About

The Apple Accessory Protocol is used for communication between the iPod and serially connected accessories (such as the Remote, iTalk, Honda Music Link etc). The protocol was introduced with the 3rd generation iPods, and at present it is assumed is also compatible with the 4th generation iPods and mini iPods. (The iPod Dock Connector provides access to this communication channel as well.)

The connections uses a standard 8N1 serial protocol. The original speed rate is 19200 baud - higher rates (up to 57600 baud) work properly (tested the 5G) but make trouble sending large amounts of data (picture blocks) faster than 38400 baud. The Honda Music Link communicates at 9600 baud.

Request/Response Structure

The request and response messages follow this general form. The mode in the message indicates which mode the command belongs to, these same mode values are used in the mode switching command below.

For example: FF 55 03 00 01 04 F8

Mode List

Mode Switching (Mode 0)

Clearly there's stuff here waiting to be figured out.

Voice Memo (Mode 1)

The Mode 1 commands are used for the voice recording functions. To initiate the recording mode on the iPod the send the switch to mode 0x01 command - this will switch the iPod display to the "Voice Memo" screen. When the record button is pressed the iPod will send the command "0x00 0x00", sending the command to switch to mode 0x01 will start the actual recording.

iPod Remote (Mode 2)

The Mode 2 commands are used by the remote control to signal button presses. When a button is pressed the appropriate code is send 66 times per second until the button is released at which point the button released command is sent.

Note that the 3 and 4 byte commands may not work for older (tested on a 3G) iPods. They are valid for newer (tested on an iPod Nano) iPods however.

Advanced Remote (Mode 4)

The mode 4 commands are used for the Advanced iPod Remote (AiR) functions. The commands can be used to remotely control much of the iPod functions. Unfortunately when switched to this mode the iPod display changes to "OK to disconnect".

Commands you send to the iPod are highlighted, responses to those commands from the iPod are not. The response is always the command + 1.

0xFF 0x55 (standard header)
0x6E (length of data in this block)
0x04 (mode of the command)
0x00 0x32 (command for picture display)
0x00 0x00 (indicates the first block (block zero))
0x01 (damned if I know.. perhaps indicates that the size info for the picture is here)
0x00 0x78 (width of picture to be displayed)
0x00 0x40 (height of picture to be displayed)
0x00 0x00 0x00 0x20 (number of bytes you're sending for each line of the display - must be a multiple of 8 and has to fit the needed bits!)
The rest is a 4 color picture, with every two bits representing a pixel.
Checksum.

0xFF 0x55 (standard header)
0x65 (length of data in this block)
0x04 (mode of the command)
0x00 0x32 (command for picture display)
0x00 0x01 (indicates the second block (increment as you go))
The rest is a 4 color picture, with every two bits representing a pixel.
Checksum.

Credits

Most of the information on this page was acquired from the Apple Accessory Protocol - wikiPodLinux document at the iPodLinux site, acquired on 29-Jan-2007 @ 14:11 EST. This information is duplicated here so that it is available whenever I need it.

The bulk of this information is due to the hard work by Christoph, you can find his results at http://stud3.tuwien.ac.at/%7Ee0026607/ipod_remote/ipod_ap.html. Generalisation of the results was the work of Otto, see http://ipodlinux.sourceforge.net/forums/viewtopic.php?p=3362#3362.