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

[Soo-Hyun Yoo]

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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