> I wish real-time ethernet solutions weren't still "on the way."
Several already exist. SERCOS III is one. Designed for stuff like motor control. http://en.wikipedia.org/wiki/SERCOS_III We found the needing two ports + proprietary hardware chip to support it fairly limiting so we crafted our own... If you throw out legacy 10bT & half duplex links (I mean, who DOESNT have 100 or 1000Mbit ethernet now a days?) there are no collisions and their exponential backoff problems to deal with. So now its just point to point latency issues. Very low w/ a typ star network hub. Regular store and forward hubs @ 100Mbit are pretty good. An example is a sixnet unmanaged switch = 5us typ delay. Thats negligible compared to the 2x1.5kB(max packet)/100Mb = ~240us latency. If you have 6-axis of velocity commands to servos * 32bit ea = 24bytes to broadcast. Even the min enet packet of 64bytes gives you 46bytes of payload. 64B/100Mbit=5.12uS. 2x that (for orig control + switch retransmission) + 5us store&forward latency = 15uS. Thats 60kHz outbound command broadcast throughput. Now, not all 6 controls can talk back at the same rate (due to min enet pack size, there would be 6x as much data coming back as out!) But in reality you need less information & less often from ea drive than to it. We haven't done final testing yet but 10kHz+ control loops seem easily doable w/ small packets while still supporting full common web stuff on an ARM cortex part. One can even use VLAN tagging to prioritize local traffic so web traffic running off the same controller/LAN doesn't drown out the control stuff during an internet attack (we haven't had to resort to that yet.) We're running under the TCP/etc layers in the stack, sending straight-old plain vanilla ethernet packets. Simple. Fast. Cheap. *STANDARD*. IEEE1588 time sync easily comes along for the ride too! Note, this is not for linux (or CNC) but RT control of some power electronics. Ethernet, to us, seemed like the no brainer solution. Its cheap, robust, and ubiquitous. All it takes is a little tinkering w/ the protocol stack. Note: this approach DOESN'T work over large WANs necessarily. But thats really not a limitation in our systems where there is currently always a local pc generating the stepper or servo signals. You can still remote into that. 'Local' RT ethernet is pretty easy to implement if you forgo the 'solution for everyone' problem the others are trying to solve. ------------------------------------------------------------------------------ Everyone hates slow websites. So do we. Make your web apps faster with AppDynamics Download AppDynamics Lite for free today: http://ad.doubleclick.net/clk;258768047;13503038;j? http://info.appdynamics.com/FreeJavaPerformanceDownload.html _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
