[etherlab-users] DC sync and arrival times of datagrams

[Jeroen Proveniers]

Hi,

A question to help me understand Ethercat a bit better, as I'm only a newbie to it;

If I connect a scope to an ET1100 SYNC0 output (configured for a cycle time of 200us) and to an output from an MCU that indicates the Syncmanager has just received new cyclic data, should I expect the two pulse trains to be exactly in sync, apart from some jitter (due to reading the SM interrupt status through the SPI PDI) and a fixed phase offset?

I just did such test today and unfortunately it seems the Ethercat frame rate is just a tad slower or faster then the sync frequency; on the scope one trace slowly rolls away instead of the two rock stable traces I expected.

In this application we use the sync pulse to latch encoder positions and set motor setpoints. The interrupt is, as expected, use to retrieve and send process data from the ESC when it becomes available.

I cannot synchronise to both the sync pulse and the SM interrupt. Whichever one I choose means once in a while a frame is either duplicated or one is missed; not to mention the constantly changing phase relationship.

Can I safely assume something is not quite right on the master side? 

I know nothing about the master side right now; a coworker is in charge of that part, but he mentioned 'fixing timing issues' a few times in the past.

I do not yet have any numbers on the actual frame rate; my estimate is that it sends about 2 to 5 frames too many or too few per second. 

Cheers,
Jeroen
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