I'm sorry but I still don't get it. And sorry for the -20/-30 mistake. > >>> Hello, > >>> > >>> I read the paper "RTnet -- A Flexible Hard Real-Time Networking > >> Framework" at > http://www.rts.uni-hannover.de/rtnet/download/RTnet-ETFA05.pdf and I > >> have a question concerning chapter 3.2, formula 3. > >>> T_slot = T_sched + t_slot - t_offset > >>> > >>> As far as I understand this formula, the slaves calculate their own > >> starting point T_slot from T_sched (the planned transmission time of > master), > >> t_slot (slot duration?) and t_offset (offset between master and slave > >> clock). To my mind, this doesn't makes any sence. Maybe I misunderstand > one of > >> the variables, but let me give a small example: > >>> T_sched = 15 (absolute time point) > >>> t_slot = 10 (duration) > >>> t_offset = 20 > >>> > >>> Then, slave's starting point would be 15+10-20=5 > >>> But how is it possible, if the master is the first one sending? I > don't > >> understand this formula. > >> > >> Oops, you stumbled over an impreciseness of the variables in that > paper. > >> They must read T_sched,master and T_slot,slave, i.e. the first one is > >> related to the master view of time while the second one was transformed > >> to the slave time base. > >> > >> Jan > >> > > But still, this doesn't makes sense to me. As I understand the paper, > formula 2 and 3 should fix potential scheduling jitters (the time difference > between scheduled and "real" sending time on the master). But how can this > be fixed, if the formula deals with the T_master_sched and not the > T_master_real (=T_master_xmit)? > > > > > > Let me give an example: > > at the same point of time (the moment the master sends sth), master > clock=10, slave clock=30 > > the master jitter may be 2, which means that T_master_sched=10 and > T_master_xmit=12 > > t_travel may be 3 > > t_slot (Slot-Length) may be 5 > > > > t_offset=12+3-35=-30 > > Hmm, my calculator prints "-20"... ;) > Okay, I agree, sorry for the mistake.
> > > > T_slave_slot=10+5+30=45 > > And that would give 35 then, which is precisely the expected value (5 us > >from the scheduled cycle start 30 us in slave time). 8) > But if slave starts sending at 35 (which means 15 from master's point of view) then it would send at the same time the master does. The slot is 5, so if master would start exactly at 10, this would work. But because of the jitter, it starts sending at 12, so if it uses his whole 5 ticks long slot, it would end at 17 and this would clash with slave, which starts sending already at 15. > > > > To my mind, this is the _scheduled_ slave slot time (the time, the slave > would send, if there wouldn't be any master's jitter), seen from slave's > point of view. > > > > But to respect the master's jitter, the formula to calculate the _real_ > slave slot time should be: > > T_slave_slot = T_master_xmit + t_slot - t_offset > > which would result in: > > T_slave_slot=12+5+30=47 > > But that would be a clear regression (apart from -30/-20 issue): You > would neglect the master jitter, ie. the fact the the cycle _should_ > have been started at 10, not 12. > So if I use this formula, I get 37 as a result, which is from master's point of view 17 and therefore right. > > > > > > Do I understand sth wrong or would you agree with that? > > > > Thanks > > Karl > > Jan > I know that the algorithm of course is right because it's working but I still don't understand this formula. So sorry for that maybe stupid questions but give me some hints please ... Karl -- von Karl Reichert "Feel free" - 10 GB Mailbox, 100 FreeSMS/Monat ... Jetzt GMX TopMail testen: http://www.gmx.net/de/go/topmail ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ RTnet-users mailing list RTnet-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/rtnet-users
