Stephen Wille Padnos wrote: > Jon Elson wrote: >>As long as you could throttle traffic on that ethernet segment, >>so a network file transfer, for instance, couldn't bog down the >>ethernet, then that would work. >> > > No, it wouldn't (not necessarily). > You'd also have to set the MTU pretty low. A single UDP packet of > 12.5kBytes takes 1ms to transmit on the wire. (12.5k * 8 bits = 100 > kbits = 1/1000 of 100 mbits/sec = 1msec) Even if you throttle large > transfers, and you can somehow convince multiple hosts on the segment to > all share the same "throttled pool" of bandwidth, a single large packet > can throw a wrench in the works. > 12.5 K bytes? I've never run a system with an MTU greater than 1536, I thought that was some generally agreed standard. My current system has it at 1500. 1536 * 8 bits = 12288, at 100 MBit/sec that would take 122 us plus overhead. Better not let more than one of those packets through per millisecond. That wouldn't really clobber the bandwidth for net transfers, though, you'd still get 1.5 MBytes/second throughput. There would not be multiple hosts, just the CNC machine and whatever was acting as a router for it. If you hooked a bunch of CNC control machines and their ethernet devices all on one segment, then you'd be asking for trouble. > >> But, I have no idea what would >>be involved in making the ethernet port driver and network stack >>RT compatible. Some time ago the RT stuff didn't do anything >>well except regularly-schenduled tasks, I think that is no >>longar a problem. But, the net driver would have to respond to >>interrupts whenever a packet came in. >> >> > > I think the base for interrupt-response threads is there - the current > HAL thread mechanism uses it to grab the timer interrupt. ADEOS can > route multiple interrupts (and multiple interrupt domains), and I'm > pretty sure RTAI also supports this. It's just not part of RTAPI or HAL. Yes, and I have no idea how hard it would be to put all that in. HAL might not need to know about it at all, but it seems RTAPI would. Now, maybe you could just start up the rtnet directly when the system boots, and let the drivers access rtnet without anybody else knowing its there. I certainly don't know all the implications, it would take some study!
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