[etherlab-users] domain process data persistence

[Florian Pose]

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Am 08.10.2012 16:42, schrieb Thomas Bitsky, Jr.:
> What you're proposing would be a mistake. The value coming back is
> the value of that object in the field. You send out a value, the
> slave reads it, then puts in the value that is actually there. If
> it just took your value and never did anything with it, you'd be
> blind to what is going on out in the field. The fieldbus isn't your
> personal memory buffer; it's feedback of what is going on in the
> real world.

That's not true. A slave will *never* change RxPDO-only data in the
frame. For pure RxPDOs (master -> slave) you will receive exactly the
data you were sending.

An exception is, if the PDOs are configured to overlap each other
(implemented in the default branch). In this case the slave will read
the RxPDOs from the frame and after that write its TxPDOs to the same
place, to save bandwidth. But this is not the point here, as I understood.

The *one and only* method to check if the slave accepted the data from
the frame (and/or wrote valid data into the frame) is the working
counter, which you can access via ecrt_domain_state()!

> Example: I turn on an output to an EL2004 output slave. When that
> value comes back to me, that bit is either still on, which means
> that the slave turned it on in the real world, or it's off, which
> means that the slave didn't turn on the output for some reason.
> But, now I know that and can do something about it. If it just left
> the value at 1, I'd have no idea that it hadn't turned on and my
> process would continue on unaware.

This is definitely wrong.

- -- 
Viele Grüße,
Florian Pose

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