Protocol

Protocol

This page is for maintainers. Normal users should start with Quick Start.

Parsec CouchLink has two small protocols:

• Runtime UDP over Wi-Fi while the bridge is streaming controller or keyboard state.
• USB-CDC setup mode while Wi-Fi credentials are being provisioned.

Runtime UDP

• Port: UDP 4242.
• Packet size: controller, keyboard, heartbeat, discovery, and diagnostic request packets are 17 bytes. Diagnostic replies can be larger.
• Addressing: the bridge broadcasts discovery, then sends unicast to the Pico.
• Watchdog: if the Pico has not received a valid packet for 100 ms, it outputs a neutral state (controller centred / all keys released).

Packet types:

Type	Meaning
0x01	Controller state.
0x02	Heartbeat.
0x03	Discovery broadcast from the bridge.
0x04	Pico ack with firmware and board identity.
0x05	GET_LOG -- bridge requests the firmware diagnostic ring. Same 17-byte shape as the others; body is reserved.
0x06	GET_USB_DIAG -- bridge requests current run-mode USB status. Same 17-byte request shape as the others; body is reserved.
0x07	REBOOT_TO_SETUP -- bridge asks run-mode firmware to reboot into setup-mode USB-CDC so Wi-Fi can be changed. Same 17-byte request shape as the others; body is reserved.
0x08	KEY_STATE -- keyboard report for the keyboard persona. The 8-byte USB HID boot report sits in the first 8 body bytes: modifier bitmap, reserved, then six key usage codes.
0x09	KEY_HEARTBEAT -- keyboard heartbeat, sent when the report is unchanged so the firmware watchdog stays fed.
0x0A	SET_PERSONA -- bridge asks run-mode firmware to persist a USB persona and reboot into it. body[0] is the persona (0 = controller, 1 = keyboard). Ignored if the Pico is already in that persona.
0x85	LOG_CHUNK -- one variable-length reply chunk to GET_LOG. 12-byte header (chunk index, flags, total chunks, payload length, lost-bytes counter) + up to 256 bytes of log payload + CRC-16. The final chunk sets the LAST_CHUNK flag bit.
0x86	USB_DIAG -- fixed 78-byte reply to GET_USB_DIAG with USB mount/suspend state, descriptor counters, IN/OUT report counters, recent timestamps, and CRC-16.

The controller fields match the standard XInput button, trigger, and stick layout so the bridge can copy the Windows XInput state directly into the packet body. Keyboard packets carry a standard USB HID boot-keyboard report, so a Pico in the keyboard persona only processes KEY_STATE/KEY_HEARTBEAT and a Pico in the controller persona only processes STATE/HEARTBEAT; the bridge sends whichever matches the persona the Pico advertised in its ack.

Compatibility is gated by protocol version. The bridge refuses to stream to a Pico that reports a different runtime protocol version. Capability bits in the ACK packet's flags byte advertise optional features without forcing a version bump: bit 0 (LOG_CHUNK_SUPPORTED) means the firmware will reply to GET_LOG; bit 1 (USB_DIAG_SUPPORTED) means it will reply to GET_USB_DIAG; bit 2 (REBOOT_TO_SETUP_SUPPORTED) means it accepts REBOOT_TO_SETUP; bit 3 (KEYBOARD_PERSONA) means the Pico is currently presenting the USB keyboard and accepts SET_PERSONA plus the keyboard packet types. Older firmware leaves these flags clear, and the bridge gates behaviour accordingly.

USB-CDC Setup

Setup mode is used before the Pico has working Wi-Fi credentials, or when credentials are cleared.

• USB VID/PID: 0x2E8A:0xCAF0.
• Transport: CDC ACM virtual COM port.
• Framing: magic, protocol version, command, payload length, sequence, payload, CRC-16.
• Password handling: the bridge clears the password buffer after sending. The Pico clears its receive buffer after writing flash.

Commands:

Command	Purpose
HELLO	Read firmware, board, and credential status.
GET_STATUS	Read Wi-Fi state and last setup error.
SET_WIFI	Store SSID and password in Pico flash.
REBOOT_TO_RUN	Reboot into runtime mode.
SELF_TEST	Firmware-side setup checks.
GET_LOG_BUFFER	Read the Pico diagnostic ring buffer.
REBOOT_TO_BOOTSEL	Reboot into the ROM BOOTSEL USB bootloader for reflashing.

HELLO_ACK keeps the first six legacy identity bytes for older hosts: protocol version, compact firmware date fields, board type, and flags. Newer firmware appends year, month, day, revision, and an optional four-character development suffix. Development builds report versions such as 2026.5.29.7-D69A; release builds report 2026.5.29.0.

USB Vendor Diag (setup mode)

Setup mode's composite also exposes a vendor-class interface (interface 2, class 0xFF) that Windows binds to WinUSB. MS OS 2.0 descriptors advertise the binding, so no INF file is needed on Windows 8.1+. The host reads the firmware diagnostic ring buffer via a vendor IN control transfer on EP0, which works regardless of CDC bulk endpoint state -- diag retrieval no longer relies on the CDC FIFO being drained.

Control transfer:

Field	Value
bmRequestType	0xC1 (vendor IN, interface)
bRequest	0x01 (GET_DIAG_LOG)
wIndex low byte	2 (interface number)
wLength	up to 4100 (4-byte header + 4 KiB ring)

Response payload matches the CDC GET_LOG_BUFFER body: a 4-byte little-endian lost-bytes counter, followed by the most-recent ring contents.

Run mode does not expose this interface -- the XInput persona is deliberately minimal to keep xusb22.sys binding stable. In run mode, diag retrieval uses UDP GET_LOG instead.

The bridge's couchlink bundle tries CDC, vendor control, and UDP in order; the first to succeed wins, and manifest.json's pico_diag_source records which path produced the captured log (setup-cdc, vendor-control, or run-udp).

USB Runtime Persona

In run mode, the Pico presents one of two USB devices, chosen by a persona byte stored alongside the Wi-Fi credentials in flash:

• Controller (default): a wired Xbox 360 controller (0x045E:0x028E, vendor class for xusb22.sys).
• Keyboard: a standard USB HID boot keyboard (0x2E8A:0xCAF1), for console games that need a keyboard such as Typing of the Dead on the Dreamcast.

Only one persona is presented at a time. Because the USB descriptors are fixed at boot, switching persona persists the new value and reboots the board. The Pico lives plugged into the console rather than the host, so the switch happens over Wi-Fi: couchlink keyboard / couchlink controller send a SET_PERSONA request, then wait for the board to rejoin Wi-Fi advertising the new persona. A record written before personas existed reads back as the controller default, so existing boards keep working without re-provisioning. Setup mode and both run-mode personas use different USB IDs so Windows does not reuse the wrong driver binding across modes.

Run-mode firmware also tracks what the USB host did after the Pico joined Wi-Fi. couchlink test usb --all asks the Pico over UDP whether the USB host completed configuration, whether the IN endpoint has accepted reports, and whether any host OUT traffic (controller rumble/LED, or keyboard LED-lock reports) has arrived. The Pico cannot read the adapter's UI or driver name, but these counters distinguish the useful cases: no USB traffic, enumeration started but not configured, configured but not polling, polling, and polling plus OUT traffic. The report counters and verdicts are labelled for the active persona (XInput or HID keyboard).