Host-link protocol

The Rytm and the Sidecar talk over the Rytm’s USB MIDI with SysEx under the non-commercial manufacturer ID 0x7D, which the stock OS ignores:

F0 7D 52 54 <cmd> <payload...> F7          ("RT")

All payload bytes are 7-bit. 14-bit values go as two bytes, high 7 bits first. Text is ASCII 0x20..0x7E (anything else _), NUL padded. A receiver ignores a command it does not know, and from the user slots on, every message carries a version byte (01) right after the command, except 26 / 27. The reference implementation and its tests are sim/protocol.py in the engine repository; the core’s C++ is tested against it.

Audio never goes over SysEx: the Overbridge stream carries it.

Voice reports (Rytm to host)

Cmd	Name	Payload
04	TRIG+PARAMS	<voice byte> <note> <velocity> <P1..P16 as 14-bit> [<tag hi> <tag lo>]
03	PARAMS	<voice byte> <P1..P16> [<tag>]: knob, LFO or slide changes, at most ~90 a second per voice
11	NOTE_OFF	<voice>: the gate closed (pad released, trig length elapsed, sequencer stopped)

• Voice byte: the physical voice (0..7) in the low 3 bits, the machine in voice & 0x78: 00 PLAITS, 08 JUNO, 10 DAISYSP, 20 DSP SAMPLE, 40 DIGIDRUM, 48 6-OP FM, 50 58 60 68 70 U1..U5. Anything else plays as PLAITS.
• P values are 14-bit: the 0..127 value, then the fraction in 1/128 steps (slides and the LFO move it). LFO modulation is included.
• List knobs (ENG and others) are equal zones: zone = min((value * N) >> 7, N - 1).
• Tag: the owner track’s sound tag. For U1..U5 it is the fingerprint of the machine the sound was made for (0 untagged). A host plays a user-slot voice only when the tag is 0 or matches.

Project, pool and names

Cmd	Dir
10	R→H	the Rytm’s project name; the pool, JUNO and FM lists are per project
0D / 0E	R→H / H→R	list a SIDECAR folder / a page of 6 entries (folders and files, sizes, names)
0F	R→H	LOAD TO PROJ on a Sidecar file: add it to the project’s pool (or JUNO / FM list)
12	R→H	assign pool slot N to a file
13	R→H	ask for the pool table and names now
14	H→R	the pool table, 6 slots a message
15	H→R	a JUNO voice’s patch sliders, so the Rytm’s knobs show the patch
16 / 17	H→R	the project’s JUNO / FM list names, 8 a message
18	R→H	ask for the FM names

User slots: 19..1E, 25

Cmd	Dir	Name
19	R→H	ASK	a slot’s metadata, a knob’s enum names, a preset page, or the installed machines
1A	H→R	METADATA	a slot’s state (EMPTY READY LOADING MISSING BAD NO_ROOM MUTED), name, short name, page-2 title, fingerprint, then the knob descriptors (look, kind, role, range, default, names)
1B	H→R	LIST NAMES	8 names a message: a knob’s enum list, the slot’s presets, or the installed machines
1C	H→R	KNOBS	knob values written back to a track (a preset’s knobs)
1D	H→R	SLOTS	the five slot generations and the installed list’s; with an answer to a 1E (READY, NOT INSTALLED, NO ROOM, STOP TO INSTALL, NOT ON CARD, BAD, LOADING)
1E	R→H	ASSIGN	assign an installed machine to a slot, clear it, or install it from the card
25	R→H	SLOT PRINTS	<seq> <flags> <n> {<slot> <fp hi> <fp lo> <count>}: every (slot, fingerprint) the project’s sounds use, at most 16; sent after a 10 and every ~5 s with the same seq (from sfx1019, in testing)

Generations (1..127 per slot) let the Rytm cache metadata and ask only when something changed. 1F is reserved.

Send FX: 20..24

Cmd	Dir	Name	Payload
20	R→H	SENDFX	track 0..11: <t> <hi> <lo>, the track’s SEND FX 1 (hi) / 2 (lo) levels. Track 12: six hi/lo pairs, each FX’s TYPE, P1..P5. Track 13: the kit’s user-effect tags. Sent in the block a changed value plays, and one track in turn every ~43 ms, so a new host has every value within ~0.6 s
21	H→R	SENDFX NAMES	the TYPE list: <replace> <total> {<id> <name 13>}, 8 a message
22	H→R	SENDFX STATE	per FX: id, state (EMPTY / READY / MISSING), a 10-character name for !NAME
23	R→H	SENDFX ASK	answered by 21, the 24s and both 22s
26	H→R	FXIN	<0 AUDIO IN | 1 the FX return on IN L/R | 7F ask> (no version byte)
27	R→H	FXIN STATE	<on>: the answer to every 26, and unasked when the Rytm’s USB port comes up (no version byte)
24	H→R	SENDFX LABELS	per TYPE: four 4-character labels for P2..P5, and up to 12 zones <knob | show<<2 | step<<3> <lo> <base> <name 5>

TYPE ids are stable: 0 OFF, 1 CHORUS, 2 COMB, 6 RINGS, 8 CLOUDSEED, 15 STUTTER, 16 CRUSH, 17 RINGMOD, 18 HALO; the other ids up to 63 are reserved for planned built-ins (a Sidecar without one reports it MISSING); 64..67 are the user-effect slots UFX1..UFX4. P1 is ROUTE on every TYPE (0..63 PRE, 64..127 POST); the Rytm shows it itself. A zone with show displays name and v - lo + base; step makes the Rytm step the knob one zone per detent.

Remote UI: 29, 2A, 30..3F

Part of the release (the rui1027d Rytm build). Every message carries <ver 1>.

Cmd	Dir	Payload
29	R→H	SRC view: <track 0..11> <on>: the stock SRC page of a Sidecar track shows or hides
2A	R→H	SRC view values: <track> <first> <n> {<int> <frac>}: P1..P8 the SRC knobs A..H, P9..P16 the SMPL page; all 16 after each 29 t 1, then the changed span each tick
30	R→H	<page> <on> <reason>: the page opened or closed (0 user, 1 host, 2 timeout, 3 local escape); page 0 SIDECAR, 1 the LFO page (TRK + LFO, also sent again while open); a close is always 0
31	R→H	<code hi> <code lo> <down>: every key while the page is open
32	R→H	<encoder> <delta + 64> <func>: encoder turns summed per 50 ms tick
38	H→R	<seq> <col0> <page0> <ncols> <npages> <packed>: a rectangle of 8x8 tiles of the 128x64 screen into staging
39	H→R	<seq>: commit staging to the screen (a partial frame never shows)
33	R→H	machines: <t0> <n> {<machine 0..52> <vb>} per track, vb 7F = not a Sidecar machine; a change at the next UI tick, all 12 after each hello and when the page opens
34	H→R	trig key LEDs: 00 gives them back to stock, 01 <c1..c16> palette colours 0..111
35	R→H	hello: <uptime hi> <lo> <caps> when the Rytm’s USB port comes up
36	H→R	splash: <ticks>: the connect screen for that long, every key and knob stock’s (0 ends it)
37	H→R	catch mask: <e0 e1 e2> <k0..k36>: which encoders and keys go to the Sidecar; the rest stay the Rytm’s
3A	H→R	<page> <on>: open or close the page; 3A 0 0 also gives the screen back from the SRC view
3B	R→H	the active kit <kit 0..127> (for the per-kit LFOs)
3D	H→R	a text item: <seq> <slot> <x> <y> <font> <flags> <chars>, drawn by the Rytm in its own font
3E	H→R	clear all text items
3F	H→R	local echo: binds an encoder to the bottom bar, which the Rytm updates itself on a turn
3C	H→R	keepalive; the Rytm leaves after 1 s without a valid 3x

The framebuffer is 1024 bytes, column-major in the Rytm’s panel layout: pixel (x, y) is byte (7 - y/8) + 8x, bit y % 8. packed carries 8-bit data in 7-bit bytes: groups of up to 7 bytes, each group preceded by a byte of their top bits.

Other

• 05..08: the looper experiment (mode, pads, slot states, mutes).
• 09, 0B, 0C: the retired folder walk; ignored.
• 02 and 01 (audio over SysEx) are reserved and unused.