The Display Data Channel or DDC is a collection of digital communication protocols between a computer display and a graphics adapter which allows the display to communicate its supported display modes to the adapter and adjust monitor parameters such as brightness and contrast from the computer host.
The standard was created by the Video Electronics Standards Association (VESA).
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The DDC standard has been superseded by E-DDC in 1999.
Prior to the DDC, the VGA standard had reserved four pins in the analog VGA connector, known as ID0, ID1, ID2 and ID3 (pins 11, 12, 4 and 15) for identification of monitor type. The first three ID pins, attached to resistors to pull one or more of them to ground (GND), allowed for the definition of up to seven monitor types, with all open (floating or NC, No Connection) - meaning "no monitor". However, only three types of monitor were defined - monochrome with a resolution of less than 1024×768 (NC, NC, GND), color with a resolution of less than 1024×768 (NC, GND, NC), and color with 1024×768 (GND, GND, NC). Pin ID3 was unused. DDC changed the purpose of these pins to incorporate a serial link interface.
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