Display Data Channel

I want to introduct something about .
The Display Data Channel or DDC is a digital connection between a computer display and a graphics adapter that allows the display to communicate its specifications to the adapter. The standard was created by the Video Electronics Standards Association (VESA).
Contents
1 Mechanical
2 Electrical
3 Logical
4 Enhanced Display Data Channel (E-DDC)
5 DDC/CI
6 Disabling DDC/CI
7 VESA Display Data Channel
8 References
9 External links
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Mechanical
The DDC link is carried on three pins data, clock and ground in a 15-pin VGA connector, a DVI connector or an HDMI connector.
Electrical
The current version of DDC, called DDC2B, is based on the I2C bus. This is a serial bus that allows multiple bus masters, although DDC2B allows only one master the graphics adapter. The monitor (e.g. a CRT or LCD) contains a read-only memory (ROM) chip programmed by the manufacturer with information about the graphics modes that the monitor can display.
8-bit I2C addressing is used when communicating with the memory in the monitor limiting the memory size to 28 bytes = 256 bytes = 2kbits. A 24C02 EEPROM would be suitable for use in the monitor.
Logical
The data in the monitor's ROM is held in a standard format called extended display identification data (EDID). This format is defined by VESA.
Enhanced Display Data Channel (E-DDC)
This version incorporated VESA DDC version 3 standard. Major changes were the addition of the E-DDC protocols, removal of DDC1 protocol and clarification to the DDC power requirements.
In Version 1 Revision 1, scope has been expanded to encompass usage in consumer electronic products and video interfaces other than VGA with text clarified in several places. Support for the P&D and FPDI-2 standards has been removed.
DDC/CI
DDC/CI (CI = command interface) is an extension to DDC specified by VESA in August 1998. It allows a computer with a suitably designed graphics adapter to adjust monitor parameters such as brightness and color balance, or to initiate degaussing. DDC/CI monitors are sometimes supplied with an external color sensor, to allow automatic calibration of the monitor's color balance. Some tilting DDC/CI monitors support an auto pivot function, where a rotation sensor in the monitor enables the operating system to keep the display upright as the monitor is moved between its portrait and landscape positions.
The latest version of the specification is "Display Data Channel Command Interface (DDC/CI) Standard, Version 1.1", October 2004.[1]
Specific command used to control monitors are defined in Monitor Control Command Set (MCCS) Standard.
Disabling DDC/CI
On Microsoft Windows (Versions XP and above), there is no software-provided option to disable plug and play monitor detection. This causes problems with computer/monitor switching applications and causes computer games to select display resolutions higher than the monitor is physically capable of displaying resulting in a garbled display.
In these circumstances, it may be necessary to remove pin 12 from the monitor VGA cable, to disable plug and play monitor detection. This allows display resolution to be selected manually and not overridden when the display adapter is removed and reinserted or a KVM switch is operated.
On XFree based systems (Linux, BSD*,...) it's possible to disable DDC by adding an option to the section "Device" in /etc/X11/xorg.conf similar to:Option "NoDDCValue"
orOption "DDC" "No"
success can vary depending on the graphics card (Tested with the VIA Xorg driver). That could solve problems with buggy drivers or software.
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 capabilities.
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 1024768, color with resolution of less than 1024768, and color with 1024768.
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 write 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.
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