I do not KNOW if a 'splitter' will do the job, may just add a new set of complications to your setup.

There are normal midi commands that should turn ON/OFF midi 'modes, i.e. OMNI and POLY. Look at the Controllers 124 to 127. They may help with just a few bytes of midi data. You dismiss SysEx, note that they may need more bytes of data, but they may have the benefit of being targetted, i.e. a SysEx message can be manufacturer specific so will be disregarded by other devices. This will require devices that have such codes available. Mfg like say Roland and Yamaha have such codes listed, other devices may not.

You might be able to do it without a splitter, if such is available that will work with your system.

Geoff

I could not locate the manual for the 303, but I assume it is similar to the 505.

https://static.roland.com/assets/media/pdf/MC-505_OM.pdf

Check page 169, "Specifying the Reception Status for Each Part (MIDI Rx Switch)". It's not a text-editable pdf, but the gist is select "Part" and enable or disable the 8 parts.

So if the 303 is similar enough, you should be able to disable parts 2 through 8 so it only responds to part 1, which by default would be on channel 1.

Found it. MIDI implementation starting on page 101, MIDI description starting on page 74. It does not specifically say you can disable parts, but I imagine it will allow you to.

https://www.cyborgstudio.com/wimpy/drummachinemp3s/roland/mc303/manual/mc303ownersmanual.pdf

Thank you both Geoff and Jason,
I have the manual, and I'm aware of the MIDI description, and I also have an "advance MIDI settings" for the unit somewhere.
Alas, these documents are not really helpful for a newbie. I have no idea how to enter the commands into the machine.
That's why I thought about going the other way around, and try to separate those three midi channels coming out of the Beatstep Pro.

It's a drag, I don't want to discard the Roland, as it's a perfectly functional machine, and have to buy a different drum machine.

Hmm, well, here is a list of the SYSEX commands that will disable the parts, so you would disable part 2 through 16 in Module Mode or 2 through 8 in Normal Mode.

They can be put in a .mid file and loaded in to the 303, and that should(?) accomplish what you wish.

https://rolandus.zendesk.com/hc/en-us/articles/201934519-MC-303-Part-Rx-On-Off-System-Exclusive-Messages

One of the forum gurus might be able to whip you up a file pretty quickly.

Alternately, this page has an in-depth description of how you can send SYSEX command from both a Mac and PC.

https://www.sweetwater.com/sweetcare/articles/how-do-i-send-and-receive-sysex-on-pc-or-mac/

You'll want only the data starting with F0 and ending with F7, one for each part to enable/disable.

Manuals and Documents

Roland MC-303 Owner's Manual (archive.org)
Roland MC-303 Support Documents (roland.com)
Arturia BeatStep Pro manuals (arturia.com)

[Note: The Roland MC-303 Owner's Manual PDF at the above archive.org link is an identical PDF file to the Roland MC-303 Owner's Manual PDF from another site that Jason provided in an earlier post.]

Sound Module Mode

I see that the Roland MC-303 has a Normal Mode and a Sound Module Mode.

• In the MC-303's Normal Mode you can use all the controls on the MC-303 to play and program it. In this mode, the messages coming in on the MIDI IN port are treated in an OMNI ON fashion: The incoming channel number is ignored, and all channel messages act on the currently selected part (whichever one of the Part buttons, R or 1 to 7, is currently lit). The MIDI Implementation information in the manual explains OMNI ON/OFF messages act only as All Notes Off commands and OMNI ON is always used in the Normal Mode.

So as rafael described, even though the Arturia BeatStep Pro is sending notes on three different channels, the Roland MC-303 in Normal Mode will effectively merge them all into one part. If the "R" button is active on the MC-303, then the BeatStep Pro's drums, bass, and melody parts will all end up playing the R part drums on the MC-303.

• In the MC-303's Sound Module Mode, only the volume knob works and all the other buttons and knobs do nothing. The incoming MIDI IN messages are sent directly to the MC-303's sound generator and it behaves in a multi-timbral fasion, that is, each channel has its own instrument sound and settings. (The MIDI Implementation information in the manual explains OMNI ON/OFF messages act only as All Notes Off commands and OMNI OFF is always used in the Sound Module Mode.)

Rafael, if you are okay with the Roland MC-303 just generating sounds and using only the Arturia BeatStep Pro to control the performance, you can try putting the Roland MC-303 into the Sound Module Mode, then each channel that is sent from the BeatStep Pro to the MC-303 can have a unique instrument sound on the MC-303. In the Owner's Manual, page 80 explains how to get into the Sound Module mode: you hold down the PLAY MODE button while you turn the power on. In the support documents, the "Advanced MIDI Programming" document is all about how to send appropriate MIDI messages to the Roalnd MC-303 when it is in the Sound Module mode.

System Exclusive messages

In the Roland Support Documents, take a look at the "MC-303 System Exclusive" document: "This PDF provides the Sys-Ex messages necessary for turning on and off the reception of MIDI note information." It is just a single page PDF file with a list of system exclusive messages that tell the Roland MC-303 to either ignore or obey the incoming note messages for each part. It seems like this might be what you want. [Note: Jason linked to a page on rolandus.zendesk.com that describes the same System Exclusive messages.]

(These messages can also be derived from System Exclusive parameter address information page 108 of the Owner's Manual, but you have to use the information in the table to manually assemble the System Exclusive messages yourself. You take the relevant "address" and "data" from the table, plug them into the "Data set 1" format of System Exclusive message shown on page 106, and then calculate the "checksum" according to page 114 and plug it into the format too. The "System Exclusive" document has already assembled each message you need, so they're ready to use.)

I notice that the Owner's Manual and the "System Exclusive" document use different assignments of the parts to the "block number" in the address. If the "System Exclusive" document has the wrong address mapping, you may have to try each message until you find the correct one for each part.

Unfortunately, it looks like the Arturia BeatStep Pro can't send System Exclusive messages. So if you want to send these System Exclusive messages to the Roland MC-303 you may need to send them from a computer or find some other device that can send them.

Device to split or filter MIDI channels

As rafael said, I think a MIDI THRU box will simply send a mirror of what's coming in to all of the outgoing MIDI THRU ports. Instead, I tried searching for a "MIDI channel filter" and found some devices that may be closer to what rafael was thinking of:

MIDI Solutions Router. "The MIDI Solutions Router is a 1-in 2-out MIDI data router/filter. Selected MIDI messages can be routed to either, both, or neither of the Router's two MIDI outputs." You can try out the freely available MIDI Solutions Programming Tools software to get a feel for the programming possibilities. See the attached picture for a setup to send channel 1 to the Out1 port, send channel 2 to the Out2 port, and block all other channels. (Because the "Ignore Priority" check box is clear, once a message matches a rule, the processing of rules stops, so the third rule ends up only matching channels 3 to 16.)

Roland MPU-103 MIDI Channel Filter Converter (PDF manual). You can turn the FILTER knob to select the single channel you want to pass and all the other channels will be blocked. It has a MIDI OUT port for the filtered messages and a MIDI THRU port with the unfiltered mirror of the original input.

CSF MIDI Processor. You can switch it to "N SOLO" to only pass through the channel N and block messages on all the other channels. It has a MIDI OUT port for the filtered messages and a MIDI THRU port with the unfiltered mirror of the original input.