The following information does NOT apply to the Select Mini V2. Will update.
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The motion controller board or mainboard is powered by a STM32F103C8T6 “ARM®Cortex®-M3 32-bit RISC core operating at a 72 MHz frequency”. See http://www.st.com/en/microcontrollers/stm32f103c8.html for full specifications.
There have been more than two variations of the Motion Controller Board but these were minor changes. One example of the minor changes is the stepper drivers originally shipped with each having their own heat sink. Sometime shortly after initial batch of Select Mini's that Monoprice started shipping to customers on May 26, 2016 the stepper drivers shared one heat sink that spanned across all four. The units that shared one heat sink may have also been in the first batch of 500 printers that were shipped.
A fan header was added to the Motion Controller board. This connector is intended for additional fan for cooling the mainboard. The fan is supposed to start if the mainboard temperature goes above 55C.
The fan header on the pictured revision will not work without a small hardware modification. The fix requires reversing the small MOSFET (SOT-23) upside down to make it correct (I ASSUME the one adjacent to the fan header). “The design engineer made a mistake that reversed the package”. The next shipment of machines was supposed to have this correction already applied. Motion Controller firmware version 26 or higher is required to use the additional fan header. There are very limited reports that this fan header works. YMMV
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