见http://www.lammertbies.nl/comm/cable/VGA-monitor.html：
The DB15VGA connector contains however 15 pins in three rows of 5 pins each. These five pins not only carry the color and synchronisation signals, but also a digital I2C interface for two-way communication between the video controller and monitor. This I2C interface makes the VGA interface very versatile. The I2C interface wasn't available on the original VGA connector definition, but has been added by the VESADDC2 definition. Via this I2C interface the video controller and monitor can exchange information about maximum capabilities like resolution and frequencies supported which prevents incompatible display modes to be selected.
又见http://en.wikipedia.org/wiki/Display_Data_Channel：
The DDC suite of standards aims to provide a "plug and play" experience for computer displays.
DDC1 and DDC2B/Ab/B+/Bi protocols are a physical link between a monitor and a video card, which was originally carried on either two or three pins in a 15-pin analog VGA connector. Extended display identification data (EDID) is a companion standard; it defines a compact binary file format describing the monitor's capabilities and supported graphics modes, stored in a read-only memory (EEPROM) chip programmed by the manufacturer of the monitor. The format uses a description block containing 128 bytes of data, with optional extension blocks to provide additional information. The most current version is Enhanced EDID (E-EDID) Release A, v2.0.
The first version of the DDC standard was adopted in August 1994. It included the EDID 1.0 format and specified DDC1, DDC2B and DDC2Ab physical links. DDC version 2, introduced in April 1996, split EDID into a separate standard and introduced the DDC2B+ protocol. DDC version 3, December 1997, introduced the DDC2Bi protocol and support for VESA Plug and Display and Flat Panel Display Interface on separate device addresses, requiring them to comply with EDID 2.0.
The DDC standard has been superseded by E-DDC in 1999.
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