----- Original Message ----- From: Andrew Warren To: Sent: Wednesday, December 20, 2000 12:36 PM Subject: Re: [EE]: USB stepper drive system > Bob Ammerman wrote: > > > Have you ever heard USB speakers stutter? > > > > Think what might happen to your hardware if a USB driven servo/stepper > > 'stuttered'. > > Bob: > > USB speakers require LOTS more data, LOTS faster, than a CNC > machine does. Right. > USB speakers use "isochronous" USB transfers. Iso transfers > don't do error-correction; they're specifically for > streaming-type applications where "guaranteed" delivery of data > is more important than actual data integrity ("guaranteed" is in > quotes because it's only really guaranteed under a real-time > operating system, not under Windows). also right. > Also, iso support on the host (i.e., PC) side is EXTREMELY > difficult to implement; "bulk" or "interrupt" transfers are MUCH > easier. absolutely true. > A USB CNC controller would use bulk or interrupt transfers and it > would only need to transfer a relatively small amount of data > once per millisecond or so. can't use bulk: bulk has _NO_ thruput guarantees on USB. interrupt is probably the way to go. > > I think you need to have enough autonomous intelligence to handle the > > inevitable 'misses' from the PC end, especially when running any > > flavor of Windoze. > You're absolutely correct; Windows DOES have problems with > real-time operations. The easiest way to show this is to just > click the Windows Start button while doing a USB loopback test > or something; the USB traffic may pause for a few SECONDS while > the Start Menu is drawn. what kind of USB traffic are we talking about here??!?? > However, there ARE other operating systems, and USB stacks are > available even for DOS. I don't think it'd be hard to build > a special-purpose CNC controller that used USB as the interface > between the PC and the machine. yeah... but if you're gonna use DOS ... Bob Ammerman RAm Systems (contract development of high performance, high function, low-level software) -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.