References

Reference Documents

Searching for DDC documentation can be confusing. The DDC protocol evolved, being part of Access Bus at one point, and shed various pieces, e.g. EDID, into separate specifications along the way. See the Wikipedia DDC page for an overview of how it morphed.

VESA Standards

VESA Display Data Channel Command Interface Standard, Version 1. August 14, 1998. This is a predecessor to the current standard, DDC/CI 1.1. The contents of this document sometimes clarify the current specification, but should not be regarded as definitive.
VESA Access.bus Specificaions Version 3.0, September 1995. Another predecessor specification.
VESA Display Data Channel Command Interface (DDC/CI) Standard Version 1.1, October 29, 2004 Also available here
VESA Monitor Control Command Set (MCCS) Standard Version 1, September 11, 1998.
VESA Monitor Control Command Set (MCCS) Standard Version 2, October 17, 2003
VESA Monitor Control Command Set (MCCS) Standard Version 2.1, May 28, 2005
VESA Monitor Control Command Set (MCCS) Standard Version 3, July 27, 2006. MCCS 3.0 proved too great a departure from Version 2.1 and did not gain acceptance.
VESA Monitor Control Command Set (MCCS) Update Document for MCCS Standard Version 3, March 20, 2007
VESA Monitor Control Command Set (MCCS) Standard Version 2.2, January 19, 2009. MCCS 2.2 incorporated features from Version 3.0, but is upwardly compatible with 2.1
VESA Monitor Control Command Set (MCCS) Standard Version 2.2a, July 13, 2011. The most recent standard. Also available here.
VESA Enhanced Extended Display Identification Data Standard (E-EDID Standard), Relase A, Revision 1, February 9, 2000
VESA Enhanced Display Data Channel (EDDC) Standard, Version 1.2, December 26, 2007
VESA Enhanced Display Data Channel (E-DDC) Standard, Version 1.3, 11 September 2017
VESA DisplayID Standard, Version 2.0, 11 September 2017
DisplayID v2.0 Errata E6, Published 3/15/20
DisplayPort Standard Version 1, Revision 1a 11 January 2008 This early version of the DisplayPort standard is available on the Internet Archive's Wayback Machine.
DisplayPort Standard Version 1, Revision 2 January 5, 2010 This early version of the DisplayPort standard is available on the Internet Archive's Wayback Machine. More recent versions of the standard appear to require VESA membership.
DisplayPort Interoperability Guideline Version 1.1, January 28, 2008 Technical specification for DisplayPort Dual-Mode (DP++)
VESA Introduces Updated Dual-Mode Standard for Higher Resolution Interoperability with HDMI Displays
VESA Standards FAQ

Many standards can be found here

USB Implementers Forum Standards

Many standards can be found here

I²C

As background, I²C is a low level, slow speed specification of a 2 wire protocol for sending and receiving bytes. DDC/CI is layered on top of that protocol, describing particular sequences of bytes that are communicated between a master device (the host computer) and a slave device (a monitor) at address x37 on an I²C bus. By analogy, DDC is as a subclass of superclass I²C, that is DDC obeys the rules of I²C, but is more specific. For example, only certain byte sequences are legal. SMBUS is another protocol based on I²C.

ddcutil reads from and writes to the userspace /dev-i2c* devices which present a file system like abstraction of an I²C bus. It leaves the bit-banging to the video device drivers, e.g. amdgpu, nouveau.

One source of confusion in DDC documentation is the specification of slave adddresses on the I²C bus. Sometimes you'll see the DDC address documented as x37, other times as the pair x6e/x6f. Leaving aside the implementation of 10 bit I²C bus addresses (uncommon), I²C slave addresses are encoded in a single byte. The high order 7 bits are used for the address per se. The low order bit is 0 for a write operation, 1 for a read operation. Much of the VESA documenation regards the entire 8 bit value as an address, which means address x37 becomes the pair x6e/x6f. Similarly, the 7 bit address for the EDID is x50; regarded as an 8 bit address, the EDID address becomes xA0/xA1. ddcutil specifies I²C slave addresses in 7 bit form, as do utilities such as i2cdetect, i2cget, and i2cset.

The following specifications and articles provide backgroud information about the I²C bus.

SMBus is a related standard. The following report can help in understanding I²C.

SMBus Made Simple, Texas Instruments

Wikipedia has several high quality pages on related specifications: They may be helpful when trying to figure out why DDC communication is not working:

DisplayPort
HDMI
Thunderbolt
USB-C. See in particular section Hardware Support for a discussion of USB-C Alternate Mode
Display Data Channel Has a useful summary of the relationships, evolution, and deprecation among releated standards,

Documentation on DP <-> HDMI/DVI conversion:

Type C Connectors

Benq: USB C Introduction
TotalPhase: How DisplayPort Alt Mode is enabled over a USB Type-C Cable
AnandTech: DisplayPort Alt Mode 2.0 Spec Releaaed Has a diagram of pin usage
Guide to USB-C Pinout and Features All About Circuits web site, December 10 2018
Wikipedia USB-C page
VESA DisplayPort Alternate Mode on USB Type-C Technical Overview Jim Choate, VESA Compliance Program Manager, September 27-28, 2016
VESA DisplayPort Alt Mode for USB Type-C Standard Feature Summary, Sept 22, 2014 PowerPoint presentation
VESA - DisplayPort Alternate Mode on USB-C Conference presentation, Jim Choate, VESA Compliance Program Manager, USB Developer Days 2019, November 10, 2019
HDMI Alt Mode USB Type-C A technical description of the spec that had essentially no adoption. Includes pin usage.
RIP HDMI Alt Mode, we hardly new ye Ars Technica, 1/12/2023 Type-C HDMI alt mode never saw adoption. What exist are Type-C DisplayPort alt mode cables, etc. that have circuitry to convert the DisplayPort signal to HDMI.
HDMI licensing administrator says the obscure HDMI Alt Mode specs are dead Another article pointing out that USB-C HDMI Alt Mode devices don't exist. What exist are Alt-Mode DisplayPort cables/connectors, which convert DisplayPort to HDMI. That is, the device with the Type-C connector emits DisplayPort signaling, not HDMI signaling. The DisplayPort to HDMI conversion is performed by circuitry in the cable/connector.
Enable Desktop Mode (HDMI Alt Mode) in kernel/bootloader or with custom ROM xda.developers.com forum discussion. A discussion of why you (usually) can't get video output from a cell phone Type-C connector. Note that the mention of "HDMI Alt Mode" in the title is misleading.
DisplayPort over USB-C Dell Knowledge Base Article
What's up with DisplayPort over USB C superuser.com, asked 3/2017, last modified 10/2020
Are all USB Type-C-to-HDMI cables active? superuser.com asked 12/2020, last modified 12/2020
Add USB-C with DisplayPort-alt-mode to your PC A nice overview of devices for adding a Type-C connector supporting DisplayPort Alt Mode to a PC whose motherboard or video card does not have such a connector.
Bi-directional DisplayPort->UDB-C, HDMI->USB-C, and HDMI->DP Cables Another nice overview of available hardware from Dan S. Charlton