The most common version, called DDC2B, is based on I²C, a serial bus. Pin 12, ID1 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. And the monitor acts as the slave device at I²C address A1h (7-bit I²C address 50h, read-only, so A1h) providing the 128 bytes to 256 bytes EDID.
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.
Both DVI and HDMI connectors feature dedicated DDC2B wires.
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.
Both DVI and HDMI connectors feature dedicated DDC2B wires.
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