Display Data Channel
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The Display Data Channel, or DDC, is a collection of
protocols Protocol may refer to: Sociology and politics * Protocol (politics), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technology ...
for digital communication between a
computer display A computer monitor is an output device that displays information in pictorial or textual form. A discrete monitor comprises a visual display, support electronics, power supply, housing, electrical connectors, and external user controls. The d ...
and a
graphics adapter A graphics card (also called a video card, display card, graphics adapter, VGA card/VGA, video adapter, display adapter, or mistakenly GPU) is an expansion card which generates a feed of output images to a display device, such as a computer moni ...
that enable the display to communicate its supported display modes to the adapter and that enable the computer host to adjust monitor parameters, such as brightness and contrast. Like modern analog VGA connectors, the
DVI Digital Visual Interface (DVI) is a video display interface developed by the Digital Display Working Group (DDWG). The digital interface is used to connect a video source, such as a video display controller, to a display device, such as a comp ...
and
DisplayPort DisplayPort (DP) is a digital display interface developed by a consortium of PC and chip manufacturers and standardized by the Video Electronics Standards Association (VESA). It is primarily used to connect a video source to a display device su ...
connectors include pins for DDC, but DisplayPort only supports DDC within its optional Dual-Mode DP ( DP++) feature in DVI/HDMI mode. The standard was created by the
Video Electronics Standards Association VESA (), formally known as Video Electronics Standards Association, is an American technical standards organization for computer display standards. The organization was incorporated in California in July 1989To retrieve the information, searc ...
(VESA).


Overview

The DDC suite of standards aims to provide
Plug and Play In computing, a plug and play (PnP) device or computer bus is one with a specification that facilitates the recognition of a hardware component in a system without the need for physical device configuration or user intervention in resolving resou ...
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 The Video Graphics Array (VGA) connector is a standard connector used for computer video output. Originating with the 1987 IBM PS/2 and its VGA graphics system, the 15-pin connector went on to become ubiquitous on PCs, as well as many monitors ...
.
Extended display identification data Extended Display Identification Data (EDID) and Enhanced EDID (E-EDID) are metadata formats for display devices to describe their capabilities to a video source (e.g. graphics card or set-top box). The data format is defined by a standard published ...
(EDID) is a companion standard; it defines a compact
binary file A binary file is a computer file that is not a text file. The term "binary file" is often used as a term meaning "non-text file". Many binary file formats contain parts that can be interpreted as text; for example, some computer document fil ...
format describing the monitor's capabilities and supported graphics modes, stored in a
read-only memory Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
(
EEPROM EEPROM (also called E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or as a ...
) 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''. The first version of the DDC standard was adopted in August 1994. It included the EDID 1.0 format and specified DDC1, DDC2B and DDC2Ab physical links. ''DDC version 2'', introduced in April 1996, split EDID into a separate standard and introduced the DDC2B+ protocol. ''DDC version 3'', December 1997, introduced the DDC2Bi protocol and support for
VESA Plug and Display VESA Plug and Display (P&D) is a video connector for digital monitors, such as flat panel displays and video projectors, ratified by VESA. It was introduced around the same time as DFP, marketed as a replacement of VESA Enhanced Video Connector ...
and Flat Panel Display Interface on separate device addresses, requiring them to comply with EDID 2.0. The DDC standard has been superseded by
E-DDC The Display Data Channel, or DDC, is a collection of protocols for digital communication between a computer display and a graphics adapter that enable the display to communicate its supported display modes to the adapter and that enable the com ...
in 1999.


Physical link

Prior to the DDC, the
VGA Video Graphics Array (VGA) is a video display controller and accompanying de facto graphics standard, first introduced with the IBM PS/2 line of computers in 1987, which became ubiquitous in the PC industry within three years. The term can no ...
standard had reserved four pins in the analog
VGA connector The Video Graphics Array (VGA) connector is a standard connector used for computer video output. Originating with the 1987 IBM PS/2 and its VGA graphics system, the 15-pin connector went on to become ubiquitous on PCs, as well as many monitors ...
, known as ID0, ID1, ID2 and ID3 (pins 11, 12, 4 and 15) for identification of monitor type. These ID pins, attached to resistors to pull one or more of them to ground (GND), allowed for the definition of the monitor type, with all open (n/c, not connected) meaning "no monitor". In the most commonly documented scheme, the ID3 pin was unused and only the 3 remaining pins were defined. The ID0 was pulled to GND by color monitors, while the monochrome monitors pulled ID1 to GND. Finally, the ID2 pulled to GND signaled a monitor capable of 1024×768 resolution, such as
IBM 8514 IBM 8514 is a graphics card manufactured by IBM and introduced with the IBM PS/2 line of personal computers in 1987. It supports a display resolution of pixels with 256 colors at 43.5  Hz ( interlaced), or at 60 Hz ( non-interlaced ...
. In this scheme, the input states of the ID pins would encode the monitor type as follows: More elaborate schemes also existed that used all of the 4 ID pins while manipulating the HSync and VSync signals in order to extract 16 bits (4 ID pin values for each of the 4 combinations of HSync and VSync states) of monitor identification. DDC changed the purpose of the ID pins to incorporate a serial link interface. 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. The DDC signal can be sent to or from a video graphics array (VGA) monitor with the I2C protocol using the master's serial clock and serial data pins.


DDC1

DDC1 is a simple, low-speed, unidirectional
serial link In telecommunication and data transmission, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are ...
protocol. Pin 12, ID1 functions as a data line that continuously transmits the 128-byte EDID block, and the data clock is synchronised with
vertical sync Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog s ...
, providing typical clock rates of 60 to 100 Hz. Very few display devices implemented this protocol.


DDC2

The most common version, called DDC2B, is based on
I²C I2C (Inter-Integrated Circuit, ), alternatively known as I2C or IIC, is a synchronous, multi-controller/multi-target (master/slave), packet switched, single-ended, serial communication bus invented in 1982 by Philips Semiconductors. It is wide ...
, a
serial bus In telecommunication and data transmission, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are ...
. Pin 12, ID1 of the VGA connector is now used as the data pin from the I²C bus, and the formerly-unused pin 15 became the I²C clock; pin 9, previously used as a mechanical key, supplied +5V DC power up to 50mA to drive the EEPROM, this allows the host to read the EDID even if the monitor is powered off. Though I²C is fully
bidirectional Bidirectional may refer to: * Bidirectional, a roadway that carries traffic moving in opposite directions * Bi-directional vehicle, a tram or train or any other vehicle that can be controlled from either end and can move forward or backward with e ...
and supports multiple bus-masters, DDC2B is unidirectional and allows only one
bus master In computing, bus mastering is a feature supported by many bus architectures that enables a device connected to the bus to initiate direct memory access (DMA) transactions. It is also referred to as first-party DMA, in contrast with third-party ...
- 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. Because this access is always a read, the first I²C octet will always be A1h. DDC2Ab is an implementation of the I²C-based 100 kbit/s ACCESS.bus interface, which allowed monitor manufacturers to support external ACCESS.bus peripherals such as a mouse or keyboard with little to no additional effort; such devices and monitors were briefly available in the mid 1990s, but disappeared with the introduction of
USB Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...
. DDC2B+ and DDC2Bi are scaled-down versions of DDC2Ab which only support monitor and graphics card devices but still allow bidirectional communication between them. DDC2 is not exclusive to the VGA connector, as both
DVI Digital Visual Interface (DVI) is a video display interface developed by the Digital Display Working Group (DDWG). The digital interface is used to connect a video source, such as a video display controller, to a display device, such as a comp ...
and
HDMI High-Definition Multimedia Interface (HDMI) is a proprietary audio/video interface for transmitting uncompressed video data and compressed or uncompressed digital audio data from an HDMI-compliant source device, such as a display controller, ...
connectors feature dedicated DDC2B wires.


DDC/CI

DDC/CI ( Command Interface) standard was introduced in August 1998. It specifies a means for a computer to send commands to the monitor, as well as receive sensor data from the monitor, over a bidirectional link. Specific commands to control monitors are defined in a separate
Monitor Control Command Set VESA Monitor Control Command Set or MCCS is a computer standard developed by Video Electronics Standards Association. It defines a binary protocol for controlling the properties of computer monitors from a host device such as PC, set-top box, etc. ...
(MCCS) standard version 1.0, released in September 1998. 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. Most DDC/CI monitors support only a small subset of MCCS commands and some have undocumented commands. 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. ''DDC/CI'' standard describes a full suite of bidirectional control protocols - DDC2Ab, DDC2Bi and DDC2B+ - in a single standard and provides a means for packaging Monitor Control Command Set commands. DDC/CI version 1.1 was adopted in October 2004. ''
Monitor Control Command Set VESA Monitor Control Command Set or MCCS is a computer standard developed by Video Electronics Standards Association. It defines a binary protocol for controlling the properties of computer monitors from a host device such as PC, set-top box, etc. ...
'' version 2.0 was adopted in October 2003. A new MCCS V3 was introduced in July 2006, though did not gain enough industry attention yet. The latest release of V2 standard is version 2.2a, adopted January 2011.


OS support for DDC/CI

Despite its ubiquity in post-2016 displays, DDC/CI is not generally used by the operating system by default for brightness control on external displays. Additional software can be used to send commands to the display, but the degree of system integration vary. Windows exposes DDC/CI as the ''Monitor Configuration'' Win32 API series.


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 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. Earlier DDC implementations used simple 8-bit data offset when communicating with the EDID memory in the monitor, limiting the storage size to 28 bytes = 256 bytes, but allowing the use of cheap 2-Kbit EEPROMs. In E-DDC, a special I²C addressing scheme was introduced, in which multiple 256-byte segments could be selected. To do this, a single 8-bit segment index is passed to the display via the I²C address 30h. (Because this access is always a write, the first I²C octet will always be 60h.) Data from the selected segment is then immediately read via the regular DDC2 address using a repeated I²C 'START' signal. However, VESA specification defines the segment index value range as 00h to 7Fh, so this only allows addressing 128 segments × 256 bytes = . The segment index register is volatile, defaulting to zero and automatically resetting to zero after each NACK or STOP. Therefore, it must be set every time access to data above the first 256-byte segment is performed. The auto-reset mechanism is to provide for backward compatibility to, for example, DDC2B hosts, otherwise they may be stuck at a segment other than 00h in some rare cases. 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 1.1'', approved March 2004, featured support for
HDMI High-Definition Multimedia Interface (HDMI) is a proprietary audio/video interface for transmitting uncompressed video data and compressed or uncompressed digital audio data from an HDMI-compliant source device, such as a display controller, ...
and consumer electronics. ''E-DDC Version 1.2'', approved December 2007, introduced support for
DisplayPort DisplayPort (DP) is a digital display interface developed by a consortium of PC and chip manufacturers and standardized by the Video Electronics Standards Association (VESA). It is primarily used to connect a video source to a display device su ...
(which has no dedicated DDC2B links and uses its bidirectional auxiliary channel for EDID and MCCS communication) and
DisplayID DisplayID is a VESA standard for metadata describing display device capabilities to the video source. It is designed to replace E-EDID standard and EDID structure v1.4. The DisplayID standard was initially released in December 2007. Version 1.1 ...
standards. ''E-DDC Version 1.3'' from September 2017 contains corrections for errata and minor clarifications.


Disabling DDC

Some
KVM switch A KVM switch (with KVM being an abbreviation for "keyboard, video, and mouse") is a hardware device that allows a user to control multiple computers from one or more sets of keyboards, video monitors, and mice. Name Switches to connect mu ...
es (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 that connect such device to multiple PCs. Microsoft Windows features a standard "Plug and Play Monitor" driver which uses the display's EDID information to construct a list of supported monitor modes. The Display Resolution control panel applet allows the user to disable this driver's Plug and Play features and manually select any resolution or refresh rate supported by the video card. Many video card manufacturers and third parties provide control applications which allow the user to select a custom display mode that does not conform to the EDID information or the monitor .INF file.


See also

*
Display Control Channel Display Control Channel (DCC) is an advanced method of implementing an on-screen display (OSD) technology on KVM switches. On-screen display technology used on KVM switches is used to show control and selection functions pertaining to the KVM, su ...
*
Extended Display Identification Data Extended Display Identification Data (EDID) and Enhanced EDID (E-EDID) are metadata formats for display devices to describe their capabilities to a video source (e.g. graphics card or set-top box). The data format is defined by a standard published ...
*
Reverse DDM Dynamic device mapping is a technology for USB KVM switches which is sometimes implemented as an alternative to standard USB keyboard and mouse emulation. Reverse DDM (short for Dynamic Device Mapping), on the other hand, is a related technology ...


Notes

{{notelist


References

* ''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


External links

* Linux *
ddcci-driver-linux
Linux kernel driver supporting backlight control for monitors supporting DDC/CI *
ddccontrol
Linux software which uses DDC/CI to control monitors supporting this protocol (seems to b
maintained on github
*
ddcutil
(formerly ddctool) Linux software for querying and changing monitor settings over DDC/CI *
MonitorDarkly
proof of concept for exploiting monitors over vendor-specific DDC/CI extensions * Windows *
softMCCS
Windows software which uses DDC/CI to control monitors supporting this protocol **
Nicomsoft WinI2C/DDC
Windows Software Development Kit (SDK) which uses I2C and DDC/CI protocols to control monitors'' (removed from support, downloadable ZIP files don't contain application) *
Monitorian
Open source application that uses DDC/CI to change brightness from a icon in the Task Bar *
Twinkle Tray
similar to Monitorian *
Win10_BrightnessSlider
similar to Monitorian * Mac *
DisplayBuddy
a Mac app for control of display functions *
NativeDisplayBrightness
a minimal DDC luminosity app for Mac OS X *
ddcctl
an open source tool for querying and changing monitor settings over DDC/CI *
MonitorControl
an open source tool for mac which uses DDC/CI to control monitors supporting this protocol *
Lunar
an open source app that uses DDC to sync a Mac's primary display brightness and contrast with external monitors VESA