Re: Real time USB2Serial devices and behaivor

[Greg KH <greg@kroah.com>]

On Wed, Mar 26, 2008 at 10:58:37PM -0400, Alan Stern wrote:
> On Wed, 26 Mar 2008, Greg KH wrote:
> 
> > > Is there any reason to think that if I created my own isochronous
> > > USB2Serial adapter and iso-usb-driver that I couldn't get determinism?
> > 
> > I strongly doubt it as others have tried and failed in the past.
> 
> I don't understand.  Isochronous transfers have pretty strict 
> transfer-time guarantees.  Why wouldn't this work?

I don't know, but the person who tried this a while ago said it wasn't
really "real-time" enough for their application (robot arm movement).

> One reason I can think of is that Iso transfers aren't reliable.  But
> then regular RS232-type serial transfers aren't reliable either.
> 
> The only other reason is that the USB stack itself has an unpredictable 
> amount of overhead.  However I think it should fall within an 
> acceptable range for RT applications.

It's all about bounding the longest latency.  Sometimes, under heavy
loads, latency can be pretty big.  But now that we have the -rt kernel,
it might be a lot better than before, so that might be possible now,
haven't tried it...

good luck,

greg k-h