Concerning remote I/O over Ethernet :
Beckhoff has also come up with a new Ethernet based fieldbus system called EtherCAT (www.ethercat.org), it is much faster than the Modbus TCP (60 ms cycle time, I read once, too slow for most control applications). You can not have a control loop running over Modbus TCP, with EtherCAT this is possible.
EtherCAT is basically not more than embedding "EtherCAT telegrams" in standard Ethernet packets (at least from the master viewpoint).
I think it is possible to write some code that supports the EtherCAT protocol stack and run this with the real time Ethernet drivers of rtnet. Actually you could do the same for the "Ethernet Powerlink" protocol(B&R company). Both protocols are supported by a lot of manufacturers. This is probably much more realistic way to have remote I/O over Ethernet using rtnet stuff, than asking manufacturers to support the rtnet protocols.
Yes, we are of course very aware of those solutions and their technical backgrounds as far as information is publically available. Unfortunately, both decided to found some "open" specification groups you have to join first to get the specs (CANopen is a better example in this respect). Furthermore, it is still not totally clear if these consortiums may accept free implementations of their stacks (some members are selling stacks and drivers...) - but I didn't studied the conditions for becoming a member in every detail.
Technically, I'm quite confident that it is possible. Powerlink likely not at the speed they achieve by using dedicated controllers. And Ethercat should be even easier - Beckhoff actually plays the classic RTAI/RTLinux game on the master, but unfortunately uses Windows with their own real-time extension (TwinCAT). I guess the day will come when we can add an Ethercat protocol module to RTnet, maybe also Powerlink as an alternative RTmac discipline (it seems to be much like our TDMA). Driving, e.g., Powerlink slaves by an RTnet master at moderate but still hard-real-time update rates should be feasible.
We spoke to both vendors several times without getting concrete responses. E.g., early in 2002 I offered B&R to implement Powerlink for free(!) as media access control protocol before Marc started his own implementation. Today I think it was not that bad they didn't reacted, because our framework appears to be more flexible and is less remote-I/O centred than most other protocols. I'm not that sure, for instance, if it is possible to stack middlewares like RTPS or RT-CORBA on top of those approaches. With RTnet - no big deal (see ORTE project for RTPS).
Neither my institute nor the cooperating Linux companies have so much to give away for free. There has to be someone (not necessarily Beckhoff or B&R themselves) supporting the work - and I'm convinced there will be someone...
Jan
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