Light indicator for TTL-level UART serial interface with a SINGLE LED?
simulate this circuit &ndash; Schematic created using CircuitLab

You could do it with ONE transistor + diodes, but transistors are cheap as dirt. A high-efficiency LED might only require R1, R2 of 270 ohms for sufficient brightness.
This circuit has the advantage that if both TX &amp; RX are active at the same time, you get DOUBLE-brightness. But you're not likely to notice this condition by eye.
Base current is small (perhaps 0.1mA) so that the TTL RX, and TTL TX lines are lightly loaded.