Karl Reichert wrote: >>> 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"... ;) > > 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) > > 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. > > > Do I understand sth wrong or would you agree with that? > > Thanks > Karl Jan
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