[etherlab-users] waiting for received frames

Ian Prochazka jprochazka at persimmontech.com
Thu Nov 21 17:38:19 CET 2013

Sometime the issue is that etherCAT master loop does not need and should 
not be running as tight as possible, because the computer needs to do 
also some other tasks. But at the same time, the master needs to react 
to slave data as quickly as possible. So master initiates transaction 
with fake/not valid transmitted data in order to get data from slaves. 
And just here it would be *very* useful to have notification when data 
are ready to be read from master, so the master (or rather the 
application above the master) could read just received data and react to 
them immediately and *only then* go for a longer EtherCAT loop period sleep.


On 11/21/2013 04:25, Martin Troxler wrote:
> Hi
> I watched that discussion about the need for a API to wait (poll) for
> received frames. I still don't get it why that is nessessary.
> Here is a clarification of how ethercat (process data transfer) works:
> - Process Data should be regarded as idempotent states, which means it
> doesn't matter how many times you read it.
> - it's a kind of huge shift register from the master through all slaves
> and back to the master where each slave (ESC-Chip) manipulates the
> stream of data on the fly.
> - Slaves (i.e. the firmware on microcontrollers) cannot delay the
> transmission of the frame. If there is no new data the ESC simply
> transfers the actual content of its local memory.
> - The frame transmission time is fixed for a given set of slaves and
> depends largely on the domain size and not on how fast the slave's
> microcontroller can update the data.
> Regards
> Martin
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