Hello. I agree, it is a shame for those patent holder to behave that way. Sure they don't even understand the way they are killing their own future business. Even more incredible is the weakness of the invention claimed by those patents.
I also admit that Powerlink is far from being the smartest and more efficient real time Ethernet implementation, but it have the advantage to only require standard Ethernet controllers for both Masters and Slaves. Please, now, let's keep those problems out of that discussion, and focus on the only Ethernet based fieldbus we can implement freely without taking risk to be censored. Edouard 2009/10/8 Anders Blomdell <[email protected]>: > Peter Puchwein wrote: >> Edouard TISSERANT wrote: >>> Dear hackers, >>> >>> I've been passing last month seeking for the best way to implement >>> Powerlink with Xenomai. I still didn't find the absolute truth, and >>> would like to confront my point of view. >>> >>> In short, Powerlink is a kind of CANopen over polled ethernet. >>> Powerlink re-use a subset of CANopen concepts for high level >>> communications, whereas on the low level, a single master cyclically >>> poll each node for data. As you can imagine, communication cycle is >>> seriously influenced by each controlled node latency answering poll >>> requests from master. >>> >>> My intention is to write an efficient Powerlink implementation for >>> Xenomai, but suitable for later reuse in future Real Time Linux >>> implementation such as RT-PREEMPT. Discussing with RT-PREEMT gurus, I >>> understood that re-using native Linux queues could never be a >>> solution, as those queues induce way too long and unpredictable >>> latencies for such real-time ethernet (those latencies also apply for >>> SocketCan). Some software hook at ethernet driver level will be >>> necessary to reroute RT-related packets directly to the concerned RT >>> stack. With this approach, the whole stack would have to stay kernel >>> side to reduce latency in answering poll requests. Implementing the >>> whole CANopen stack kernel side is generally a bad idea, as it >>> complicate access to object dictionary from application, usually in >>> user space. >>> >>> RTnet proves that Real-Time ethernet can be implemented through the >>> socket paradigm. I believe that Time Division Medium Access (TDMA) of >>> RTnet could be interchanged with polled ethernet medium access >>> discipline from Powerlink. Thus, CANopen implementation part could >>> stay user side, and could pre-fill answers to poll requests, avoiding >>> long latencies. But what is the future of RTnet ? Will it be ported to >>> RT-PREEMPT or will it enter official Xenomai tree ? Will available >>> RTnet's drivers set continue to grow ? Would it be theoretically >>> possible for RTnet to support Linux ethernet drivers through a hook >>> mechanism as mentioned in previous paragraph ? >>> >>> My feeling is that, yes, we should try to fit implementation of >>> Powerlink data link layer inside RTnet as another MAC discipline, and >>> use a modified version of CanFestival for CANopen aspects, keeping it >>> a user space library. Please comment. >>> >>> Best regards, >>> >>> Edouard >>> >> >> >> Hi Edouard, >> >> we are successfully using the EtherCAT-Master (www.etherlab.org) with >> xenomai. We've choosen EtherCAT because of the high availability of >> components. >> With the features CoE (Can over Ethercat) and EoE (Ethernet over >> Ethercat) you have a lot of possibilities. >> Most common network drivers are already implemented for use with >> realtime implementations like Xenomai. > Beware that Beckhoff does not allow open-source implementations of the > EtherCAT > stack, they require you to sign a "shared source license" that prohibits > anybody > to use the code for any other purpose than a "standard compliant EtherCAT > master". We scrapped all our EtherCAT development (which was demonstrated at > JavaOne 2007) because of that very prohibitive license. > > /Anders > > -- > Anders Blomdell Email: [email protected] > Department of Automatic Control > Lund University Phone: +46 46 222 4625 > P.O. Box 118 Fax: +46 46 138118 > SE-221 00 Lund, Sweden > -- LOLITech 24, rue Pierre Evrat 88100 Saint-Dié des Vosges tel: +33 3 29 57 60 42 mob: +33 6 10 49 76 41 [email protected] http://www.lolitech.fr _______________________________________________ Xenomai-help mailing list [email protected] https://mail.gna.org/listinfo/xenomai-help
