tim77777:
Should there be a diode input clamp on the transmit line, to protect the uart pin on the microntroller.
It is not needed because the transmitter only ever sees the LED and seriese resistor from the equipment it is connected to.
You have to watch however that the 3V3 signal pin can provide the appropriate amount of current, some processors are limited in the source current certain pins can supply. It has to cope with the current required if the signals are swapped over, that is the resistor is in seriese with a diode and not an LED.