a key
connects a row to a column. If a low is output on a row and a key in
that row is pressed, then the low will appear on the column, which
contains that key and can be redetected at the input port. If one knows
the row and the column of the pressed key, one then knows which key
is pressed.
Therefore, the keyboard controller has to do three steps for the detection
of a key-press. (1) Scan the rows (2) Sense the columns and (3) detect
the key-press.
Keyboard Matrix
Scanning
The process of finding out which key is pressed by reading the row and column value is
called keyboard scanning.
Scan Codes
The keyboard sends a special code called as scan code to the computer. The computer
converts this scan code as proper ASCII codes with the help of the BIOS program. Scan
code generated when the key is closed (pressed) is called as ‘make code’ and the scan
code released when the key is open (when not pressed) is called as ‘break code’.
Connector
The most common keyboard connectors are:
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5-pin DIN (Deustche Industrie Norm) connector
6-pin IBM PS/2 mini-DIN connector
4-pin USB (Universal Serial Bus) connector
Internal connector (for laptops)
Inside the Keyboard
The processor in a keyboard has to understand several things that
are important to the utility of the keyboard, such as:
• Position of the key in the key matrix.
• The amount of bounce and how to filter it.
• The speed at which to transmit the typematics
Trouble Shooting
BIOS Upgrade: This is a known problem with Toshiba 8500 desktop systems and the
Microsoft Natural Keyboard. In virtually all cases (including the Toshiba 8500), the PC
keyboard controller BIOS recognizes the keyboard during the Power-On Self Test
(POST), but it does not recognize some of the keys—including certain function keys
and Windows-specific keys. One will need to try a generic keyboard or upgrade the
system’s keyboard controller BIOS
One or more Windows-specific keys don’t work
BIOS: This is almost always a limitation of the keyboard-controller BIOS. For
example, a Jetkey keyboard controller BIOS (v.3.0) will not recognize the right
Windows key on a Microsoft Natural Keyboard. One will need to try a generic
keyboard or upgrade the system’s keyboard- controller BIOS.
Trouble Shooting
Remote-control programs don’t work after installing
Driver Software: keyboard drivers Many PC “remote-control” programs (e.g.,
PC Anywhere, ReachOut, and Carbon Copy) use keyboard and mouse drivers
that are simply not compatible with the keyboard’s specific drivers. For
example, the remote-control programs listed will not work when IntelliType
software is installed for the Microsoft Natural Keyboard. One will need to
disable the remote-control software, install patches for the remote-control
software that will properly support the keyboard or replace the keyboard with
a more generic model.
Assigned key sounds do not work
Software Problem: When one assign sounds to keystrokes (under the Options tab
in the Keyboard tool in the Control panel), the sounds might play when one
presses the assigned keys. This problem is known to occur with some
programmable keyboards when HiJaak Pro or HiJaak 95 Graphics Suite
installed on the computer. These products might load a device driver named
“Runner” that disables programmable keyboard sounds. One might be able to
work around the problem by closing the “Runner” task:
Press <Ctrl>+<Alt>+<Del> to open the Close program dialog box.
If “Runner” is listed, click Runner, then click End task.
Trouble Shooting
One cannot use Windows-specific keys to start task switching
Update SYSTEM.INI software other than TASKSW16.EXE One can start the desired
task switching software using <Ctrl>+<Esc>, or by double clicking the desktop.
Chances are that the Windows-specific key will not start any other task switching utility
if TASKSW16.EXE can be found on the path. One will need to update the task
switching program reference in SYSTEM.INI. Load SYSTEM.INI into any text editor,
and modify the line that reads:
TASKMAN=TASKSW16.EXE
to read
TASKMAN=<task manager>
where <task manager> is the name of the executable file that one wants to start when
one presses the Windows key. Rename the TASKSW16.EXE file (e.g.,
TASKSW16.OLD) or move it to a directory that is not in the path. Save and close the
SYSTEM.INI file, then restart the computer.
Trouble Shooting
Disable Pen: This can happen with some programmable keyboards when
pen software is installed on the system. One should be able to correct
the problem by disabling the pen device:
The NumLock feature might not activate when the NumLock key is
pressed
1 Click Start, select Settings, then click Control panel.
2 Doubleclick the System icon and select the Device manager tab.
3 Doubleclick the Ports entry to expand it.
4 Doubleclick the port to which the pen (or touch-screen) device is
connected.
5 In the Device usage area on the General tab, click the Original
configuration (current) check box to clear it (if one is using OSR2,
click the Disable in this hardware profile
check box to select it).
6. Click OK, then restart the system when prompted.