ELSA Synergy II, User Manual

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All about graphics
ELSA SYNERGY II
21
VESA DDC (Display Data Channel)
The Display Data Channel provides a serial data channel between the monitor and the
graphics board, as long as both support DDC and the monitor cable includes the addi-
tional DDC wire. This feature allows the monitor data to be sent automatically to the
graphics board (e.g. name, type, max. horizontal frequency, timing definitions etc.) or
even for the graphics board to send instruction to the monitor.
There are various standards; DDC2B and DDC2AB.
DDC2B
A bi-directional data channel based on the I2C access-bus protocol is used for the com-
munication between monitor and 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 (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 Display Identification
File).
DDC2AB
With DDC2AB additional to DDC2B, the computer can send commands for controlling the
monitor, e.g. for adjusting the screen position or the brightness (similar to ACCESS bus).
Modern monitors and graphics boards no longer use this standard.
The pin assignment of the VGA D-shell socket can be found in the chapter 'Technical
data'.