DIY prototype RPN/TENKEY programmable calc with printer <:3)--

[macieksoft]

Blue wire is for STM32 default UART bootloader. It is used to program the chip. You connect it to the USB>UART interface and you can load the program into the chip. You also have to flip that switch below it (and obviously change the jumpers on board to put STM32 into bootloader mode), because normally pin RX1 is used to receive data from ATMEGA-16 that is the keyboard controller. So the blue wire is normally not connected to anything, because when switch is flipped left, RX1 goes to ATMEGA and not to blue wire.