This time I will explain the electronic parts. But first of all, a disclaimer, I'm not an electrician nor an electric engineering. This design is based on my limited knowledge on basic electronics. So take this post with a grain of salt.
My printer is Anet A8v2, it does not have documentation for anything beside the basic installation and it only have 3 empty port/connector.
Two things are common in such electronic boards (of course there are exceptions):
There should be be a connector for debugging the MCU.
Board with STM32 MCUs are more likely to have a reset connector.
With that two characteristics, I made a guess that the 2 pins connector are reset pins, and the 4 pins connector are for debugging.
Which leave only 1 connector unknown, the 6 pins connector. Since there are not any documentation for this, the only way to know what is the connector function is to trace it manually.
Fortunately, nowadays, mobile phones are very common and they have camera(s). With the help of a image processing software we can trace it without pinching our eyes.
After tracing, I know that the 4 pins connector is directly connected to the MCU, and 2 are power pins. We can use this to expand the functionality of the printer.
That's it for now. I'll talk more on my next post.