Resolved! In the STM32F4 example, it says I have to enable or disable TRANSMITTER_BOARD, but I don't know where or how to do that.
The example I'm trying to work with comes from: STM32Cube_FW_F4_V1.25.0\Projects\STM32F4-Discovery\Examples\UART\UART_TwoBoards_ComPollingTo enable in the compiler means the board is the transmitter and disable means the board is the receiver. The co...