Finally got everything working! The final problem was 100% my inability to read. As Orgia suggested, and catskull alluded to, my "link cable" was the final issue. I for some reason was using pin 19 instead of 16, so fixing this mistake got it talking immediately.

This doesn't account for my initial problem (and rich's), so I'll have to continue investigating if I want to use trash's most recent builds, but I'm happy to have it making noise in the meantime. If I find out anything more that may be useful to anyone else with similar issues, I'll follow up here.

Thanks again to everybody who offered help and advice - I'll post the finished build in another post when I'm done!

I'm running Windows 10 on this computer, which is also why I couldn't use the exact MIDI keyboard program you recommended above. From what I've read, I can probably go through the unnecessary software devices and uninstall them manually to make them disappear, but I probably won't bother doing that until I've got the rest working.

I don't have a link cable, and am intending to build the teensy inside the GBP, so at the moment I have the 4 connections that would usually be through the link cable hard wired between the teensy and the GBP. I've triple checked these, and they seem to line up as expected (double checked with a meter and everything), but I'll unsolder them completely and see what behavior changes. There may be an aspect to hard wiring these connections that I'm overlooking in comparison to using a broken out link cable. Some error on my part here might also account for the difference in behavior between your fork and the standard build, but I don't know enough yet to be sure.

Thanks again for all the input. I'll be updating this thread whether I solve my issues or not in the hopes it can help others who run into similar issues.

Thank you Trash80 and catskull for the help!

I tried both the version Trash80 posted, as well as catskull's fork, and neither worked off the bat. After trying swapping the usb cable, as well as using several different midi sending programs (I was using Reaper, but also tried several free keyboard programs as suggested), I still wasn't having any luck.

I eventually tried swapping the Arduino USB Type to "All Of The Above," and I've finally gotten the LEDs to respond to the midi out from Reaper. I haven't gotten it to talk properly to the GBP yet, but there's plenty of things I can try and troubleshoot on that before I give up, first being my wiring.

A couple quirks I'm running into though on the Arduino side:

1. Due to the "All of the Above" USB setting, the Teensy enumerates a ton of virtual devices, and clogs up midi selection menus due to this. A small price to pay, but I'll be trying the other USB types to see if any others work as All of the Above does.

2. I still could not get the version that Trash80 linked to to work as of yet. Currently only catskull's fork is responding for me.

3. On all versions I have tried to use, using the mode button does not loop back to 1 after 7. If I click past 7, no lights will light up from mode changes, and I have to use a clear EEPROM sketch to empty the memory. The only way I've found to avoid this is to set the number of modes in the main Arduinoboy.ino sketch to 6, in which case it goes to 6 then wraps back to 1 no problem. Is this indicative of anything specific I'm doing wrong?

4. When trying to use the USB type "MIDI", I often was getting driver crashes whenever I tried to access the Teensy midi device in any program. This doesn't appear to be happening anymore with All of the Above selected, but I thought it was worth noting.

Thanks again for all the help, I'm very new to all of this still. If anybody has any insight to the strange behavior, I'd love to hear it. In the meantime, I'm going to making sure my serial connections to the GBP are correct.

Thanks for all the suggestions catskull. I'm not sure about richy, but I haven't had any luck yet. I get all LEDs on boot still, but they never seem to respond to any input but a mode change.

The usbMode shouldn't be enabled, correct? … y.ino#L112
I've tried it on and off just in case, but it makes no difference.

I've tried the byte change that you suggested, as well as trying output on channels 1-16, but still haven't had any luck.

Is the 1.3 release of Arduinoboy what I should be using for this? I'm new to both github and Arduino so I'm just making sure I'm not missing something obvious here. As far as I can tell, 1.3 supports Teensy and USB support according the documentation.

Ultimately, if I can't get it working, I'll purchase one of your prebuilt Teensyboys which look awesome - I was just hoping to learn something from getting this running if I can.

Thanks again for the help.

I've been attempting a very similar setup to you, and am getting the same problems. Did you ever find a resolution to this? I'll follow up if I manage to get mine working