VGA video interface
Introduction on the VGA interface
Although strict VGA monitors and graphic cards haven't been sold for more than ten years,
VGA, short for Video Graphics Array remains
the best known standard for graphics on IBM PC compatible computers.
There are two reasons for this. First of all, the basic VGA display
modes of 80x25 character mode and 640x480 in graphics mode are supported by all
modern graphic cards, independent of the extended
modes supported by these cards. Therefore all computers start in one of these basic VGA modes and only
after the operating system
has loaded the device specific graphic card drivers the operating system will switch to a higher resolution mode. Second, despite the
higher resolution and color depth, the connector used on most computers to connect the monitor with the computer is still the same as
defined by the VGA standard. Therefore most people will talk about a VGA display and controller,
even if it is a XGA, super VGA or whatever
higher resolution version available nowadays.
Before VGA was introduced in 1987, several different display adapters were used
in IBM compatible PCs. The MDA monochrome display adapter was designed
for text mode, but manufacturer Hercules soon introduced an MDA compatible monochrome clone card
capable of displaying graphics.
Pure graphics could be viewed with a CGA color graphics adapter. This adapter was nice for playing games,
but general computer use like word processing
was difficult because of the low text resolution with only fourty characters per line. The EGA
enhanced graphics adapter tried to combine
both text modes and higher resolution graphic modes, but the 640x350 graphics mode suffered from the problem
that the pixels were not
square when a display with a width:height ratio of 4:3 was used. VGA was introduced by IBM
to solve all those problems in one new
Other than with MDA, CGA and EGA, the A in VGA
is not used for the word adapter, but for array. This is because IBM wanted to define a video
standard that would not only could be implemented as a separate video card in an extension slot, but also as an on-board chip
on the motherboard. In fact the first PS/2 series were shipped with an on-board VGA chip.
Current use of the VGA connector
Although modern display interface definitions like DVI are slowely replacing the VGA interface,
most lower resolution and low-cost
graphic card and display implementations still use the 15 pins DB15 VGA interface
as connection between the video graphics controller and
monitor. The size of the connector equals the size of the DB9 serial interface connector. The
DB15 VGA connector contains however
15 pins in three rows of 5 pins each. These five pins not only carry the color and synchronisation signals,
but also a digital
I2C interface for two-way communication between the video controller and monitor.
This I2C interface makes the VGA interface
very versatile. The I2C interface wasn't available on the original VGA connector definition, but has been added by
the VESA DDC2 definition.
Via this I2C interface the video controller and monitor can exchange information about maximum capabilities
like resolution and frequencies supported which prevents incompatible display modes to be selected.
VGA DB15 connector pinout
The pin layout of the VGA interface connector is shown in the figure below.
Three pins are used to carry the three basic RGB
color signals red, green and blue and two pins carry the horizontal and vertical sync signal.
The red, green and blue
signal lines have their own ground return line. The picture shows the VGA DDC2 connector including
the I2C SLC clock and SDA data
lines for exchanging digital data between the video controller and the display.
VGA connector pinout
|9||VDC||5 VDC supply (fused)|
|12||SDA||DDC / I2C data|
|15||SCL||DDC / I2C clock|
In theory, there is no difference between theory and practice.
But, in practice, there is.
JAN L.A. VAN DE SNEPSCHEUT