John L. Craddock wrote: > > The most common way of establishing real time determinism is to eliminate > the collisions inherent in ethernet. This is done by establising a master / > slave strategy with the master calling the shots in time-slots (TDMA). This > requires an accurate distributed clock arrangement with a synchronising > procedure. It is commonly referred to as isochronous transmission. The > introduction of high resolution timers in Linux opened the way to achieving > this strategy. The open source implementation is at www.rtnet.org . It works > on fast ethernet (100mbps) and gigabit ethernet; so the possibilities already > are in the underlying structure supporting EMC2 in RTAI, also Xenomai. > The difficulty at present is that the system only works between a linux > master and a linux slave whereas the commercial and other so-called > open-source implementations have embeded the slave into asics or fpgas where > the IP cores are proprietary. > However, for the motion control application, this is not really needed, as there is ONE master and one or several slaves. But, the slaves would never initiate a transfer. If the master were set to address each slave and wait for the response before addressing another slave, there would be no possibility of collision, making the protocol much simpler. This might require an FPGA ethernet stack which could respond within microseconds, instead of a micro which might take hundreds of us to prepare a packet and send it.
It has been suggested that a lower-level component of the rtnet package could be used to manage the ethernet mostly as a multi-point serial port, with really minimal overhead. I have no idea how complicated that might be. Jon ------------------------------------------------------------------------------ The ultimate all-in-one performance toolkit: Intel(R) Parallel Studio XE: Pinpoint memory and threading errors before they happen. Find and fix more than 250 security defects in the development cycle. Locate bottlenecks in serial and parallel code that limit performance. http://p.sf.net/sfu/intel-dev2devfeb _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
