(Note: imported verbatim from old website)

Reverse-engineered using this method.

General data transfer

The DDC bus is nothing else than an I2C bus.

DDC2 informations, like monitor supported refresh rates (EDID), are transmitted using the I2C address 0x50.

DDC/CI packets are transmitted using the I2C address 0x37.

For more informations on I2C bus, see I2C specifications at Phillips.

Timing

Any application using the DDC/CI bus should make a pause of at least 30ms between every packet.

Write packet

Data Description
0x6E I2C address : 0x37, writing
0x51 Sub-address.
0x80 OR n Last 4 bits indicates the number of following bytes (excluding checksum)
… n bytes
Checksum Simple XOR of all preceding bytes (including the first)

Read packet

First, send a write packet, then, when reading, the monitor returns this:

Data Description
0x6F I2C address : 0x37, reading
0x6E ACK of the packet previously written.
0x80 OR n (sometimes, simply n) Last 7 bits indicates the number of following bytes (excluding checksum)
… n bytes
Checksum XOR of all preceding bytes (including the first), expect the second, which is replaced by the subaddress (0x51).

Commands

Read a value

First, send a 2 bytes write packet:

Data Description
0x01 Read a value
0xXX Index of the value to read (see below for monitor-specific indexes)

Then, read the answer (8 bytes read packet):

Data Description
0x02 Value follows
0x00 NULL-byte
0xXX Index of the value which is read
0x00 NULL-byte
0x00 NULL-byte
0xZZ Maximum value (usually 0xFE)
0x00 NULL-byte
0xYY Current value

Modify a value

Send a 4 bytes write packet:

Data Description
0x03 Change a value
0xXX Index of the value to change (see below for monitor specific indexes)
0x00 NULL-byte
0xYY New value, in the range 0x00-0xFE

Monitor specific indexes

Mitsubishi Diamond Pro 2060u

Monitor informations

Manufacturer: MEL Model: 4513

Index Description Index Description
0x00 Degauss (value must be 0x01) 0x26 Pincushion balance
0x?? Auto-size 0x46 Top pincushion amplitude
0x?? Reset factory settings 0x48 Top pincushion balance
0x10 Brightness 0xE6 Center pincushion amplitude (value 0x00-0x01 ???, not working)
0x12 Contrast 0x4A Bottom pincushion amplitude
0x16 Red 0x4C Bottom pincushion balance
0x18 Green 0x3A Vertical linearity
0x1A Blue 0x3C Vertical linearity balance
0x22 Width 0x28 Horizontal static convergence
0x32 Height 0x38 Vertical static convergence
0x30 Vertical position 0x56 Moire cancel
0x20 Horizontal position 0xE8 Top left screen purity
0x24 Pincushion amplitude 0xE9 Top right screen purity
0x42 Pincushion phase 0xEA Bottom left screen purity
0x40 Keystone balance 0xEB Bottom right screen purity

Others?

If you have other monitors, you can find these values using this method.

DDC is a trademark owned by VESA (Video Electronics Standard Association).

I2C is a trademark owned by Philips.