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
