I have recently done this for my own home AC control project. My Daikin ducted system had an "external control board" option so I ordered one of those for that but my Daikin splits are IR remote control only (or so the installer said).

Note: The following results are for my installed model only, and any other model (or maybe even a later version of the same model) might have differences.

1. The remote control is "stateful". It stores all the AC settings itself and when you press a button it updates the internal state and then sends a packet with the complete state via IR. This means that it is not possible to form a packet that just turns the unit on/off. Every packet sets the full state of everything: on/off, mode, temperature, etc.

2. At the lowest level, the IR was pulsed at approx 38kHz when ON. Duty cycle was about 25%, but you could vary this as you wish. Let the time 1/38000 sec be one "slot".

3. Next level up: A zero bit is 16 slots ON followed by 16 slots OFF. A one bit is 16 slots ON followed by 48 slots OFF.

4. Packet format is:

4.1 Start condition of 128 slots ON, 64 slots OFF.

4.2 Next is 152 bits, as follows (multibit fields are LSB first):

(0-39) = 0x11 0xDA 0x27 0x00 0x00
(40) = Main power: 0->Off, 1->On
(41) = On timer: 0->Disabled, 1->Enabled
(42) = Off timer: 0->Disabled, 1->Enabled
(43) = 0
(44-47) = Mode: 0->Auto, 2->Dry, 3->Cool, 4->Heat, 6->Fan
(48-55) = Temp: temp in degrees * 2, except for dry mode where this value is 0xC0
(56-63) = 0x00
(64-67) = Vertical swing: 0x0->Off, 0xF->On
(68-71) = Fan speed: 0x3->Fan-1, 0x4->Fan-2, 0x5->Fan-3, 0x6->Fan-4, 0x7->Fan-5, 0xA->Auto, 0xB->Night (Dry mode puts the fan in auto)
(72-75) = Horizontal swing: 0x0->Off, 0xF->On
(76-79) = 0x0
(80-91) = On timer time in minutes if set, else 0x000
(92-103) = Off timer time in minutes if set, else 0x000
(104) = Powerful mode: 0->Disabled, 1->Enabled
(105-108) = 0x0
(109) = Silent mode: 0->Disabled, 1->Enabled
(110) = 0
(111) = Home leave mode: 0->Disabled, 1->Enabled
(112-127) = 0x00 0xC0
(128) = 0
(129) = Sensor: 0->Disabled, 1->Enabled
(130-143) = 0
(144-151) = Simple checksum of all 144 previous bits

5. Good luck. You might be able to get this to work with a limited number of models. If you need to work with ANY Daikin model then be sure to sign the client to an ongoing "maintenance contract".