Prior to 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 called EDID, could be sent to the graphics card in this fashion.
Eventually even 128 bytes proved to be too little. While looking at methods to address the problem, VESA decided to once again redefine the pins, and created E-DDC (E for Extended). ID1 was now used as the data pin from the I2C bus, and the formerly unused pin 15 became the I2C clock. The data itself was in the same format as the earlier DDC, so changes to the computer-side drivers were minimal.
Using this system the monitor could exchange considerable information with the graphics card. 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 pin. E-DDC included this as an option, allowing monitor manufacturers to provide A.b support for no additional effort. There was a brief appearance of such monitors in the mid 1990s, but they disappeared with the wider use of USB.
See also: EDID
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