Joybus Protocol

The Joybus Protocol is a proprietary, non-standard, serial protocol by which the N64's Peripheral Interface (as well as the GameCube and Game Boy Advance) communicates with controllers, controller accessories (Controller Pak, Rumble Pak, etc.), keyboards, mice, game cartridges, and other devices plugged into the console.

Operation
The protocol utilizes four types of bits: Zero, One, the Console Stop Bit, and the Controller Stop Bit. Zero, One, and the Controller Stop Bits are 4μs long, while the Console Stop Bit is 3μs. Communication is always initiated by the console, by sending an 8 bit (one byte) command to the device plugged in (not necessarily always a standard controller). A Console Stop Bit will usually follow after this command byte unless there is more data.

The controller or device will read this command and any extra data, and then respond appropriately. For example, when the console wishes to write to a Controller Pak, it will send 0x03, followed by a 2 byte address, 32 bytes of data, and then the stop bit.

Command List
Because all commands are exactly 8 bits long, there can be a total of 256 commands. However, only about 16 are known for the N64, and a few more for GameCube and Game Boy Advance. Games also have the ability to define custom commands with or without extra data, but official controllers or devices will not understand them.

$†$ Real Time Clock

0xFF - Reset / Info
While identical to 0x00 in what data is returned to the console, if a device has an intended reset function, it should be performed when this command is sent. The N64 controller for example will reset the internal position of its analog stick to (0, 0), essentially recalibrating it. Other devices may have similar functionality, or none at all, but either way they should still send back the same data as if the 0x00 command was sent.

0x00 - Info
This command requests information about the device. The N64 a 2-byte identifier and 1 byte of extra data. $†$ Requires verification yet.