Sapphire Pro (Robin Nano) UART connection to TMC2209

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This is not a downloadable item, just a concise guide on how to establish an UART connection between the TwoTrees Sapphire Pro (MKS Robin Nano) and TMC2209 stepper drivers. Since finding information on this topic can be challenging, the quickest way to achieve this is by using a specific Marlin version: https://github.com/le3tspeak/Marlin-2.0.X-Sapphire-PRO. Alternatively, you can use this more customized version: https://github.com/Miraculix200/Marlin-SapphirePro-2209-UART/tree/SapphirePro-2209-UART-4pin. To set up the UART connection, follow these steps: 1. Remove all jumpers under the stepper driver. 2. Connect the RX/TX pins of the stepper driver together. 3. Attach them to a free pin on the Robin Nano (e.g., PA6). 4. Define TMC2209 in Configuration.h. 5. Uncomment the *_SERIAL_*_PIN lines in pins_MKS_ROBIN_NANO.h, based on the pin(s) you used. 6. Compile and flash. This method utilizes a dedicated pin for each stepper driver. It is possible to make all stepper drivers share one pin by assigning a unique hardware address to each driver. However, I have not attempted this yet as I only require it for one 2209 at present. You do not necessarily need to solder or crimp anything; DuPont jumper wires can be used if you manage to create a 2-pin-to-1-pin connection and your TMC2209 has two top pins for RX/TX (e.g., the Fysetc TMC2209 v2.1). To determine the pin names of the Robin Nano, refer to this resource: https://github.com/makerbase-mks/MKS-Robin-Nano/blob/master/hardware/MKS%20Robin%20Nano%20V1.2_004/MKS%20Robin%20Nano%20V1.2_004%20PIN.pdf. Note that not all free pins can be used for the UART connection; only some are suitable. You assume full responsibility when implementing this setup. Some recommendations suggest using a 1K resistor in the circuit, but I also saw someone doing it without a resistor, so I opted not to use one either. If this approach proves fatal to my board, I will update this text accordingly.