from adafruit_ble.uart import UARTServer

uart = UARTServer()

while True:
    uart.start_advertising()

    # Wait for a connection
    while not uart.connected:
        pass

    while uart.connected:
        # Returns b'' if nothing was read.
        one_byte = uart.read(1)
        if one_byte:
            uart.write(one_byte)

    # When disconnected, arrive here. Go back to the top
    # and start advertising again.