USB MIDI controllers (such as Launchpad Mini Mk II for example) are common and often quite low in cost.

To interface such a controller with a Eurorack synth system, often a host computer and a MIDI to CV interface might be used. The host computer would take USB MIDI data from the MIDI controller, perhaps store and manipulate that data in some way (e.g. a sequence), using a MIDI to CV converter to then control a Eurorack synth system.

It would be useful to use USB MIDI controllers with Eurorack synth systems without needing a computer and MIDI to CV interface in between the two.

Teensy 3.6 is a great microcontroller that can be programmed using the Arduino IDE. A very useful feature of the Teensy 3.6 is the USB host port.

Using the USB host port, the Teensy 3.6 can act as a host device for a USB MIDI controller. This means that a computer is not required to interpret the USB MIDI messages from a USB MIDI controller.

The Teensy can receive MIDI data sent over USB from the MIDI controller (e.g. faders, knobs, buttons) and can also send MIDI data back to the MIDI controller over USB (e.g. LEDs for buttons, displays).

At the same time, the Teensy can send and receive voltages to and from a Eurorack system. This includes control voltages, gates, triggers and clock signals as both inputs and outputs.

By programming the Teensy, a relationship between USB MIDI data from the controller and the voltages to and from a Eurorack system can be set up.”