> I avoid Windows for real-time applications if at all possible, but my main reason is uptime of the OS :>

sorry, i don't understand "uptime of the OS :>"

you avoid Windows for real-time applications: did you already made some ?

> Does the PC control the stepper moters of the CNC machine directly or is there a controller in the CNC already? or do you have an ISA/PCI card sitting in the PC?

not only stepper motor, cc motor as well.

the cnc is a software cnc.
there is an external board based on a CPLD, plugged on the ECP Parallel port.
It provides only low level services: io, wired security, some wired logic and power management.

> (not taking into acount development cost of the board and associated software).

that is the problem, i wonder if it is cheaper than making a Windows device driver. the big if is: can a device driver do the job ?

And a PC Motherboard is quite a powerfull and cheap board..

> Yes. However you may find adding an external controller is cheaper then trying to get your windows program to run fast enough.

that is the question.

I just know some windows program/device driver use to make software MPEG or MP3 decompression.

that is real time and quite fast isn't it ?

In fact, i know there is special PC cnc board: see
www.galilmc.com, ni.com
and other.

But we have some 'virtual cnc' classes i hesitate to trow away.

I'm just exploring all the possibilities. I try to evaluate what Windows can do before taking a decision.

Thanks and Regards

Roland