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.
Ian
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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