F ollowing is LoopBack Program to Write and Read from UART while (flag) { mraa_uart_write(uart, buffer, sizeof(buffer)); sleep(5); mraa_uart_read(uart,buffer1,sizeof(buffer1)); printf("Data Read %s\n",buffer1); sleep(5); buffer1[0]='\0'; } I tried with /dev/ttyS0 /ttyS1 /ttyS2 /ttyS3 and /dev/ttyS4. I short Port-4 pins (Pin no.1 and Pin no.2) pins