Display Data Channel

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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).

Contents

[edit] Scope

The DDC suite of standards aims to provide "plug and play" experience for 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, but recently made way to DVI and HDMI connectors which feature dedicated DDC2B wires.

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.

A separate DDC/CI standard defines means to exchange monitor control command messages over DDC2Ab/B+/Bi bidirectional links.

[edit] Physical link

Prior to the DDC, the VGA standard had reserved three pins, known as ID0, ID1, and ID2 (pins 9, 12, and 15) for indentification of monitor type. The pins, attached to resistors to pull one or more of them high, allowed for the definition of up to seven monitor types, with all zero meaning "no monitor". However, only three types of monitor were defined - monochrome with resolution of less than 1024×768, color with resolution of less than 1024×768, and color with 1024×768 - and pin ID2 was unused.

DDC changed the purpose of these pins to incoprorate a serial link interface into VGA connector.

[edit] DDC1

DDC1 is a simple, low-speed unidirectional serial link protocol. Pin 12 (ID2) functions as a data line that continuoulsy transmits the 128-byte EDID block, and data clock which is syncronised with vertical sync, providing typical clock rates of 60 to 100 Hz.

Very few display devices implemented this protocol.

[edit] DDC2B

The most common version, called DDC2B, is based on I²C, a serial bus. Pin 12, ID1 is now used as the data pin from the I²C bus, and the formerly unused pin 15 became the I²C clock; pin 9 supplied +5V DC power. Though I²C is fully bidirectional and supports multiple bus-masters, DDC2B is unidirectional and allows only one bus master - the graphics adapter.

DDC2Ab is an implementation of I²C-based 100 kbit/s ACCESS.bus interface, which allowed monitor manufacturers to support external ACCESS.bus peripherals such as mouse or keyboard with little upto no additional effort; such devices and monitors were briefly available in the mid 1990s, but disappeared with the introduction of USB.

DDC2B+ and DDC2Bi are scaled down versions of DDC2AB which only support monitor and graphics card devices but still allow bidirectional communication between them.

[edit] DDC/CI

DDC/CI (Command Interface) specifies a means for computer to send commands to the monitor, as well as receive sensor data from the monitor. It requires a bidirectional link such as DDC2Bi.

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.

Specific commands used to control monitors are defined in a separate Monitor Control Command Set (MCCS) standard.

[edit] Revision history

The first version of DDC was adopted in August 1994. It included EDID format and specified DDC1, DDC2B and DDC2Ab physical links.

DDC version 2, introduced in April 1996, split EDID into a separate standard and introduced DDC2B+ protocol.

DDC version 3, December 1997, introduced DDC2Bi protocol, and support for VESA Plug and Display and Flat Panel Display Interface on separate device adresses, requiring them to comply with EDID 2.0.

DDC/CI was introduced in August 1998. It described a full suite of bidirectional control protocols - DDC2Ab, DDC2Bi and DDC2B+ in a single standard and provided a means for packaging Monitor Control Command Set commands. This release coincided with MCCS standard. DDC/CI version 1.1 was adopted in October 2004[1].

Monitor Control Command Set Standard v2.0 was adopted in October 2003. Version 3.0 was introduced in July 2006, but never gained industry attention and was superseded by the most current version 2.2, adopted February 2009.

[edit] E-DDC

Enhanced Display Data Channel (E-DDC) is the most recent revision of the DDC standard. Version 1 was introduced in September 1999 and featured addition of a segment pointer which allowed up to 32 Kbytes of display information storage for use by Enhanced EDID (E-EDID) standard. Earlier DDC implementations used simple 8-bit I²C addressing when communicating with the memory in the monitor, limiting the storage size to 28 bytes = 256 bytes, but allowing the use of cheap 2 Kbit EEPROMs.

Other important changes were removal of DDC1 and DDC2AB protocols, deprecation of separate VESA P&D and FDDI device adresses, and clarification to the DDC power requirements.

E-DDC Version 1.1, approved March 2004, featured support for HDMI and consumer electronics.

E-DDC Version 1.2, approved December 2007, introduced support for DisplayPort (which has no dedicted DDC2B links and uses its bidirectional auxiliary channel for EDID and MCCS communication) and DisplayID standards.

[edit] See also

[edit] References

  1. ^ VESA Standards Summaries: Display Data Channel Command Interface (DDC/CI) Standard, Version 1.1 (VESA document VESA-2004-10)
  • Extended Display Identification Data (EDID) Standard, Version 3, 1997, VESA
  • VESA Standards FAQ
  • Display interfaces: fundamentals. Bob Myers, Robert L. Myers, Society for Information Display

[edit] External links

Retrieved from "http://en.wikipedia.org/wiki/Display_Data_Channel"
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