Karl Reichert wrote: > Jan Kiszka wrote: >> Karl Reichert wrote: >>> Hello, >>> >>> I have again a question concerning the master's jitter - the >>> difference between scheduled and real transmission time. Just to make >>> sure I understood it right ... >>> >>> The offset between global and local clock is calculated without >>> respect to master's jitter. Same with transmission time. So we only >>> need to respect master's jitter if we are calculating a time relative >>> to a Synchronisation frame, right?! >> Nope. RTnet implements an atomic transmission time stamp that allows to >> compensate most of the sender's software-related TX jitter, and this >> information is applied to all calculations. It would make no sense to >> restrict its usage only on internal frame timing. >> >> Jan > > But where do I need it then (except for maybe backup masters)? As you told me > before, the offset between TDMA slots has to be chosen big enough to > compensate also master's jitter. So why do we need to know then master's > jitter (if we forget about backup master for a moment)?
Two reasons: - Avoid accumulated jitters. If the master is late + some slave is late on the already delayed cycle, you end up with even higher jitters on the wire (critical if you have slots at the end of the cycle). - The synchronised global time may be used for much more than TDMA. If you use it to time-stamp external events on distributed nodes, you want to reduce the jitter as much as feasible. Jan
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