Re: [CAD_CAM_EDM_DRO] Re: Indexer LPT and granularity

> I'm marginally familiar with VMware and, with my limited knowledge, it would
> not work for this. It sets up a virtual machine under linux (or windows)
> which makes available the host machine's resources. So an output to a
> parallel port by windows will pass through the linux I/O routine, causing
> double the timing problems.

I think Fitch was proposing to have linux (acturally the rtl stuff) do
more or all of the motion and tight timing I/O and have windows only
run the user interface and maybe G-code parser and maybe some higher level
planning stuff - as well as be there for running CAD and CAM programs.

> I think linux and windows aren't good examples here. Both have very good GUI
> interfaces but neither is designed for real time applications. (EMC works
> only due to a real time kernel patch). Is there a good, relatively
> inexpensive, real time operating system available?

Actually, there is a good free realtime operating system available,
and EMC uses it. Contrary to popular belief, it is not Linux. Rather
there is a realtime operating system that runs on the bare computer.
It runs realtime tasks like the EMC motion stuff. And with whatever
time is left over, it runs linux, because that is a convenient
environment in which to interact with the user, to setup the realtime
tasks from, and to run programs under that tell the realtime tasks
what to do. Linux itself is not made into a realtime operating
system, rather it runs underneath one.

It is my impression that the kinds of slight changes linux needed to
tolerate this treatment would prevent running windows in a similar
manner without the assitance of microsoft in making those changes. If
I understand correctly, EMC ran on such a platform originally, but the
realtime stuff could not be distributed for user modification since it
was licensed from microsoft. For that, and perhaps performance
reasons, they went to RTL & Linux. However if windows can run under
VMware, then maybe it can run under something like the RTL schedular
(although there is potentially a patent issue - RTL takes advantage of
a free patent license that only permits use with 'free' client
operating systems)

There are other realtime operating systems, from outfits like Wind
River, but it is unclear what exactly you gain in return for going to
these proprietary solutions. RTL, Linux, and EMC are quite easy to
install... it only gets complicated when you try to deal with some
video cards and peripherals that one would never dream of supporting
in a simple realtime system.

Chris Stratton

-------------------------------------------------
Christopher C. Stratton
Engineer, Instrument Maker, and Horn Player
ccs@... 617 628 1062
http://web.mit.edu/~stratton/www/brassbuild.html