I was browsing OSH park's shared circuits when I saw this:
Logic Analyser Buffer.
Since a true 12v RS232 signal will exceed the Salese's voltage limits, something like this could be useful. The diode and resistor seem straight forward enough. The three pin device I assume has a zener in it that shunts current to ground if the voltage exceeds 30 volts.
The part in the picture is labeled 215, but I can't figure out what it is.
If I can figure it out I was going to make a version of this which plugs directly into my Saleae. Or perhaps one with a DB9 tap.
Best Answer
It looks to be a Fairchild BZX84-C3V6 Zener diode.
So the circuit is like this (per channel):
simulate this circuit – Schematic created using CircuitLab
The resistors appear to be 1206 types, which would be inadequate to withstand +30V continuously (about 1.5W dissipation).