RE: EMC for windows

[jmkasunich-at-ra.rockwell.com]

"Erik Keller" wrote:

> There is one counter-example which shows that under certain
> circumstances windows will work in real-time: Mach 1, which
> is the twin of EMC hosted under windows.  My theory on why
> that works is that, by definition, a step and direction system
> is open loop stable.  Thus, when bill gates has internet
> explorer dump your hard drive over the internet to Redmond,
> nothing bad happens, because no step/dir pulses are output.

It's not that simple.  If the steppers were moving fast when
windoze started talking to Redmond, the pulse train would
stop abruptly.  It wouldn't send the table flying, but it would
cause lost steps because the motors wouldn't stop instantly.

Since Mach1 manages to avoid lost steps, Art must be doing
something more sophisticated.  I think he uses a real-time
device driver to handle the low level stuff.  The driver runs
at a higher priority than almost everything else in the
computer, so even when windoze itself takes a nap, the driver
keeps doing its thing.

In other words, it's not "impossible" to do CNC under windows,
just difficult.  Unmodified windows is no better or worse than
unmodified Linux for real-time stuff.  With Linux, the answer
is the real-time extensions (RT-Linux or RTAI).  With windows,
the answer is a real time low level driver.

John Kasunich