"Real time is when its fast enough." Yes, but the issue is that the OS developer, not you, decide what is fast enough. What is fast enough to drive an USB bus may not be fast enough to produce a 411 nSec pulse on a LPT port pin. I appreciate the "magic" of Windows and Linux and for the applications they were developed for they both do a fair job. However, the discussion is about how to make the PC timing behave absolutely predicatable and that is - by definition - not possible with a multitasking OS. Thus when the absolute width of a pulse is critical - no multitasking.

Have fun,

Erik