[etherlab-users] Jitter when trying to use EtherLab Master at hard real-time 1 kHz under RT-PREEMPT

[Soo-Hyun Yoo]

On Thu, Mar 1, 2012 at 1:51 PM, Graeme Foot <GraemeF at touchcut.com> wrote:

> ** ** **
>
> Hi,****
>
> ** **
>
> There is an RTDM patch for EtherLabs EtherCAT master for use with RTAI /
> Xenomi.  It allows certain ethercat calls to be made from user space from a
> real-time context.
>

Thank you for the suggestion, and I would be happy with that if user space
operation was the only design goal for our software stack, but we want to
minimize the number of software components we have to configure and
compile. It would be great if there was a solution for RT_PREEMPT.

****
>
> ** **
>
> I don't know know about the RT_PREEMPT side of things.****
>
> ** **
>
> Regards,****
>
> Graeme.****
>
> ** **
>  ------------------------------
>
> *From:* etherlab-users-bounces at etherlab.org [mailto:
> etherlab-users-bounces at etherlab.org] *On Behalf Of *Soo-Hyun Yoo
> *Sent:* Friday, 2 March 2012 09:00
> *To:* etherlab-users at etherlab.org
> *Subject:* [etherlab-users] Jitter when trying to use EtherLab Master at
> hard real-time 1 kHz under RT-PREEMPT****
>
> ** **
>
> Hello,****
>
> ** **
>
> I am using EtherLab version 1.5.0 in Xubuntu 10.04, kernel v2.6.31-11-rt
> (RT_PREEMPT-patched kernel, installed via aptitude), and Orocos to provide
> a real-time user space environment in which to run software controllers at
> hard real-time 1 kHz.****
>
> ** **
>
> Unfortunately, I am having problems with jitter on the EtherCAT bus.
> Oscilloscope readings show jitter as high as 400 or 500 us, possibly
> higher. Comparatively, our old EtherCAT master running in kernel space
> under RTAI v3.8.1 has jitter under 50 us at most.****
>
> ** **
>
> From what I have read in mailing list threads like this one (
> http://lists.etherlab.org/pipermail/etherlab-users/2009/000535.html), it
> sounds like the EtherLab Master is capable of real-time user space
> operation in an RT_PREEMPT-patched kernel, but only capable of
> real-time kernel space operation under RTAI/LXRT (
> http://www.mail-archive.com/etherlab-users@etherlab.org/msg00588.html).***
> *
>
> ** **
>
> My tenuous understanding of this is that since the EtherCAT character
> device tries to use the kernel's ioctl calls, the kernel's ioctl
> "interface" (?) needs to be patched. Since RT_PREEMPT is the only patch
> that does so, while LXRT provides its own interface, the former is the only
> one capable of real-time user space operation while the latter is limited
> to kernel space. Based on this assumption, I am hoping that the EtherLab
> Master is capable of real-time operation in user space under RT_PREEMPT.**
> **
>
> ** **
>
> What can I do to lessen this jitter? Does anyone have a working EtherCAT
> master running at 1 kHz with jitter under 50 us?****
>
> ** **
>
> Soo-Hyun Yoo****
>
> Dynamic Robotics Lab****
>
> ****Oregon** **State** **University********
>
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