So to get USB midi working I needed an in-circuit programmer for an Arduino. It’s very easy to make one from an Arduino, I used a nano I had with a DB9 connector since I have a lot of those laying around.
There are a lot of descriptions on how to do this, the key thing I learned is that to use the nano you definitely need the 10 uf cap between reset and ground.
Here’s what my programmer looks like:
and here it is installed in a DB9 shell:
