GameCube Joybus Protocol & Keyboard Reference¶

Comprehensive Joybus protocol reference with reverse-engineered keyboard support

Documented by Robert Dale Smith (2022-2025) Based on joybus-pio by JonnyHaystack

This document provides a comprehensive technical reference for the GameCube Joybus controller protocol, with detailed coverage of the reverse-engineered keyboard protocol.

Overview¶

The GameCube Joybus protocol (also known as "GC-Joybus" or "SI protocol") is a bidirectional serial protocol developed by Nintendo for communication between the GameCube console and its peripherals. The protocol is used for:

Standard GameCube controllers
WaveBird wireless controllers (with receiver)
GameCube Keyboard (for Phantasy Star Online Episode I & II)
GameCube to GBA link cable
Other licensed peripherals

Key Characteristics¶

Single-wire bidirectional: Data line is tri-state (console or controller drives)
250 kbit/s nominal bitrate: 4us per bit
9-bit byte encoding: 8 data bits + 1 stop bit
Command-response protocol: Console sends command, controller responds
Multiple report modes: 6 different analog/button configurations
Rumble support: Integrated motor control via stop bit

Historical Context¶

The Joybus protocol was originally developed for the Nintendo 64 and adapted for GameCube with higher bandwidth and more sophisticated features. The keyboard accessory was released only in Japan for Phantasy Star Online Episode I & II, making it a rare peripheral with minimal documentation.

Physical Layer¶

Connector Pinout¶

The GameCube controller port uses a proprietary 6-pin connector:

Pin	Name	Direction	Description
1	VCC	-	+5V power (from console)
2	DATA	Bidirectional	Tri-state data line (3.3V logic)
3	GND	-	Ground
4	GND	-	Ground (shield)
5	N/C	-	Not connected
6	3.3V	-	+3.3V power (from console, optional)

Electrical Characteristics¶

Logic levels: 3.3V CMOS (0V = LOW, 3.3V = HIGH)
Data line: Open-drain with ~1k pull-up resistor on console side
Idle state: Data line HIGH (pulled up)
Drive strength: Both console and controller can actively drive the line LOW
Power: 5V @ ~200mA max (with rumble active)

Tri-State Operation¶

Phase	Behavior
Console transmit	Console drives data line LOW/HIGH
Console receive	Console releases line, controller drives
Controller transmit	Controller drives data line LOW/HIGH
Controller receive	Controller releases line (pulled HIGH)

Critical: Both sides must release the line (go tri-state) when not transmitting to avoid bus contention.

Joybus Protocol Basics¶

Bit Encoding¶

Each bit is transmitted as a 4us period with 3 phases:

Logical 0:

    1us      2us      1us
   +-----+         +-----
   | LOW |   LOW   | HIGH  (pulled up after release)
---+     +---------+

Logical 1:

    1us      2us      1us
   +-----+---------+-----
   | LOW |  HIGH   | HIGH
---+     +---------+

Encoding rule: - All bits start with 1us LOW pulse - Bit value determines 2us data period (LOW = 0, HIGH = 1) - Followed by 1us delay before next bit

Sampling point: Receiver samples at 1us after falling edge (middle of 2us data period)

9-Bit Byte Encoding¶

The Joybus protocol uses 9-bit bytes to support multi-byte messages without inter-byte gaps:

Bit(s)	Field	Description
0-7	Data byte	MSB first
8	Stop bit	0 = continue, 1 = stop

Example (3-byte command 0x40 0x03 0x00):

Byte	Value	Stop Bit	Meaning
0	0x40	0	Continue to next byte
1	0x03	0	Continue to next byte
2	0x00	1	End of command

Command-Response Cycle¶

The console sends a command (1-3 bytes) to the controller, then waits for a 4us reply delay. The controller then sends its response (length varies by command) back to the console.

Reply delay: Controller must wait 4us (1 bit period) after receiving command before responding.

Timing Constraints¶

Command timeout: 50us (if no response received, console retries)
Inter-byte gap: None (9-bit encoding allows continuous transmission)
Reset timeout: 130us (if line stays LOW/HIGH too long, protocol resets)

Controller Protocol¶

Command Set¶

Command	Hex	Bytes	Description	Response
PROBE	0x00	1	Probe device type	3 bytes (status)
RESET	0xFF	1	Reset device	3 bytes (status)
ORIGIN	0x41	1	Calibration request	10 bytes (origin report)
RECALIBRATE	0x42	3	Recalibration	10 bytes (origin report)
POLL	0x40	3	Poll controller state	8 bytes (controller report)
KEYBOARD	0x54	3	Poll keyboard state	8 bytes (keyboard report)
GAME_ID	0x1D	11	Read game disc ID	(varies)

Device Identification¶

PROBE / RESET Response (3 bytes):

Bytes 0-1 contain the device type as a big-endian 16-bit value:

Value	Device
0x0009	Standard controller
0x0900	WaveBird receiver (no controller paired)
0x0920	WaveBird receiver (controller paired)
0x2008	Keyboard
0x0800	Steering wheel
0x0200	Bongos

Byte 2 contains status flags:

Bit	Description
0	Rumble motor supported
1	Standard controller
2-7	Reserved

Example (standard controller with rumble): The response bytes are 0x09 0x00 0x03, where bytes 0-1 (0x0900) identify the device type and byte 2 (0x03) indicates both rumble support and standard controller flags are set.

Controller Poll Command¶

Command: 0x40 0x03 0x00 (8 bytes requested, rumble stop bit)

Rumble control via stop bit: - 0x40 0x03 | 0 -> Rumble OFF (stop bit = 0) - 0x40 0x03 | 1 -> Rumble ON (stop bit = 1)

Byte 1 (0x03): Report mode - 0x00 = Mode 0 (4-bit triggers, 4-bit analog A/B) - 0x01 = Mode 1 (4-bit C-stick, full triggers) - 0x02 = Mode 2 (4-bit C-stick, 4-bit triggers) - 0x03 = Mode 3 (standard - full 8-bit everything) - 0x04 = Mode 4 (full C-stick, full analog A/B)

Byte 2 (0x00): Reserved (always 0x00)

Controller Report Format (Mode 3)¶

8 bytes (default mode used by most games):

Byte	Content	Description
0	Button bits: A, B, X, Y, Start, Origin, ErrLatch, ErrStat	Face buttons and status flags (MSB to LSB)
1	Button bits: DL, DR, DD, DU, Z, R, L, High1	D-pad, triggers, and protocol marker (MSB to LSB)
2	Left stick X	0x00 = left, 0x80 = center, 0xFF = right
3	Left stick Y	0x00 = down, 0x80 = center, 0xFF = up
4	C-stick X	Same range as left stick
5	C-stick Y	Same range as left stick
6	L analog	0x00 = released, 0xFF = fully pressed
7	R analog	0x00 = released, 0xFF = fully pressed

Bit fields: - All buttons: Active HIGH (1 = pressed) - Origin bit: LOW after console sends ORIGIN command - High1 bit: Always set to 1 (protocol marker)

Origin Calibration¶

ORIGIN Command (0x41): Requests the controller's neutral position (calibration data).

Response (10 bytes): Bytes 0-7 contain the current controller state in the same format as a POLL response. Bytes 8-9 are reserved and always 0x00.

The console uses this to establish the controller's center position for analog sticks and triggers. Games typically request this on boot or when the controller is first connected.

Keyboard Protocol (Reverse-Engineered)¶

Discovery¶

The GameCube keyboard was released exclusively in Japan for Phantasy Star Online Episode I & II. The protocol was completely undocumented, requiring hardware analysis and iterative testing to decode.

Key discovery: The keyboard uses command 0x54 (not documented anywhere publicly) and returns an 8-byte report with 3 simultaneous keypresses and a rolling counter with XOR checksum.

Keyboard Poll Command¶

Command: 0x54 0x00 0x00 (8 bytes requested)

Unlike the controller, the keyboard does not support rumble, so the stop bit has no effect.

Keyboard Report Format¶

8 bytes:

Byte	Content	Description
0	Counter (4 bits), Unknown (2 bits), ErrLatch, ErrStat	High nibble is a rolling counter; low 2 bits are error flags
1	Unknown	Reserved / undetermined
2	Unknown	Reserved / undetermined
3	Unknown	Reserved / undetermined
4	Keypress 1	GameCube keycode of first simultaneous key
5	Keypress 2	GameCube keycode of second simultaneous key
6	Keypress 3	GameCube keycode of third simultaneous key
7	Checksum	XOR of keypress 1, keypress 2, keypress 3, and counter

Counter: 4-bit value (0-15) that increments with each report. Used in checksum calculation.

Checksum algorithm: The checksum is computed by XOR-ing all three keypress bytes together with the 4-bit counter value. This prevents data corruption and validates that the report is intact.

Keyboard Keycodes¶

The GameCube keyboard uses proprietary keycodes (0x00-0x61), different from USB HID:

Control Keys¶

Keycode	Name	Description
0x00	NONE	No key pressed
0x06	HOME	Home (Fn + Up)
0x07	END	End (Fn + Right)
0x08	PAGEUP	Page Up (Fn + Left)
0x09	PAGEDOWN	Page Down (Fn + Down)
0x0A	SCROLLLOCK	Scroll Lock (Fn + Insert)

Alphanumeric Keys¶

Range	Keys
0x10-0x29	A-Z
0x2A-0x33	0-9

Function Keys¶

Range	Keys
0x40-0x4B	F1-F12

Special Keys¶

Keycode	Name	PSO Mapping (Normal / Shift)
0x34	MINUS	- / =
0x35	CARET	^ / ~
0x36	YEN	\ / \|
0x37	AT	@ / `
0x38	LEFTBRACKET	[ / {
0x39	SEMICOLON	; / +
0x3A	COLON	: / *
0x3B	RIGHTBRACKET	] / }
0x3C	COMMA	, / <
0x3D	PERIOD	. / >
0x3E	SLASH	/ / ?
0x3F	BACKSLASH	\ / _

System Keys¶

Keycode	Name
0x4C	ESC
0x4D	INSERT
0x4E	DELETE
0x4F	GRAVE
0x50	BACKSPACE
0x51	TAB
0x53	CAPSLOCK
0x54	LEFTSHIFT
0x55	RIGHTSHIFT
0x56	LEFTCTRL
0x57	LEFTALT
0x59	SPACE
0x61	ENTER

Arrow Keys¶

Keycode	Name
0x5C	LEFT
0x5D	DOWN
0x5E	UP
0x5F	RIGHT

Japanese Layout Keys¶

Keycode	Name	Description
0x58	LEFTUNK1	Muhenkan
0x5A	RIGHTUNK1	Henkan/Zenkouho
0x5B	RIGHTUNK2	Hiragana/Katakana

These keys are specific to Japanese keyboards and may not have direct equivalents on Western keyboards.

Keyboard Mode Switching¶

The GameCube console determines the device type from the PROBE response. When the console detects a keyboard (device type 0x2008), it sends 0x54 keyboard poll commands instead of 0x40 controller poll commands.

To emulate a keyboard, a device must: 1. Respond to PROBE with device type 0x2008 2. Handle 0x54 poll commands 3. Return 8-byte keyboard reports with valid checksums

To switch between controller and keyboard emulation at runtime, the device changes its PROBE response device type and adjusts its poll response format accordingly.

Phantasy Star Online Key Mappings¶

The keyboard was designed for PSO Episode I & II. Here are the in-game key mappings:

Function	Key	Keycode
Move forward	W	0x26
Move backward	S	0x22
Strafe left	A	0x10
Strafe right	D	0x13
Jump	Space	0x59
Action	Enter	0x61
Menu	ESC	0x4C
Chat	/	0x3E
Inventory	I	0x18
Map	M	0x1C

Timing Requirements¶

Critical Timing Constraints¶

Joybus bit period: 4us (250 kbit/s) - T1: 1us LOW pulse - T2: 2us data period - T3: 1us delay

Console timing tolerance: +/-0.5us per bit (measured)

Reply Delay¶

Controller must wait 4us (1 bit period) after receiving a command before replying. This prevents bus contention between console and controller.

Timeout Values¶

Parameter	Value	Description
Bit period	5us	Per bit including margins
Receive timeout	50us	No byte received = command timed out
Reset timeout	130us	Line held too long = protocol error, reset

Implementation Notes¶

Digital-Only Trigger Support¶

Modern controllers have different trigger types: - Analog triggers: Xbox, PlayStation (send both digital bit + analog value) - Digital-only triggers: Switch Pro, PS3 (send only digital bit, analog = 0)

When emulating a GameCube controller with a digital-only source, the digital button press should be converted to a full analog value (255) so that games expecting analog trigger data function correctly.

Mouse to Analog Stick Mapping¶

USB mice typically report at 125-1000Hz, while the GameCube polls controllers at ~60Hz. When mapping mouse deltas to analog stick values, the higher-frequency mouse reports should be accumulated between GameCube poll cycles and clamped to the +/-127 analog range to produce smooth input.

Quick Reference¶

Command Summary¶

Command	Hex	Description
PROBE	0x00	Probe device type -> 3 bytes (status)
RESET	0xFF	Reset device -> 3 bytes (status)
ORIGIN	0x41	Calibration request -> 10 bytes
RECALIBRATE	0x42	Recalibration -> 10 bytes
POLL	0x40	Poll controller -> 8 bytes
KEYBOARD	0x54	Poll keyboard -> 8 bytes

Device Types¶

Value	Device
0x0009	Standard controller
0x0900	WaveBird (no pairing)
0x0920	WaveBird (paired)
0x2008	Keyboard
0x0800	Steering wheel
0x0200	Bongos

Timing Summary¶

Parameter	Value
Bit period	4us
T1 (LOW pulse)	1us
T2 (data period)	2us
T3 (delay)	1us
Reply delay	4us
Receive timeout	50us
Reset timeout	130us

Acknowledgments¶

JonnyHaystack - joybus-pio library
Nintendo - Original Joybus protocol design
Phantasy Star Online community - Keyboard protocol hints

References¶

The GameCube keyboard protocol documented here represents original reverse-engineering work. The 0x54 keyboard command and 8-byte report structure with XOR checksum were discovered through hardware analysis and testing with Phantasy Star Online Episode I & II.