I wanted to interface Rpi with the function generator Keithley 3390. My goal is to capture the signal which Keithley 3390 generates and plot it using Rpi, i.e, to function it as an oscilloscope. I went through the reference manual of Keithley 3390 but could not come up with a solution. Please help.

 

Just do it the usual way; sample the signal.

Did you try Google? A couple promising looking hits.

http://www.instructables.com/id/PiScope-Raspberry-Pi-based-Oscilloscope/

http://www.bitscope.com/pi/