capable of a 640 x 200 display using 2 colors.
Because of these limitations, the CGA adapter is
generally considered obsolete.
Enhanced Graphics Adapter (EGA)
The enhanced graphics adapter (EGA) superseded
the CGA adapter and drives an RGB monitor. The
EGA provides 16 colors at a resolution of 320 x 200
or 640 x 200. The character box for text is 8 x 14
instead of the 8 x 8 used with the CGA card.
The EGA card comes with 64K of video memory
that is expandable to 256K using a graphics memory
expansion card. This card adds an additional 64K of
video memory.
The EGA card also uses 128K of
RAM from the computer’s RAM. The video is stored
just above the 640K boundary. Video memory is used
to refresh the display, freeing up the CPU chip for
other operations.
Video Graphics Array (VGA)
The video graphics array (VGA) adapter card
overcame the limitations earlier adapters had in
displaying high quality color. The earlier adapters
used digital signals to control the three electron guns
of the CRT. Each gun was either turned on or off by
these signals and limited the display to 8 colors. By
adding a high and a low intensity signal, the number
of colors that could be displayed was doubled to 16.
The VGA card generates analog signals to control
the electron guns and, therefore, can control the
intensity of each gun at varying levels. Current VGA
cards are capable of displaying 256 colors and
generating 262,144 (256K) colors. Since the VGA
generates analog signals, be sure the monitor is
capable of accepting these signals.
The VGA card displays text in a 9 x 16 character
box and has a resolution of 640 x 480.
Super Video Graphics Adapter (SVGA)
Super video graphics array (SVGA) is a term
used to describe graphic adapters that have exceeded
those of the VGA system. As of now, there is no set
standard for SVGA. Resolutions for SVGA vary by
manufacturer but 800 x 600 and 1024 x 780 are
common.
Some SVGA cards work on a 60-Hz
vertical scan rate and some use 70 Hz. Once an
SVGA card is installed, a software driver that
describes the specifications of that card needs to be
installed.
Extended Graphics Array (XGA)
The extended graphics array (XGA) is a
refinement of the VGA standard. The XGA system
provides a 32-bit bus master for micro channel-based
systems. The bus master has its own processor that
allows it to operate independently of the motherboard,
freeing the main processor.
The XGA system also provides greater resolution
and more colors than the VGA system. The XGA can
hold up to 1M of video memory. Resolution is
variable, depending on the mode selected. Maximum
resolution is 1024 x 768, with the capability of
displaying 256 colors from a palette of 262,144
colors. The XGA can also display 65,536 colors at a
resolution of 640 x 480, providing almost
photographic quality color.
Video Adapter Maintenance
As with the monitor, maintenance of video driver
cards is generally limited to replacement of the card.
Special test equipment is available for component
level repair of some video drivers.
LIQUID CRYSTAL DISPLAYS
The development of laptop and notebook
computers required a high resolution flat screen
display with low power consumption. The most
popular are passive and active matrix liquid crystal
displays (LCDs).
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