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robert-burger / libethercat

EtherCAT master library. This library is used to build a deterministic fieldbus network with EtherCAT components.
https://www.dlr.de/rm
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Recommended way to access process data? #7

Closed marcfir closed 4 months ago

marcfir commented 4 months ago

How do you normally read and write the input data? I think sync in the cyclic_task or async with a double or triple buffer are both possible?

If I am correct, the process data memory is defined in the pd_group https://github.com/robert-burger/libethercat/blob/d9c124be9059a71cc81b2941cf2ec3a9b1736b1a/include/libethercat/ec.h#L91 You can access the pd via the pd_groups https://github.com/robert-burger/libethercat/blob/d9c124be9059a71cc81b2941cf2ec3a9b1736b1a/include/libethercat/ec.h#L213 or via the pointers in the slave structures https://github.com/robert-burger/libethercat/blob/d9c124be9059a71cc81b2941cf2ec3a9b1736b1a/include/libethercat/slave.h#L276-L292

Access to pd is secured by a lock https://github.com/robert-burger/libethercat/blob/d9c124be9059a71cc81b2941cf2ec3a9b1736b1a/src/ec.c#L1434-L1449 https://github.com/robert-burger/libethercat/blob/d9c124be9059a71cc81b2941cf2ec3a9b1736b1a/src/ec.c#L1547-L1588

robert-burger commented 4 months ago

How do you normally read and write the input data? I think sync in the cyclic_task

Yes, that's one option. Or you can register a callback function which gets called when the cyclic group lrw frame has returned. https://github.com/robert-burger/libethercat/blob/d9c124be9059a71cc81b2941cf2ec3a9b1736b1a/src/ec.c#L1452

or async with a double or triple buffer are both possible?

But the you need to do some work which is not included a.t.m. (implement triple-buffer, copy data in cyclic_task to/from triple buffer).

If I am correct, the process data memory is defined in the pd_group

https://github.com/robert-burger/libethercat/blob/d9c124be9059a71cc81b2941cf2ec3a9b1736b1a/include/libethercat/ec.h#L91

You can access the pd via the pd_groups or via the pointers in the slave structures

correct.

Access to pd is secured by a lock

https://github.com/robert-burger/libethercat/blob/d9c124be9059a71cc81b2941cf2ec3a9b1736b1a/src/ec.c#L1434-L1449

https://github.com/robert-burger/libethercat/blob/d9c124be9059a71cc81b2941cf2ec3a9b1736b1a/src/ec.c#L1547-L1588

No, that's only a lock for the cyclic group datagram. The process data buffers in the groups have no lock because i always make synchronous access via the above mentioned callbacks. At DLR we have a bigger hardware abstraction framework running where these are indeed copied to a triple-buffer.

marcfir commented 4 months ago

Thanks for the clarification. That makes sense.