BigTreeTech S42B Servo with 0.9° XY steppers
on LPC Port of RepRapFirmware

Prepare

Hardware

• 2 BTT S42B

• 2 0.9° steppers

• ST-Link v2 mini, a USB dongle programmer to update S42B firmware

• 8 bolts m3 x 40, or a suitable length for stepper and S42B

• (optional) 8~16 washers m3, to adjust the height of S42B

Firmware

Download or git clone the source code of Setar's 0.9° stepper firmware from this GitHub repository.

TrueStep S42B firmware implements a better LCD UI than BTT's original UI. Setar's firmware is a fork of TrueStep firmware with modification for 0.9° steppers.

IDE

VSCode with PlatformIO plugin

Driver Slot Requirement by RRF

Insert S42B slot adapters at the last slots as LPC/STM port of RepRapFirmware requires TMC drivers to be inserted in the first few continuous slots of LPC/STM boards. In this case stepper.numSmartDrivers = 2 in board.txt.

The assignment of driver slot is not necessary if you use Marlin or Klipper.

With SKR v1.4 no adapters are needed, just plug the cable in the the Z-CLS and E0-CLS sockets (or those sockets after TMC drivers).

Connection

• The distance between S42B and magnet might affect calibration result. Adjust the height of S42B with washers.

• Short circuit might happen (the small display and S42B don't work) if the pins behind the PCB touch the stepper. Trim the pins if necessary.

• Connect: PC/Mac USB port <--> ST-Link programmer <--> (Dupont connectors) <--> S42B

• Dupont connectors

  • 3.3V

  • GND

  • SWCLK

  • SWDIO

• Follow the steps in setar's repository. Compile the firmware source code using PlatformIO and upload the binary to S42B.

RRF config.g

• M584 X2 Y3 Z0 E1 ; Map S42B X & Y to slot 2 & 3. Map Z & E to slot 0 & 1.

• M92 X256.5 Y256.5 Z4000.00 E392.00 ; XY values are a result of manual measurement (but not sure about the reason. They were 200 steps/mm while using TMC driver.)

P.S. The steps/mm values are a result of manual measurement but not sure about the reason. They were 200 steps/mm while using TMC driver at 16 micro steps with 2GT 16T pulleys.

Calibration Is A Must

• Must calibrate before print, or you will get skewed prints.

• Must re-calibrate if the driving current in S42B has been changed.

• The direction of polarity of round magnets should be radial (lays in the XY plane after installation), not axial (along the Z axis as common round magnets do).