VESA Display Data Channel

VESA Display Data Channel (or DDC) is a VESA standard that defines how to read certain pins in a standard SVGA monitor to query the monitor's cabilities.

Prior the DDC, the SVGA cable included three pins that were used for this purpose, known as ID0, ID1 and ID2. These were attached to resistors to pull one or more of them high, allowing for the definition of up to seven monitor types (all zero meant "no monitor"). However ID2 was not used, so the standard defined only three monitors; monochrome with resolution of less than 1024x768, color with resolution of less than 1024x768, and color with 1024x768.

The introduction of DDC dramatically improved the capabilities of the system. Instead of using the pins to define the monitor, the purpose of the pins was changed and ID1 was used as a serial line. Whenever the monitor received a vertical sync signal (every 60th of a second or so) it would read out its capabilities onto the ID1 pin. Up to 128 bytes of data, stored in compact format, could be sent to the graphics card in this fashion.

Eventually even this 128 bytes proved to be too little. While looking at methods to address the problem, VESA created the E-CCD standard. ID1 was again re-defined, this time to be the data pin from the I²C bus. The formerly unused pin 15 became the I²C clock.

Using this system the monitor could exchange considerable information with the graphics card. The data itself was in the same format as the earlier DDC, so changes to the computer-side drivers were minimal. In addition, the graphics card could send information back to the monitor. This allowed software on the computer to control the monitor, bypassing the typically hard to use in-monitor controls. I²C could easily be expanded into a full ACCESS.bus with the addition of +5V, which the E-DDC included as a standard, allowing for monitor manufacturers to provide A.b support for no additional effort.

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