Computer Peripherals 439
SUMMARY
No computer would be a complete working system without a few essential input and output devices. Key-
board, display, mouse and printer are some of these essential devices to make any computer usable by humans.
Keyboards may be taken as embedded systems, capable of sensing closure or release of any key(s)
arranged in the form of a two-dimensional array, scanned continuously by a group of input and output
lines of the port of the microcontroller. Every key, either pressed or released, is debounced and its scan
code is sent to the host through interrupt by serial communication, mostly USB these days.
Mouse might be of opto-mechanical or optical type, depending upon of the technology used to deter-
mine its movements over a plain surface. The latest X and Y components of the movement along with
its button-status is sent periodically to the host through serial communication, mostly USB.
Three varieties of printers are widely used with computers to generate the hard copies of documents.
The dot-matrix type of printer depends on its solenoid-driven vertical array of pins, whose number may
be 7 or 24. Using an inked ribbon between pins and the paper, the printer is capable of generating char-
acters by positioning pins properly and projecting or retrieving those, as per the pattern to be generated.
In inkjet type of printers small ink-dots are printed on the paper by evaporating a small quantity of liquid
ink and by adjusting the positions of the print-head and the paper, characters are generated. For laser
type printers, over the surface of a rotating drum the image is generated by laser beam discharging the
electrostatic toner powder, which, after further rotation, transfers the image over the paper.
Monitors or display devices are either made of CRT or LCD. In either case, they display characters
through formation of two-dimensional array of pixels, whose visible or invisible conditions depend
upon the geometric characteristic of the character to be displayed. In CRT, the electronic gun, whose
position is controlled by varying voltage between horizontal planes and vertical planes, illuminates the
phosphorus coating on the screen, depending upon the speci c display requirements of different pixels.
For LCD type display, which are lighter in weight and atter in thickness, it uses two plates between
which the lique ed crystals are sandwiched. The state of these tiny crystals are changed to solid by
application of voltage or passing current through thin lm transistors (TFT) spread evenly on both front
and back plates of the screen.
POINTS TO REMEMBER
R Key pressing and key releasing codes are different for IBM PC keyboards.
R All key actions must be properly debounced to ensure valid key codes.
R Optical mice use a small camera and DSP to monitor its own movements.
R Toners are dry powders attracted by positively charged particles for laser printers.
R LCD type at screen display panels use TFT for generating pixels at each grid point.
QUICKSAND CORNER
As the term computer not only means the host but
also all its peripheral devices interfaced with it,
therefore an adequate knowledge of present day
computer peripheral is essential for any computer
expert. The rapid change of technology have intro-
duced and would keep on introducing new type of
peripheral devices, providing us the scope to be
better equipped with additional advantages.
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