Physical Link
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.
Read more about this topic: Display Data Channel
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