display data channel (VESA) (DDC)
The Display Data Channel(DDC) was specified by the Video Electronics Standards Association( VESA) for the control of monitors. The monitor, the operating system and the graphics card exchange technical data via the DDC lines, thus enabling plug-and-play. The DDC lines in themselves form an I2C bus via which the internal communication of the peripheral devices with the central unit takes place.
In order to fulfill the plug-and-play, the graphics cardchecks the parameters transmitted by the monitor and sets an appropriate graphics mode or refresh rate, provided that the corresponding function is supported by the operating system, such as Windows 98 and Windows 2000, or by the drivers of the graphics cards. The signal is transmittedserially via the monitor cable, or via the HDMI cable in the case of HDMI.
There are several DDC versions: In the simple DDC1 version, the monitor's identification data is constantly transmitted unidirectionally to the graphics card as Extended Display Identification Data( EDID). The 128 byte long data records contain information about which standard resolutions and which DPMS modes are supported. Further identification data includes the screen size and the manufacturer specification.
The version DDC2 knows two versions. In the version DDC2B a bidirectional communication between monitor and graphic card is supported, whereby the graphic card can request the characteristic data of the monitor, which are supplied over the VESA Display Information File( VDIF). The monitor can therefore respond to specific requests from the graphics card. This mode is used most frequently.
In the DDC2AB version, additional data for controlling the monitor is transmitted bidirectionally. In addition, the monitor can be controlled in brightness, contrast, etc. from the graphics card using special software.