On Tuesday 30 November 2004 13:05, Joe Pfeiffer wrote: >Gene Heskett writes: > > The point being that for real time control of stepper motors, the > > step signals need to go out at a steady beat, whether that beat > > is 10 times a second, or 50,000 times a second, and any variation > > in between. In other words, when the motor is to be stepped, and > > an outb is done at a certain rate, that certain rate must be > > maintained, and cannot be subjected to any resync to a fixed > > clock that might be built into the protocol. They cannot go out > > 10 packets at a time, (your "multiple packets per frame" above) > > and then wait for the next time the port can be energized, > > thereby skipping the time slots those 10 packets represent, and > > then sending them as a single burst of data. Motors, being > > subject to inertia effects, simply cannot have their input > > signals subjected to the loss of coherency that we are reading > > into the present information regarding the usb standard thats > > published, albeit rather hard to come by if you are not a member > > of the usb consortium. > >First, the little thing: I didn't have much trouble getting the > spec, and I'm not a member of the consortium: >http://www.usb.org/developers/docs/usb_20.zip
Great, at 9.2 megs that has got to be far more complete than what I have here now, thanks. >Anyway, yes, you've given me a much clearer picture of the problem. > >As a bit of background, I'm no expert in the area, but I am > currently teaching a senior project class doing a USB-based DMX-512 > controller. DMX-512, in turn, is a theatrical lighting controller > standard built on top of RS-485 (which in turn is basically > RS-232with different voltage levels); its timing requirements are > nowhere near as precise as driving a stepper motor, but I still > couldn't see taking an approach of just using an RS-485 port and > driving it from the computer nor using a commercial USB-to-RS232 > adapter and hacking it. Both of these approaches have been tried > (and there was even an article in Circuit Cellar a couple of months > ago showing how to do the latter). I've also got some experience > with stepper motors, though I've never tried to drive one from a > PC. Humm, DMX, is that the one where the controller sends a pulse train to the power box that is a seriel sample on a 256 bit scale of where each dimmer slider is at? I've been baby sitting a pair of CD-80's for the studio light control for the last 20 years. Darned control panel is worn out though, the sliders are noisey as all get out. Now out of my responsibility, they've added a whole kit of HI flourescents to the light kit now. Cuts the AC load by about 20 tons! And also, when they fade up, every UPS in the place goes on a beeping spree until they are at operationg brightness. The ciruit boards in the cd-80 packs are problems though, a bunch of cmos stuff that is NOT adequately protected from the high power stuff, so it goes away everytime mother nature throws a fit about that stuff not being butter. (old tv commercial) >So... what we've done is to put an SGS-Thompson ST7 microprocessor > in a box with an RS-485 driver. The ST7 understands USB natively, > and has a whole raft of peripherals on-board. The host sends > dimmer levels to the adapter; the adapter keeps track of all 512 > levels and generates the DMX protocol. Total project cost was > about $150 (including having the board fabbed by PCBExpress), not > counting a programmer for the ST7which cost about $250 but should > be useable on into the future. I'm afraid my hardware design had > two bugs and some relativeley harmless other small goofs so I'd be > a bit embarrassed to send it to you as-is, but I'm hoping to do a > version 1.1 over Christmas break if you'd be interested. > >If I were tring to do stepper motor control, I'd probably think in >terms of an adapter like that one. Since I expect you want multiple >motors moving synchronously, I'd try to find the micro that could >handle as many outputs as possible, and then think of USB commands >that would do something like give the time they should take to do >something, and then give the number of steps several motors should >take. Then the details of stepping would be off-loaded from the > host. > >Anyway, take this for whatever it's worth... It is a good discussion Joe, no doubt. And one that needed to be done just for the groups education. It has certainly given me some food for thought, thanks. -- 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) 99.29% setiathome rank, not too shabby for a WV hillbilly Yahoo.com attorneys please note, additions to this message by Gene Heskett are: Copyright 2004 by Maurice Eugene Heskett, all rights reserved. ------------------------------------------------------- SF email is sponsored by - The IT Product Guide Read honest & candid reviews on hundreds of IT Products from real users. Discover which products truly live up to the hype. Start reading now. http://productguide.itmanagersjournal.com/ _______________________________________________ [EMAIL PROTECTED] To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-users
