On Tuesday, April 05, 2011 01:34:49 PM Bill Gatliff did opine: > Guys: > > On Tue, Apr 5, 2011 at 9:12 AM, gene heskett <gheskett@xxxxxxxx> wrote: > > I can head you off at the pass Ryan, by advising you that the > > latencies associated with USB (10 milliseconds and up) make this a > > very frustrating undertaking. ïOvershoots and wrecked parts will be > > the general order of the day because you cannot control your machine > > in 'real time' via usb. > > Why not use one of the off-the-shelf USB stepper motor controller chips? > > Or, if you still want to go the bit-bang route, a USB GPIO expansion > chip? > > > b.g. Because you still have the random latency of the usb involved. The machine could overshoot 20 thousandths of an inch while the command to stop that axis, and start moving another was in the pipeline. This is all in the specs for usb. However, I have not had the pleasure of reading the usb3.0 specs yet. I would suggest that if you want to do this via usb, then concentrate your efforts into figuring out a way, perhaps looking like the isolcpu command, where a usb3.0 port could be reserved, and hidden from the rest of the system, so that it might be assigned to be used ONLY with the RTAI facilities commonly used for real time control in 15-50 u-sec time slices like we do with a parport based hardware now. Figure that out so that USB can actually be used, and there are quite a few folks on the emc-user list that will bow and scrape when you walk into the room. It is a very common question from the new bee's, and one we have to explain to them all, one by one. If indeed, there was a usable solution made available, we would be more than happy to put it in our wiki, and point the new bees to it. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) <http://tinyurl.com/ddg5bz> <http://www.cantrip.org/gatto.html> Asking a working writer what he thinks about critics is like asking a lamp-post how it feels about dogs. -- Christopher Hampton -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
