How to shorten fall time of NPN transistor
It looks like whatever's providing TX_DATA_5V is an open-drain/open-collector output. You'll want to add a pullup resistor, like so:







simulate this circuit &ndash; Schematic created using CircuitLab

This resistor will pull the line high when the signal is high, and is necessary because open-drain/open-collector outputs can only drive the signal low. If you want a faster fall time, you can reduce the value of R4. Be careful you don't end up sinking too much current into your microcontroller's output though.