Wiring the SKR mini V1.1 board
DISCLAIMER: Please don't wire this board up if you're not familiar with handling electronic devices! Instead, get a pro to do it for you. Otherwise you may end up doing harm to the mainboard or even yourself! I am not responsible for any damage or loss you may cause.
From version 1.5x of the SMuFF firmware on, you'll be able to utilize another controller board for the SMuFF. I've picked the SKR mini V1.1, because it's an affordable powerful 32-Bit board with the option to use any stepper motor driver you like. Since it's 32-Bit, all operations run much faster than on a 8-Bit board. Especially on the stepper motor movements you'll spot the difference.
Please notice: In order to program the firmware onto this device, you'll need an STLink USB stick, which gets connected to the ISCP interface of the board!
This image shows you the SKR mini V1.1 board layout with the connectors you need to wire it up to the SMuFF.
The Stepper motors connect to the X- Y- and E- sockets accordingly, whereas X is the stepper motor of the Selector, Y is the stepper motor of the Revolver and E is the stepper motor of the Feeder. In contrast to the Wanhao i3 mini, we're not utilizing the Z stepper motor, because the STEP and DIR pins of this socket may be used for an additional serial port (Serial 3).
This board is able to take any stepper motor driver, so feel free to pick one of your favorite drivers. Since the firmware isn't doing any fancy stuff with those drivers, it is sufficient to run them in A4988 mode (controlled through the STEP/DIR signal pins).
Before you connect your motors, make sure you have set up the Vref correctly by using the according potentiometers on the stepper driver and that you have set the stepper mode jumpers to 16 microsteps.
The endstops for the Selector, the Revolver and the Feeder go to the endstop sockets X-, Y- and Z-.
The Fan socket can be used to connect a PWM driven Fan to cool the drivers while operating but it's optional as long as you don't use some high power drivers which are producing a lot of heat.
The Serial1 socket is used to establish a serial connection with your 3D printer (usually this socket is equipped with the touch screen display). Please notice: Never attach this socket directly to a RS-232 / V.24 connector, which is common for most PCs. Such connectors use a signal level of +12V/-12V and will damage your board immediately.
The communication between your 3D printer (or PC for testing purposes) and the SMuFF runs either over Serial 1 or Serial 3. Please note that on the SKR you can't utilize USB for this task. In case of the 3D Printer you don't need any sort of converter, since 3D Printer controllers usually have a compatible TTL serial port for communication with peripheral devices. In most cases you'll need a cross-over connection between the TX and RX signals of the 3D Printer and the SMuFF.
In order to communicate with your PC (say for testing), you'll need a separate USB to TTL Serial interface connected between your PC and the SMuFF (either Serial 1 or Serial 3). This connection also needs a cross-over wiring, which means: The TX signal of the USB to TTL interface goes to RX on the SKR and the RX signal goes to TX of the SKR. You also have to wire the GND signal from the USB to TTL interface to the SKR.
Baudrates can be easily configured within the Settings menu of the SMuFF. Since this board has two serial ports available, it's possible to connect both, your 3D printer as well as your PC to it, if you need to.
The SKR mini V1.1 board is equipped with the standard (Marlin) EXP1/EXP2 connectors, which usually are being used to connect a Display/Rotary encoder unit for input/output.
Unfortunately, the SPI bus on this connector goes to SPI3 on the STM32, which makes it unusable for the U8G2 library unless the library itself gets modified to support SPI3 directly.
So, instead of messing around with libraries, I've decided to replace the display unit with an customized OLED TWI/I2C interface. Details about this board you'll find here. To connect the display simply connect a 5x2 cable between EXP1 and the connector on the display board. If you want to be able resetting the controller from the display board, you have to wire up a single cable wire from the micro switch to the RESET pin on EXP2 as well.
If you intend utilizing a servo as the filament wiper as shown here, you have to wire the servo as follows:
- take the supply voltage and GND from a separate buck converter (step down converter) which you'll feed from the main +12/24V power input of the board, to power the servo (Red/Brown wire on the servo). Set up the buck converter to a output voltage that satisfies your servo (usually in the range from +5V to +8V) but make sure, you're not overpowering it. Set up the voltage before you connect your servo to it.
- on the connector named Servo 1 in the picture above, connect the leftmost pin as the signal (PWM) pin (Orange wire on the servo).
- the Servo 2 connector is being used for driving the servo of the SMuFF Servo Variant. Same procedure here: Use an external buck converter to power the Servo. If you use both servos, you'll only need one buck converter but make sure it's able to deliver enough current for both servos (something in the ballpark of 3A should be enough, depending on the servos you use).
To test the Servo 1 connection, simply send a G12 command to the SMuFF controller over the serial interface.
To test the Servo 2 connection, you have to set up the SMuFF for servo mode within the Settings -> Steppers -> Revolver menu by changing the Use Servo option to true.
You, of course, may want to have an enclosure for the new controller. Have a look at this thing on Thingyverse. There you'll find all the pieces needed.