Peter, Thanks for that exceptional explanation... heck I even understand it now :)
John On 7 Jan 2010 at 7:41, Peter C. Wallace wrote: > On Thu, 7 Jan 2010, John Thornton wrote: > > > Date: Thu, 07 Jan 2010 06:21:26 -0600 > > From: John Thornton <bjt...@gmail.com> > > Reply-To: "Enhanced Machine Controller (EMC)" > > <emc-users@lists.sourceforge.net> > > To: Enhanced Machine Controller EEMC" > <emc-users@lists.sourceforge.net> > > Subject: Re: [Emc-users] Mesa 5i20 5i23 > > > > What is DMA? > > > > John > > > Direct Memory Access. > > In the case of the FPGA cards what it basically means is that the > card itself > transfers its data (encoder counts, current step, I/O pin status, > PWM outputs, > step rates) to or from the host CPUs memory on its own rather than > the host > CPU doing a series of programmed access cycles (this is called > Programmed I/O) > > The advantage of DMA over Programmed I/O is that with PCI, theres > quite a bit > of overhead in single accesses, while burst accesses are much faster > (burst > accesses being a block of transfers grouped together) DMA uses burst > accesses > so it can be about 20 times faster. > > Practically this is not of much use at 1 KHz servo rates with normal > 3-5 axis > CNC machines, however there are places where a 1 KHz servo rate is > no where > fast enough. This has to do with the mechanical bandwidth of the > system > controlled by the servo. A rule of thumb is that the sample rate > should be > 10 to 30 times the system bandwidth. A large CNC machine with ball > screws > may have a bandwidth of 50 - 100 Hz, so 1 KHz sample rate is fine. > A small system with a linear motor may have a 350 Hz bandwidth. Here > you may > want a 4 - 8 KHz sample rate. At these high sample rates, without > DMA, the CPU > will be spending a significant portion of its time doing card access > cycles. > > The access time problem also comes up where many axis are used (we > had a > customer set up a system with 40 Axis recently) > > > > On 6 Jan 2010 at 14:21, Sebastian Kuzminsky wrote: > > > >> John Thornton wrote: > >>> The 5i23 will not gain you anything for use with EMC. > >> > >> There is currently nothing to be gained by using the 5i23 over > the > >> 5i20, > >> but there are two possible future advantages that may become > real > >> some > >> time. One is that the 5i23 has a 400 Kgate FPGA (vs the 200 > Kgate > >> FPGA > >> on the 5i20), the other is that the 5i23's PCI chipset can > support > >> DMA. > >> > >> There are currently no hostmot2 firmwares that don't fit in 200 > >> Kgates, > >> and the EMC2 hostmot2 driver does not currently use DMA. But > who > >> knows > >> that 2010 will bring? > >> > >> > >> -- > >> Sebastian Kuzminsky > >> > >> > -------------------------------------------------------------------- > >> ---------- > >> This SF.Net email is sponsored by the Verizon Developer > Community > >> Take advantage of Verizon's best-in-class app development > support > >> A streamlined, 14 day to market process makes app distribution > fast > >> and easy > >> Join now and get one step closer to millions of Verizon > customers > >> http://p.sf.net/sfu/verizon-dev2dev > >> _______________________________________________ > >> Emc-users mailing list > >> Emc-users@lists.sourceforge.net > >> https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > > > > -------------------------------------------------------------------- > ---------- > > This SF.Net email is sponsored by the Verizon Developer > Community > > Take advantage of Verizon's best-in-class app development > support > > A streamlined, 14 day to market process makes app distribution > fast and easy > > Join now and get one step closer to millions of Verizon > customers > > http://p.sf.net/sfu/verizon-dev2dev > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > Peter Wallace > Mesa Electronics > > (\__/) > (='.'=) This is Bunny. Copy and paste bunny into your > (")_(") signature to help him gain world domination. > > > -------------------------------------------------------------------- > ---------- > This SF.Net email is sponsored by the Verizon Developer Community > Take advantage of Verizon's best-in-class app development support > A streamlined, 14 day to market process makes app distribution fast > and easy > Join now and get one step closer to millions of Verizon customers > http://p.sf.net/sfu/verizon-dev2dev > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ This SF.Net email is sponsored by the Verizon Developer Community Take advantage of Verizon's best-in-class app development support A streamlined, 14 day to market process makes app distribution fast and easy Join now and get one step closer to millions of Verizon customers http://p.sf.net/sfu/verizon-dev2dev _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
