RS232-TX voltage translator
First, that 1N4148 is just a diode, not a zener.

Second, just draw out a schematic if you can't follow the circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

Looking at it, strictly speaking the 1N4148 doesn't seem to be necessary, but it does make the leads line up more evenly to terminate to the DB9.