On Tuesday, April 05, 2011 06:53:25 PM Alan Cox did opine: > > Because you still have the random latency of the usb involved. The > > machine > > USB latency isn't random in ISO transactions, its pretty precise. > That has not been my experience, but that is also dated. > > 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. > > If you are trying to hit sub 1mS deadlines on a PC you are in the wrong > game, you can get stalls that big on the bus off the graphics card, off > all sorts of things. Which is why, generally, the graphics card brand is moot for this, but only the vesa driver is used. Realtime latency tests using the nvidia driver often disclose part wrecking stalls of 100's of milliseconds. Rip it out and use the vesa driver and those same stalls might reach .5 milliseconds worst case. That is still enough to disturb a stepper but rarely dings the part unless moving at max speeds, where the loss of a timely step signal effectively stops the motor in its tracks, and it is not capable of resuming full speed on the next step. My machine cam move at 35ipm, but works at 10-15 because those stalls are a non-problem at the lower speeds. > > 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. > > The robotics people handle this all fine - but they use reasonable smart > controllers so they don't have a PC playing "I am a very expensive and > completely crap microcontroller" emc can handle anything we have thrown at it so far, including carving Toyotas racing engine blocks. Up to 9 axis's of totally coordinated motions. That is not a crap controller. > Or take a look at reprap .. 8) I have. Its a poor solution looking for a good problem. Sure, it will develop into something usable in time, but that time is not today. -- 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> The only way to get rid of a temptation is to yield to it. -- Oscar Wilde -- 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
