Karl Reichert wrote: >>> 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. >> >> I think I see the misunderstanding: t_slot is not the slot size of the >> master slot here. It is the slot _offset_ (relative to the cycle start) >> of some arbitrary slave (or the master - then t_offset would just be 0). >> That offset must account for a) the time the preceding slot requires at >> least and b) the potential jitter of the preceding slot user. > Okay, I think this misunderstanding seems to be in formula 2, not 3. You say, > the offset must account also the jitter. But to my mind it doesn't. > Let's try an example with above numbers: > a) not jitter occurs: t_offset = 10 + 3 - 33 = -20 > b) jitter is 2: t_offset = 12 + 3 - 35 = -20 > T_slave_recv changes from 33 to 35, because of master's jitter of 2 the data > arrives 2 ticks later at slave. > So, as a result, the offset does not take any jitter into account.
Which is precisely the intention: Make the clock offset estimation _independent_ of the synchronisation signal sender jittery. Now you can use the _scheduled_ cycle beginning (based on the master clock) which comes along with the sync frame to calculate the _scheduled_ slot beginning of some slave (based on the respective local slave clock). Jan
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