Sending/receiving MIDI and other data over the same port

[dayunbao]

I have a series of MIDI controllers that are built around Raspberry Pi Picos. The MIDI controllers both connect to the host device and send/receive MIDI data over USB. I'm currently using Mido on the host device to handle all of the incoming and outgoing MIDI messages. What I'd like to do is also send other kinds of data over the same ports that Mido is using. I need to do this in a way that is non-blocking. What would be the best way to do this, if it's possible?

To be more specific, I have a sequencer module that connects to the host device and sends Note On/Off, Control Change, and System Real-Time messages. It also receives SysEx messages from the host device. I've been keeping the MIDI controllers rather "dumb" up to this point. So they aren't aware of what else is going on in the system, except that they've been connected to the host device, and need to send certain kinds of MIDI messages when a button is pushed, a knob turned, etc. Now I'd like the host device to be able to send the MIDI controllers non-MIDI messages. I want the host device to send the sequencer MIDI controller song/track number information, so that it can be displayed on the sequencer. Would I be able to use something like PySerial to also connect to the same port Mido is using, and send data over it without interfering with the MIDI messages?

[rdoursenaud]

If you intend on using the MIDI path, implementing your own System Exclusive is the way.

Otherwise, you could expose another USB endpoint like HID, CDC and/or any other class compliant device depending on your use case. I’ve seen many synth/control surfaces manufacturers do that.

[dayunbao]

If you intend on using the MIDI path, implementing your own System Exclusive is the way.

Otherwise, you could expose another USB endpoint like HID, CDC and/or any other class compliant device depending on your use case. I’ve seen many synth/control surfaces manufacturers do that.

I think using System Exclusive messages is going to be the simplest.

It seems that Mido only sends the 3-byte type of SysEx message, right? So I 'll have to convert the string to bytes, loop over them, and send them 3 bytes at a time?

[rdoursenaud]

System Exclusive Messages data can be any arbitrary length: https://mido.readthedocs.io/en/latest/messages/index.html#system-exclusive-messages

Per specification, the first byte (or first 3-bytes) should be a "Manufacturer ID".
Your receiver should ignore messages without the expected IDs.
For a research application, you may want to use the "Non Commercial" ID which is 0x7D.
For commercial applications, you must register one with the MIDI Association.

From now on you define what the bytes mean.

0xF0 <Manufacturer ID> <Your custom protocol> ... 0xF7.

Something many manufacturers do is have a "Device ID" to target only specific devices followed by a Command, Parameter(s) scheme.

0xF0 <Manufacturer ID> <Device ID> <Command ID> <Parameter(s)> ... 0xF7.

The only constraint is that these should be MIDI compatible and thus fit 7 bits. The eigth bit is reserved for channel messages.
You may convert 8 bit values to and from a 7 bit stream with any appropriate scheme.

You may want to read the MIDI specification page 34 and the following for inspiration.

Good luck!