System Exclusive

System Exclusive (SysEx) is MIDI's escape hatch: an arbitrary-length message whose meaning is defined by a manufacturer, or by the MIDI Association for the "Universal" messages every device can share.

Framing

A SysEx message opens with F0 (Start of Exclusive), carries an ID that says who defines the payload, then any number of 7-bit data bytes, and closes with F7 (End of Exclusive, EOX).

F0 id dd dd dd … F7

• Every byte between F0 and F7 must be 00–7F. Binary payloads (samples, firmware) are packed into 7-bit groups, a common scheme being 7 bytes of 8-bit data spread over 8 MIDI bytes with a leading byte that holds the high bits.
• There is no length field. The receiver reads until F7. Any other status byte except a System Real-Time message also ends the SysEx, and the message is treated as terminated.
• System Real-Time messages (F8–FF) may appear in the middle of a SysEx stream, for example a F8 clock tick, and must not end it. Parsers should handle them and keep collecting SysEx bytes.
• SysEx carries no channel and clears running status (see Messages).

Manufacturer IDs

The byte after F0 identifies the owner of the message. One-byte IDs 01–7C were assigned in MIDI's early years. When those ran out, 00 became a prefix for three-byte IDs (00 xx xx). Three IDs are reserved for special uses:

Some well-known IDs

The full, current list lives on midi.org.

Universal System Exclusive

Universal messages share a fixed structure:

F0 7E|7F dev sub-id#1 sub-id#2 data … F7

dev is the device ID (often called the SysEx channel), 00–7F. Devices usually default to 7F or to a user-set value. 7F in a message means "all call": every device should respond.

Device Inquiry (Identity Request / Reply)

The quickest way to find out what is on the other end of a cable:

F0 7E 7F 06 01 F7      Identity Request, sent to all devices

A device that supports it answers with:

F0 7E dev 06 02 mm ff ff dd dd ss ss ss ss F7

General MIDI system messages

A receiver needs time to reset after GM System On. Leave a gap (commonly 100–200 ms) before sending further messages. See General MIDI.

Master Volume

F0 7F 7F 04 01 ll mm F7

A 14-bit value sent LSB first: ll is the low 7 bits and mm the high 7 bits. F0 7F 7F 04 01 7F 7F F7 sets maximum volume. Unlike CC 7, it applies to the whole device, not one channel.

Manufacturer-specific messages

Everything after the manufacturer ID belongs to that manufacturer. Many follow a similar pattern: device ID, model ID, command (for example "data set" or "data request"), an address in the device's parameter memory, data, and a checksum. Two widely used resets:

Roland checksum

Roland "Data Set 1" (DT1, command 12) and "Request Data 1" (RQ1, command 11) messages end with a checksum computed over the address and data bytes. The sum of address + data + checksum must be 0 modulo 128:

checksum = (128 − (sum of address and data bytes mod 128)) mod 128

Worked example: the GS Reset above.

40 + 00 + 7F + 00 = 0xBF = 191. 191 mod 128 = 63. 128 − 63 = 65 = 0x41.

In practice

• Pace your dumps. Many devices have small receive buffers. Large dumps often need a few milliseconds between messages, or a handshake (ACK/NAK) where the device supports one.
• USB and network transports split SysEx into packets. USB-MIDI 1.0 uses code index numbers 4–7 to mark continuation and end packets, and UMP carries SysEx in 64-bit Data Messages. Your API usually reassembles them for you. See Transports and MIDI 2.0.
• In Standard MIDI Files, SysEx is stored with a length prefix (F0 len …), not framed by F7 alone. See SMF SysEx events.

const access = await navigator.requestMIDIAccess({ sysex: true });