Single LED indicator for activity in both serial communication RX and TX lines. Assume standard USART type logic levels with active LOW for RX line and active HIGH for TX line.
Activity LED is lit when P-MOSFET gate is pulled down to LOW voltage either from RX line directly or by HIGH voltage level in TX line inverted by N-MOSFET. Diode at RX line input prevents disturbances from TX line activity. The capacitor prolongs the LED lit time making single short pulses visible as well. Optional 22 ohm resistors at inputs do edge smoothing and ringing suppression.
To minimize total footprint area, use passives in 0402 or smaller packages, diode in SOD323 or smaller package and dual P-N-channel MOSFETs in SOT-363 or smaller packages (DMC3400SDW in real life application).
