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Foot controller protocol

How a foot controller talks to SignalPatch. The firmware lives in its own repository. Input is ordinary MIDI, so any controller works through MIDI learn. What this adds is feedback: SignalPatch tells the controller what each switch means right now, so its LEDs and display match the Board. A controller that ignores the SysEx still works.

The reference hardware is an ESP32-S3 with TinyUSB, class compliant, so no drivers are needed.

Input (controller → SignalPatch)

Control Message Notes
Footswitch n (1-8) Note On 60+n-1 (press), Note Off (release), channel 10 Momentary. SignalPatch toggles a stomp on Note On, fires a command on the rising edge, loads a slot on Note On.
Bank / slot buttons Program Change 0-4 Direct slot select.
Expression 1 / 2 CC 11 / CC 4 0-127, 7-bit is enough for a pedal. "Calibrate the pedal" on the knob's menu learns the heel/toe span (saved with the mapping; swapped ends invert).
Encoder 1-4 CC 20-23, relative (64 ± delta) Learn the knob with "MIDI learn as relative encoder": each detent nudges it 1/128 of its range, n > 1 for fast spins.
Tap tempo Note On 48, channel 10 Mapped like any footswitch: the drum machine's TAP button (command "tap", four taps averaged) or the looper's REC.

Channel 10 keeps the controller's notes away from anything driving a synth.

Feedback (SignalPatch → controller)

SysEx, manufacturer id 0x7D (non-commercial), device id 0x53 ('S').

F0 7D 53 <command> <payload...> F7
command payload meaning
0x01 switch (0-7), state (0 off / 1 on / 2 blink) LED for a footswitch
0x02 switch (0-7), label as 7-bit ASCII, up to 8 chars text under a switch
0x03 slot (0-4), active (0/1) which rig slot is live
0x04 name as 7-bit ASCII, up to 16 chars current rig name for the display
0x7F side (0x00 controller, 0x01 SignalPatch) hello

The controller sends F0 7D 53 7F 00 F7 on its input port when it boots (and whenever it wants the state again). SignalPatch answers on the output port with the same name as that input: F0 7D 53 7F 01 F7, then the whole state (eight LED + label pairs, five slot messages, the rig name). After that it sends only what changed, checked ten times a second: a stomp, a looper going into record (LED 2 = blink), a slot change, a rename. Labels are the pedal's name (8 chars), REC <pedal>, RIG n or the group's name. The side byte is what keeps a looped-back port (ALSA's Midi Through) from answering itself.

Try it without hardware: aseqdump -p "Midi Through" in one terminal, aseqsend -p "Midi Through" F0 7D 53 7F 00 F7 in another.

Notes

  • A response curve for an expression pedal belongs to the knob it drives: set it in the inspector.
  • Not done yet: a view in the app that shows the eight switches the way the pedal does.

Hardware sketch

  • ESP32-S3 (the SuperMini kit already in use), TinyUSB MIDI.
  • 8 momentary footswitches with a WS2812 LED each; 2 × 1/4" TRS expression inputs (ADC); 4 encoders; a small OLED for labels and the rig name.
  • Everything debounced in firmware; switches send Note On/Off, never toggles, so SignalPatch owns the state.