Display Data Channel
Encyclopedia
The Display Data Channel or DDC is a collection of digital communication protocols between a computer display
and a graphics adapter
that enables the display to communicate its supported display modes to the adapter and to enable the computer host to adjust monitor parameters, such as brightness and contrast.
The standard was created by the Video Electronics Standards Association
(VESA).
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. 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 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The DDC standard has been superseded by E-DDC in 1999.
, 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.
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, providing typical clock rates of 60 to 100 Hz.
Very few display devices implemented this protocol.
, a serial bus
. 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
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
. 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
.
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
and HDMI
connectors feature dedicated DDC2B wires.
) 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
(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
version 2.0 was adopted in October 2003. A new MCCS V3 was introduced in July 2006, though didn't gain enough industry attention as of yet. The latest release of V2 standard is version 2.2, adopted February 2009.
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 = 32KB. 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
and consumer electronics.
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.
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 from the analog VGA cables that connects 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.
Computer display
A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...
and a graphics adapter
Video card
A video card, Graphics Card, or Graphics adapter is an expansion card which generates output images to a display. Most video cards offer various functions such as accelerated rendering of 3D scenes and 2D graphics, MPEG-2/MPEG-4 decoding, TV output, or the ability to connect multiple monitors...
that enables the display to communicate its supported display modes to the adapter and to enable the computer host to adjust monitor parameters, such as brightness and contrast.
The standard was created by the Video Electronics Standards Association
VESA
VESA is an international standards body for computer graphics founded in 1989 by NEC Home Electronics and eight other video display adapter manufacturers.VESA's initial goal was to produce a standard for 800×600 SVGA resolution video displays...
(VESA).
Overview
The DDC suite of standards aims to provide a "plug and play" experience 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
VGA connector
A Video Graphics Array connector is a three-row 15-pin DE-15 connector. The 15-pin VGA connector is found on many video cards, computer monitors, and some high definition television sets...
.
Extended display identification data
Extended display identification data
Extended display identification data is a data structure provided by a digital display to describe its capabilities to a video source . It is what enables a modern personal computer to know what kinds of monitors are connected to it. EDID is defined by a standard published by the Video...
(EDID) is a companion standard; it defines a compact binary file
Binary file
A binary file is a computer file which may contain any type of data, encoded in binary form for computer storage and processing purposes; for example, computer document files containing formatted text...
format describing the monitor's capabilities and supported graphics modes, stored in a read-only memory
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
(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.
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 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 in 1999.
Physical link
Prior to the DDC, the VGA standard had reserved four pins in the analog VGA connectorVGA connector
A Video Graphics Array connector is a three-row 15-pin DE-15 connector. The 15-pin VGA connector is found on many video cards, computer monitors, and some high definition television sets...
, 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.
DDC1
DDC1 is a simple, low-speed, unidirectional serial linkSerial communications
In telecommunication and computer science, 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 sent as a whole, on a link with several parallel channels...
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, 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 I²CI²C
I²C is a multi-master serial single-ended computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, cellphone, or other electronic device. Since the mid 1990s, several competitors I²C ("i-squared cee" or "i-two cee"; Inter-Integrated Circuit;...
, a serial bus
Serial communications
In telecommunication and computer science, 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 sent as a whole, on a link with several parallel channels...
. 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
Two-way communication
Two-way communication - uses communication to negotiate with the public, resolve conflict, and promote mutual understanding and respect between the organization and its public. Two-Way Communication in Public Relations...
and supports multiple bus-masters
Bus mastering
In computing, bus mastering is a feature supported by many bus architectures that enables a device connected to the bus to initiate transactions...
, DDC2B is unidirectional and allows only one bus master
Bus mastering
In computing, bus mastering is a feature supported by many bus architectures that enables a device connected to the bus to initiate transactions...
- 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
Extended display identification data
Extended display identification data is a data structure provided by a digital display to describe its capabilities to a video source . It is what enables a modern personal computer to know what kinds of monitors are connected to it. EDID is defined by a standard published by the Video...
. 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
ACCESS.bus
ACCESS.bus is a peripheral-interconnect computer bus developed by Philips in the early 1990s. It is similar in purpose to USB, in that it allows low-speed devices to be added or removed from a computer on the fly...
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 developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....
.
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
The Digital Visual Interface is a video interface standard covering the transmission of video between a source device and a display device. The DVI standard has achieved widespread acceptance in the PC industry, both in desktop PCs and monitors...
and HDMI
High-Definition Multimedia Interface
HDMI is a compact audio/video interface for transmitting uncompressed digital data. It is a digital alternative to consumer analog standards, such as radio frequency coaxial cable, composite video, S-Video, SCART, component video, D-Terminal, or VGA...
connectors feature dedicated DDC2B wires.
DDC/CI
DDC/CI (Command InterfaceInterface (computer science)
In the field of computer science, an interface is a tool and concept that refers to a point of interaction between components, and is applicable at the level of both hardware and software...
) 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
Monitor Control Command Set
VESA Monitor Command Control 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
Page orientation
Page orientation is the way in which a rectangular page is oriented for normal viewing. The two most common types of orientation are 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
Monitor Control Command Set
VESA Monitor Command Control 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 didn't gain enough industry attention as of yet. The latest release of V2 standard is version 2.2, adopted February 2009.
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 = 32KB. 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
HDMI
HDMI is a compact audio/video interface for transmitting uncompressed digital data. It is a digital alternative to consumer analog standards, such as radio frequency coaxial cable, composite video, S-Video, SCART, component video, D-Terminal, or VGA...
and consumer electronics.
E-DDC Version 1.2, approved December 2007, introduced support for DisplayPort
DisplayPort
DisplayPort is a digital display interface standard produced by the Video Electronics Standards Association . The specification defines a royalty-free digital interconnect for audio and video. The interface is primarily used to connect a video source to a display device such as a computer monitor...
(which has no dedicated DDC2B links and uses its bidirectional auxiliary channel for EDID and MCCS communication) and DisplayID
DisplayID
DisplayID is a VESA standard designed to replace E-EDID standard and EDID structure v1.4. The current version 1.1 was released in March, 2009.DisplayID features variable-length structures which encompass all existing EDID extensions as well as new extensions for 3D displays and embedded displays...
standards.
Disabling DDC
Some KVM switchKVM switch
A KVM switch is a hardware device that allows a user to control multiple computers from a single keyboard, video monitor and mouse. Although multiple computers are connected to the KVM, typically a smaller number of computers can be controlled at any given time...
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 from the analog VGA cables that connects 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.
External links
- Linux
- ddccontrol: Linux software which uses DDC/CI to control monitors supporting this protocol. (Seems to be not supported and not maintained at this time).
- 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.
- Display Tuner: Windows software which uses DDC/CI to control monitors supporting this protocol.
- ScreenBright: Small Windows software which uses DDC/CI to control monitors supporting this protocol.