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On 5/6/2013 4:16 AM, Christophe Grellier wrote:
> Hello, First, let me say that I am pretty new to CNC. I use a 3
> axis mill (like this one <http://www.cnc-shop.ch/cnc6040.htm>) for
> 2 years now And English is not my native language ( i'm french ) so
> I may have misunderstood things. Please feel free to correct me if
> I'm wrong.
> 
> My current breakout board died because of a motor short-circuit. I
> was running it with ubuntu 12.04, xenomai kernel and 
> "rtos-integration-preview3" branch. So I decided to go for a more
> reliable, better quality hardware, and bought a Mesa 5i25 / 7i76
> and Gecko G203V drives.
> 
> From what I understood, the 5i25 does what you call "hardware
> stepgen" and the computer latency numbers are much less critical
> than "software stepgen". right ? So I installed a Debian Wheezy
> with the rt_preempt kernel ( I was fed up with ubuntu 12.04 /
> xenomai that didn't let me install fglrx graphic driver ).
> 
> So my main question is : what version of LinuxCNC can I install ? -
> the stable 2.5.2 ? From what I understood, it is RTAI-only. -
> master branch ? - rtos-integration-preview3 ? I am looking for
> something pretty stable, that can work with the rt_preempt kernel.

Then I would stick with the rtos-integration-preview3 branch you were
using previously.  Build details for Debian Wheeze with an rt kernel
are on the wiki:

http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Debian_Wheezy_Linux-Rt_Compile_LinuxCNC

> Would I gain anything at using a xenomai or rtai kernel that have
> better latency numbers ?

Yes, but if you've got a Mesa card, you don't gain that much on an x86
platform.  Unless you're really pushing your servo rate, rtai and
xenomai are IMHO really only necessary on x86 if you're trying to do
software stepgen.  But make sure you test your board, latency numbers
for rt_preempt can vary a *LOT* based on BIOS settings and your
particular hardware (since for rt_preempt to work well, the driver
code needs to be written to work well under SMP).

Now if you were on an ARM, rt_preempt still has horrid worst-case
latency (milliseconds, or 1000000+ nS), so you need something like
xenomai *and* hardware help.

- -- 
Charles Steinkuehler
[email protected]
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