Display Data Channel - E-DDC

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 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.