The 6n137 can be driven directly by 3.3V logic with a 220 ohm resistor in series with the emitter diode on a parallel port pins 2 through 9 of the db25, the control output pins are the issue (1,14,16,17) as in many computers the don't have much power to source current. In that instance you probably will have to provide an isolated supply voltage for a resistive pullup to 3.3V or 5V depending on the computer. Now, you say 3.3V, there is a lot of confusion about laptops. Actually many laptops aren't 3.3V logic on their parallel port when they are powered by AC power. And you will find many older destops, that only put out 3.3V in the HI state. The 1284 spec for parallel ports wether laptop or desktop, specifies output highs as 2.4V or higher into certain loads. If the laptop says its 1284, then it will work the same as most desktops. Laptop issues many times are incorrectly diagnosed as being a port problem, when in fact it's automatic CPU power saving changing timing.