[Xenomai-help] OT. A question about constraints in realtime

[Leopold Palomo Avellaneda <leo@domain.hid>]

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Dear people,

first of all this is an Off topic, so please don't be angry with me. If you 
don't want to answer, move this message to /dev/null. But I have a problem 
with a device and maybe some people in this list could give me some idea.

I'm working in a robotics lab and we have a device (a haptic from Sensable) 
that it's connected to the parallel port. Of course that the Sensable company 
doesn't not publish open source drivers, etc. By now it's an sterile 
discussion.

Anyway, they provide a driver that it's a lib (.so). Also there's some library 
that can act with the device using that driver. Excepting the driver, for the 
other parts I have some source code.

I have had several problems with this driver. Firstly, the driver only can 
works if the parport module detects the EPP mode. Sadly, the linux parport 
module is more or less unmaintained and fails in many boxes to detect the EPP 
mode, although the bios is configured to.

After that, you can read encoders of the device (using the driver) but when 
you send forces to the engines, it suffers some kind of chattering and the 
driver disable the force because the loop of control expend too much time. 

Asking in some forum I have arrived to the conclusion that the driver has some 
kind of timer and if it doesn't not receive any order in 1ms, then the force 
is disabled.

So, my question are:
- Why the windows driver works so well? the scheduler in Windows do that if a 
task need to have a constraint of 1ms they give it the control and that's 
all?
- Why a linux driver doesn't not work? the load of the box is quiet normal. 
I'm not compiling or solving eigen systems. Or is possible that some device 
is breaking some access to the parport, or whatever?
- I have tried to use the realtime kernel -rt but same result. 
- Can I do something with xenomai to avoid it?

The sensable people claims that their driver works with some versions of Linux 
(till fedora 4 and some suse) but I'm not be able to run it in a debian etch. 
I don't know if the distros provide a kernel with some kind of modifications 
that solve it, but I don't think so.


Well, thanks in advance and I'm sorry for the noise. Really, the realtime 
stuff is something not obvious...

Best regards,

Leo


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