Re: real time?

TADGUNINC@... wrote:

> From: TADGUNINC@...
>
> Fortunately I have the support of my girl friend in learning all this new
> stuff about Linux EMC and cnc stuff, not to mention everyone on the list!!!!
> So time for another no brainer for you guys...what does real time mean and
> how do the window based cad programs differ from Linux?

Linux, by itself, is not a real time system, either. The definition of real time is
that tasks that are necessary can get the CPU time they need to run, without
any possibility of being interfered with by disk accesses, memory paging, other
tasks, even processor error handling. A secondary aspect is that interrupt
latency should be small. The real time patches to Linux definitely satisfy both
of these needs. The real time scheduler parcels out CPU time to Linux only
when the real time modules don't need it. As long as the real time processes
don't hog the cpu, you never even know they are there.

Windows-based CAD programs don't do motion control, so they don't
need to be real-time. There are some motion programs that DO control
machines, and unless they are supported by real-time extension packages,
the motion will be choppy, with the possibility of just sitting there (with
steppers) or leaving the machine moving (with servos) for several seconds
at a time. VERY bad!

And, the guys at NIST tried out some real-time extensions for Windows
NT, and the results were that the did work, but the interrupt latency was
VERY bad. Many times worse than the RT Linux.

Jon