Elsa Victory Erazor LT User Manual

The 16-bit graphics modes are organized differently.  Most common are (R-G-B) 5-6-5
(e.g.  XGA) and 6-6-4 (e.g.  i860).  5-6-5 means that 5 bits are used for each of red and
blue and 6 bits are used for green.  In the case of 6-6-4,  6 bits are used for red and green
and 4 bits for blue.  Both ways of assigning the bits correspond to the color sensitivity of
the human eye: this is highest for green and lowest for blue.  65,536 different colors can
be displayed.

The TrueColor mode is more complex, using 24 bits per pixel.  Here, 8 bits are available
for each color component (256 levels), resulting in 16.7 million different color hues.  There
are more colors available than pixels on the screen (1.3 million pixels at a resolution of
1280 x 1024 ).

VESA DDC refers to a serial data channel between the monitor and the graphics board.
Required for this is that both components support DDC and that the monitor cable
includes the additional DDC line.  An extended monitor cable is used so that the can send
data about its technical specification, such as the name, model, maximum horizontal fre-
quency, timing definitions etc.  or receive commands from the graphics board.

There are various standards; DDC1, DDC2B, and DDC2AB.

Only the monitor can send data (unidirectional).  A line in the monitor cable is used to
send a continuous data stream from the monitor to the graphics board.  In the case of a
standard IBM VGA compatible 15-pin monitor connector, pin 12 (formerly used as monitor
ID bit 1) is used for data transmission, and the Vertical Sync signal of pin 14 is used as
transmission clock (VCLK).  An EDID data set (Extended Display Identification) of
128 bytes is sent repeatedly, from which the major monitor data can be read in the com-
puter.  The computer can then read the most important data, e.g.  the monitor size, the
extent of DPMS support and a list of the most important VESA monitor timings sup-
ported, and some freely definable monitor timings.

The data channel is based on the I2C bus type with the access bus protocol and can be
operated in both directions (bidirectionally between monitor and board).  In the case of a
standard IBM VGA compatible 15-pin monitor connector, pin 12 (formerly used as monitor
ID bit 1) is used for data transmission (SDA), and the pin 15 (formerly used as monitor ID
bit 3) is used as transmission clock (SCL).  The graphics board can request the short EDID
information (see DDC1) as well as the more comprehensive VDIF information (VESA Dis-
play Identification File).