The simplest possible UART Loopback is a single wire connecting the receive input side to the transmit output side: Most development boards come with some integrated USB-UART functionality. If not, ...

#define ECHO_TEST_TXD (CONFIG_EXAMPLE_UART_TXD) // UART TX pin #define ECHO_TEST_RXD (CONFIG_EXAMPLE_UART_RXD) // UART RX pin #define ECHO_TEST_RTS (UART_PIN_NO ...