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The MaxSTM Version 3.6

The image below shows OnStep telescope controllers built on the MaxSTM Version 3.6 PCB.  This board uses either the STM32F411CE at 100MHz or STM32F401CC at 84MHz, you can see a comparison of hardware features here.  The PCB is designed to fit inside this Aluminum Enclosure Electronic DIY PCB Instrument Project Box Case (120x97x40mm) to facilitate a complete professional looking solution.

The MaxSTM kit includes 3D printed panels to fit the above enclosure and has all components required to build a two axis goto controller.  The kit doesn't include components to populate the Focuser/Rotator section to save expense and effort for those who don't need that feature.  This decision also affords the possibility to move the included TMC2130 drivers into the focuser/rotator section and upgrade the mount Axis1/2 stepper drivers to TMC5160's for those who need higher current handling.

The MaxSTM kit does not require any flashing or firmware upload for most users.  When finished building/assembling simply apply power (use an old laptop supply or battery etc.) and connect your PC or cellphone/tablet to the WiFi access point. Then point your web browser at address, click on the Config webpage, and enter the mount/stepper motor parameters in the Advanced section.  Each kit is partially assembled then powered up, flashed with the default firmware, and tested for connectivity before shipping.

The MaxSTM kit comes pre-configured for maximum flexibility and has the following features and parts included:

  • The onboard STM32F401CC micro-controller has easily sufficient RAM and flash memory resources (including 8KB of high write endurance EEPROM) for this application.  It also has a floating point unit for fast alignment model calculations.  This micro-controller's performance is in the same general class as the ESP32, that is, it's fast enough for OnStep in almost any use case.  The STM32F411CE is included as a zero cost upgrade as availability permits.
  • WeMos D1 Mini Pro for WiFi.
  • DS3231 RTC.
  • Rugged full sized USB port.
  • ST4 port with SHC support.
  • DB15 connector with active support for adding your:
    • Incremental quadrature (A/B) encoders.
    • BME280 ambient conditions sensor.
    • 2 dew heaters and 2 switches (some are setup as 1-Wire devices.)
    • DS18B20's for spot temperature; focuser TCF, etc.
    • A spare serial port.
  • 1/8" stereo mini jack has the limit sense input and a variable brightness illuminated reticle output.
  • Two TMC2130 stepper drivers and both RJ45 and screw-down terminals are included (pick one or the other.)  Uses your leads/connectors going to your Axis1/2 bipolar stepper motors (not included.)
  • Power jack.  Optionally add the included power switch; requires your own wiring and possibly soldering to the PCB.


  • The MaxSTM kit's main micro-controller comes with firmware installed and it usually DOESN'T NEED TO BE FLASHED AGAIN, you configure at runtime from a webpage instead.  However, for some advanced features and configuration changes you must flash the controller using STM32Duino and DFU firmware upload across USB.
  • The MaxSTM kit's WeMos D1 Mini Pro comes with firmware installed and SHOULD NEVER BE FLASHED AGAIN.  The firmware that allows OnStep runtime configuration is closed source and only available through me.  Note that you are free to install the similar open source version of the WiFi Addon firmware but you will loose the ability to configure OnStep at runtime if you do so.  If taking this route you will need to wipe all settings in OnStep's EEPROM with the :ENVRESET# command (in the Arduino Serial Monitor) so Config.h settings are used.

Below is a fully assembled MaxSTM3.6 for reference.

  • Differences from the MaxSTM Kit:
    • Standard WeMos D1 Mini instead of the Pro model with external antenna.
    • Focuser/Rotator section is populated with socket headers (but no stepper drivers,) RJ22 jacks, and a 12V switching regulator.  These are not included.