Download Peripheral 1

Peripherals
Looking at
Keyboards
Mouse
Joystick
Keyboard
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This is probably one of the most important peripheral (along
with the monitor).
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The keyboard is organised in the same ways as a game of
battleships, each key is a switch connecting a vertical and
horizontal wire, giving a unique location for where the two lines
crossed.
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When a key is pressed a signal is produced on two
connections to a device called the keyboard encoder.
The encoder deals with key bouncing and produces a
unique code (a scan code for that key)
The encoder sends the code to the computer.
A further device, the keyboard controller, receives code
from the keyboard.
The controller performs all the necessary operations to
translate the scan code into an ASCII code and then
send it on to the Data Bus.
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Key press is converted to unique code in terms of the connections
made, the code is sent to the computer decoded and a ASCII code
for the key is put onto the data bus.
Physical construction:
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Mechanical keys - simplest switch. Pushing the switch
makes contact between two plates, completing the
circuit. Releasing the switch a spring pushes the
switch back up and the circuit is broken.
Membrane keys - similar to the mechanical keys, but
works by pushing a rod through a insulated sheet.
Capacitive keys - When a key is pressed two plates
are brought closer together (but not touching), the
capacitance changes and this is detected.
Key Bounce
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Digital logic works by being on or off.
When a key is pressed or released it can vibrate
which can lead to the key appearing as if it had been
pressed several times.
This is key bounce.
To overcome this the keyboard encoder contains debounce circuitry. One a voltage change is detected it
prevents for a short period any further changes being
detected.
Mechanical mouse
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A ball sits inside a case and can move any direction.
Inside the case there are two rollers at right angles to
each other, these detect changes in that axis.
Each roller is connected to a rotating sensor.
The rotating sensor is done by a disc rotating around
making and breaking contact.
It has a relatively short working lifetime because it is a
mechanical device.
Opto-mechanical mouse:
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The same roller system as the mechanical device is
used, but the rotating sensor in the mechanical mouse
is replaced by an optical system.
The disc is opaque with transparent strips, one side of
the disc there is a light source and the other side of
the disc is a detector. As the disc rotates the light is
detected through the strips producing electrical pulses.
The more the mouse moves the more pulses produced.
Optical Mouse
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The early optical mice presented a practical problem,
needing a special mat. When these mats go missing
the mouse can not be used. Modern ones don’t need
special mats.
A light source in the mouse shines on the surface and
is reflected back to a sensor in the mouse.
As the mouse is moved the changes detected on the
surface are detected and converted in electrical
pulses.
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Advantages
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Less wear and tear
Less open at the bottom so does not pick up as
much dirt.
Modern version can be used on a variety of
surfaces
Greater accuracy than a conventional mouse.
Joystick
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A conventional joystick usually connects
through the sound-card or an add-on card.
Using a 15-pin sub D-socket, handling:
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Four press switches
Four resistive inputs
Mechanically links two variable resistors that
alter depending on the direction of motion,
therefore altering voltages.
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Taken from http://inf33-www.informatik.unibwmuenchen.de/infothek/computer/tutorial/eprmhtml/eprm/253.htm
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Changes
– USB connected joysticks
– Force-feedback.
Summary
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When a key is pressed, it makes certain connection
to a chip (keyboard encoder), which sends a code to
the computer.
In the computer a further chip (keyboard controller)
takes this code and converts it into something the
computer can use.
Three keyboard types
– Mechanical,
– Capacitive,
– Membrane.
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Three mouse types
– mechanical
– optical
– optomechanical
A conventional joystick usually connects through the
sound-card or an add-on card. Using a 15-pin sub Dsocket, Mechanically links two variable resistors that
alter depending on the direction of motion, therefore
altering voltages.
Sources for further reading
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Chalk et al (2004) pages 142-148
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Dick (2002) PC support handbook pages 294304.
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Taken from http://inf33-www.informatik.unibwmuenchen.de/infothek/computer/tutorial/eprmhtml/epr
m/253.htm