The most common version, called '''DDC2B''', is based on [[I²C]], a [[Serial communications|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 [[Two-way communication|bidirectional]] and supports multiple [[Bus mastering|bus-masters]], DDC2B is unidirectional and allows only one [[Bus mastering|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 [[Extended display identification data|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 [[Universal Serial Bus|USB]].
