Tutorial: Mellow FLY Gemini V2 RRF installation

Hi guys, today I’m going to take you through the process of setting up your Mellow Fly Gemini V2 board using Rep Rap Firmware (RRF). RRF has introduced the ability to combine a SBC with a control board now, taking a similar architecture to Klipper. The Gemini is a hybrid board combining a single board computer (SBC) equivalent to a Raspberry Pi into the same printed circuit board as the printer controller. This reduces wiring and part count, plus given the current Raspberry Pi scalping it is very cost effective too.

The Docs

The documentation for the FLY boards is a bit scattered so I’ve tried to compile a list of useful places here.

In the box

The Fly Gemini with your kit comes with the following items in the box

  • Fly Gemini V2 board
  • JST connectors for every port on the board
  • Pre-crimped wires for the smaller JST sized connectors
  • Crimp ferrules for the power supply wires
  • Jumpers for the motor drivers and fan voltages
  • Fuses for the power inputs and bed output
  • 4 x TMC2209 stepper drivers
  • Micro USB with the SBC firmware pre-installed
  • WiFi dongle
  • Instruction manual

Preparing the board

Add screw terminal label to the cover of the screw terminals. The Bed negative input should be on the bottom (screw terminal 1) as in the image below.

Plug your WiFi dongle into the USB ports. Do NOT put wifi dongle in bottom right USB port (when looking at the USB ports) as this port is shared with the connection to the STM32 controller chip and you will NOT be able to communicate with the microcontroller.

Plug in the 3 fuses (Red automotive fuses) into the slots around the screw terminal block. They are all the same rating so the locations don’t matter.

Add jumper for the stepper motors (UART configuration) and the jumper for the fan voltage (12v) (as indicated by small green boxes on the diagram below). Then plug in the fan MOSFET into the connector indicated by the pink square. Lastly plug in your 4 motor drivers (blue squares).

Because this board can be used with RRF or Klipper and in both CAN and SBC modes there are some switches used to configure the internal setup. On the 8 pole switch you want 1-6 set to off and 7-8 turn on. For the 5 pole switch you want all 5 turned to on.


SBC image install

Download the OS image here and use raspberry pi imager to burn it onto the SD card.

Once the sdcard is complete, remove it from your PC, and plug it into the SBC SD slot (The one next to the USB A ports).

Prepare MCU firmware

Download the latest firmware for the MCU from here. You need to download the STM32F4-SBC variant.

Copy the file to the micro SD card you are using and rename it to firmware.bin (Don’t accidentally end up with firmware.bin.bin which is a common mistake!).

Configurating RRF

Now your board is all setup it’s time to configure your printer. This will be highly specific to what your are installing the board on but I’ll cover a generic 3 axis COREXY printer.

Head over to the free configurator tool. Choose custom configuration. Click next.

On the next page choose the Fly-Gemini-V2.0 board and the latest stable image. At the time of writing this it was version 3.0. Choose your printer geometry and kinematics.

Continue to click next through the dialogs. For a basic installation most of the rest of the default settings will work fine to get you started. One important part is to select the stepper drivers you are using (most common is tmc2209).

Once you reach the end you need to download the configuration bundle as a zip file.

Extract the zip file and copy the sys folder onto your SD card.

Put the SD card in the remaining micro SD card slot on the Gemini.

Setup the software

Next, connect the Gemini to your PC, via the USB-C port. When you do this, a COM serial device should appear (much like an Arduino if you are familiar with those). You now require Putty, which can be installed from https://www.putty.org/.

Open device browser to check which COM port the Gemini board has appeared as. For me the USB serial device appeared as COM6.

Open PuTTY and configure the connection as per below. You can add a name to the saved sessions box and save it for future use!

Press open and a command line interface should appear. If nothing seems to be happening you may need to hit the reset button just next to the USB C connector. You will then be asked to log in. The default user in root and the password is 1234. Change the password when prompted to something secure. The dev recommends zsh over bash. Personally I would choose bash because it is the most common and I am the most familiar with it. It shouldn’t affect the rest of the tutorial but some screens will visually look a bit different

Now set up the WiFi connection by entering the command below


Navigate to activate a connection

Navigate to the network you are interested in and press enter

Enter the password then press ok. If successful the network will appear with a * next to it indicating you are connected.

Exit out of the program back to the command line. Enter the following command to get information about the network. Look for the WLX section and then the IP address next to the ‘inet’ line. Copy this IP address for use later.


Next we need to just make sure that rrf is up to date so go ahead and run the command below. It may take some time.

rrf_upgrade latest-stable

Open your browser and navigate to the IP address you wrote down earlier. You should be greeted by the DWC interface with no errors! Good luck and have fun!

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