The DDC suite of standards aims to provide Plug and Play and DPMS power management experiences 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.
However, during the transition, the change was not backwards-compatible and video cards using the old scheme could have problems if a DDC-capable monitor was connected.
Though I²C is fully bidirectional and supports multiple bus-masters, DDC2B is unidirectional and allows only one bus master—the graphics adapter.
The monitor acts as a slave device at the 7-bit I²C address 50h, and provides 128-256 bytes of read-only EDID.
DDC2Ab is an implementation of the I²C-based 100-kbit/s ACCESS.bus interface, which made it possible for monitor manufacturers to support external ACCESS.bus peripherals such as a mouse or keyboard with little to no additional effort.
DDC2B+ and DDC2Bi are scaled-down versions of DDC2Ab which only support monitor and graphics card devices but still allow bidirectional communication between them.
Many manufacturers did not pay attention to DDC/CI in the past, but now almost all monitors support such general MCCS commands as brightness and contrast management.
Version 1 was introduced in September 1999 and featured the addition of a segment pointer which allowed up to 32 Kbytes of display information storage for use by the Enhanced EDID (E-EDID) standard.
Other important changes were removal of the DDC1 and DDC2Ab protocols, deprecation of separate VESA P&D and FPDI device addresses, and clarifications to the DDC power requirements.
E-DDC Version 1.2, approved December 2007, introduced support for DisplayPort (which has no dedicated DDC2B links and uses its bidirectional auxiliary channel for EDID and MCCS communication) and DisplayID standards.
Some KVM switches (keyboard-video-mouse) and video extenders handle DDC traffic incorrectly, making it necessary to disable monitor plug and play features in the operating system, and maybe even physically remove pin 12 (serial data pin) from the analog VGA cables[12] that connect such device to multiple PCs.
[13] Many video card manufacturers and third parties provide control applications which can be used to select a custom display mode that does not conform to the EDID information or the monitor .INF file.