PC based CNC system architecture/Black Box

hi Jeff, just some comments on your comments:

Jeff Barlow wrote:

> Multi-axis motor control is clearly a real time process. Generic PC
> hardware and OS's were not designed as real time processing tools.

The original design intention of the hardware is of course irrelevant.
We use it because it does the job cheaper than any other alternative.
Also, you would not use a generic OS for Real Time control, just as
you would not use a hammer to replace a screw driver!

> EMC (for example) is proof that a PC can be successfully pressed in to
> service as a real time controller.

You bet.

> Among the costs I see are: The installation and system integration is
> messy enough to drive many folks away;

agreed

> the RT subsystem negatively impacts the performance of the user interface;

not with Linux/EMC - I would challenge you to notice any change (of the
RTL working vs being turned off), with a suitably sized cpu.

> the performance of the stepper drive version is rather limiting.

all the world is a compromise ...

> We seem to be in general agrement that hooking motor drivers directly to
> the PC printer port doesn't really cut it.

I think the general agreement is the contrary - for the reduced complexity
the parallel port Step & Direction works quite well. It could always be
better, of course. Depends how much you improve the situation vs
how much you complicate (and spend).

> It is tempting, for many of us, to try to go off by ourself and design
> this "black box"....
> To put that another way, I would like to see us more fully define the
> functional role of this "black box" before we get too carried away with
> figuring out just how it works.

cost benefit. I actually see the biggest problem as hardware timers,
and if I had any time, I would put a number of timers and bit I/O
lines onto a DIMM stick that fits into a free memory slot (and emulates
memory I/0, but of course is Not memory) where the cpu has the absolute
minimal latency to respond to interrupts, and do I/O to the outside world.

Doug Fortune