- VESA Display Data Channel
VESA Display Data Channel (or DDC) is a
VESAstandard that defines how to read certain pins in a standard SVGAmonitor 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 1024×768, color with resolution of less than 1024×768, and color with 1024×768.
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.
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
I²Cbus, 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.buswith 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.
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 the KVM switch is operated.
* [http://documents.epanorama.net/documents/pc/vga_bd15.html VGA analogue display connector]
* [http://www.vesa.org/Standards/summary/2004_3.htm VESA E-DDC Standard]
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