RS232 Communication Voltage Level
The Wikipedia article has an image that's fairly representative of what you should see in real life: 



The rise and fall times are non-zero, the high and low levels are not necessarily all that stable or smooth (especially with charge-pump drivers) and there's a bit of overshoot/undershoot. 

But if you look at the part of the waveforms between +3 and -3V (and a fair ways beyond that) they're very clean. +/-3V are the detection levels of the receiver, so it doesn't care what happens beyond those levels, +15 is the same as +3. The additional voltage gives you noise immunity.