[Xenomai-help] rt usb

[Xenomai-help] rt usb

[Roland Tollenaar]

Hi,

I was just wondering whether any work was going on for real-time usb?

This would enable the use of PCI adaptors over USB so that PCI-IO cards 
(like the peak-can card) can be used from laptops. CAN over USB is also 
rather valuable.

Is any documentation available which clarifies why usb is a problem and 
how big the problem is (perhaps I can risk it?:) )

regards,

Roland

Re: [Xenomai-help] rt usb

[Jan Kiszka]

Roland Tollenaar wrote:
> Hi,
> 
> I was just wondering whether any work was going on for real-time usb?
> 
> This would enable the use of PCI adaptors over USB so that PCI-IO cards 
> (like the peak-can card) can be used from laptops. CAN over USB is also 
> rather valuable.

http://developer.berlios.de/projects/usb4rt
https://gna.org/projects/usb20rt

Both projects are generally not directly usable and are currently in a 
dormant state.

> 
> Is any documentation available which clarifies why usb is a problem and 
> how big the problem is (perhaps I can risk it?:) )

No longer at hand (a former student of mine did his diploma thesis about 
usb4rt). Generally, there is no real show-stopper, we "just" need a 
real-time-aware stack that provides appropriate API (e.g. to 
preallocated required resources) and that is bug-free... :)

Jan

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux

Re: [Xenomai-help] rt usb

[Roland Tollenaar]

Hi Jan,

>> Is any documentation available which clarifies why usb is a problem 
>> and how big the problem is (perhaps I can risk it?:) )
> 
> No longer at hand (a former student of mine did his diploma thesis about 
> usb4rt). 
Pity.

>Generally, there is no real show-stopper, we "just" need a 
> real-time-aware stack that provides appropriate API (e.g. to 

So, if I run CAN over USB from a rt thread there is a chance that even 
though it is not rt-safe, it may not screw up latency?

I realize that this will seem pointless and is technically incorrect but 
  if for example the USB device is not addressed by any other processes, 
what is the chance that I will have problems?

Regards,

Roland





> 
> Jan
> 

Re: [Xenomai-help] rt usb

[Jan Kiszka]

Roland Tollenaar wrote:
> Hi Jan,
> 
>>> Is any documentation available which clarifies why usb is a problem 
>>> and how big the problem is (perhaps I can risk it?:) )
>>
>> No longer at hand (a former student of mine did his diploma thesis 
>> about usb4rt). 
> Pity.
> 
>> Generally, there is no real show-stopper, we "just" need a 
>> real-time-aware stack that provides appropriate API (e.g. to 
> 
> So, if I run CAN over USB from a rt thread there is a chance that even 
> though it is not rt-safe, it may not screw up latency?
> 
> I realize that this will seem pointless and is technically incorrect but 
>  if for example the USB device is not addressed by any other processes, 
> what is the chance that I will have problems?

The things is just as with RTnet vs. standard Linux networking: You need 
a stack like the mentioned ones to provide rt-safe access to the USB 
hardware. For CAN (e.g. the Peak dongle), you then need a high-level USB 
driver that connects the RT-USB and RT-CAN stacks. But there is no point 
in trying to tunnel your USB requests through the Linux USB stack, I 
case you meant this.

Jan

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux

Re: [Xenomai-help] rt usb

[Anders Blomdell]

Jan Kiszka wrote:
> Roland Tollenaar wrote:
>> Hi,
>>
>> I was just wondering whether any work was going on for real-time usb?
>>
>> This would enable the use of PCI adaptors over USB so that PCI-IO cards 
>> (like the peak-can card) can be used from laptops. CAN over USB is also 
>> rather valuable.
> 
> http://developer.berlios.de/projects/usb4rt
> https://gna.org/projects/usb20rt
> 
> Both projects are generally not directly usable and are currently in a 
> dormant state.
> 
>> Is any documentation available which clarifies why usb is a problem and 
>> how big the problem is (perhaps I can risk it?:) )
> 
> No longer at hand (a former student of mine did his diploma thesis about 
> usb4rt). Generally, there is no real show-stopper, we "just" need a 
> real-time-aware stack that provides appropriate API (e.g. to 
> preallocated required resources) and that is bug-free... :)
One problem with USB is the inherent 1ms(full speed)/125us(high speed) cycles,
which (AFAICT) prohibits closing a control loop any faster than that. On top of
that at least some USB I/O-cards (somewhat off-topic, since post was about CAN
over USB) tend to pack multiple values into one USB-packet, which leads to
larger delays.

I would investigate Ethernet-CAN gateways, perhaps brew my own using
http://www.hitex.com/str9-comstick/

/Anders


-- 
Anders Blomdell                  Email: anders.blomdell@domain.hid
Department of Automatic Control
Lund University                  Phone:    +46 46 222 4625
P.O. Box 118                     Fax:      +46 46 138118
SE-221 00 Lund, Sweden