dingusppc/zdocs/adb.md

The Apple Desktop Bus is a bit-serial peripheral bus, Apple themselves cited a 2-MHz Motorola 68HC11 microcontroller as an example platform to implement the ADB standard with. Its transfer speed is usually around 10.0 kilobits per second, roughly comparable to a PS/2 port at 12.0 kilobits per second.

The device commands are in the form of single byte strings. The first four bits are to signal which of the 16 devices are to be used. The next two bits are for which action to execute (talk, listen, flush, or reset). These are followed by two bits which determine the register to reference (register 0 is usually a communications register, while register 3 is used for device info).

Commands

Command Name	Number
DEVCMD_CHANGE_ID_AND_ENABLE	0x00
DEVCMD_CHANGE_ID	0xFD
DEVCMD_CHANGE_ID_AND_ACT	0xFE
DEVCMD_SELF_TEST	0xFF

Devices

Device Type	Example	Default Address
Protection		0x1
Encoded	Keyboard	0x2
Relative-position	Mouse	0x3
Absolute-position	Tablet	0x4
Data transfer	Modem	0x5
Other		0x6
Other		0x7