[Etherlab-users] Configuring EL7047 stepper driver

[Gavin Lambert]

On 3 August 2021 6:07 pm, quoth Fontana Nicola:
> This, although a bit mind twisting, is functionally equivalent to the current
> one, i.e. both `ecrt_slave_config_pdos` and `autoregister_pdos` are called
> for all modules but the EK1100.

You're correct of course; I guess my brain was skidding over the return value comparison.

> Yes, I'm not interested in storing byte or bit offsets. With "hoping"
> you mean that libethercat can (at least theoretically) leave byte holes inside
> the process image? That would certainly create issues here, as I am implying
> everything is packed.
> 
> > (Which, to be fair, it always should -- until you move devices around
> > in your network or upgrade a device to a later revision with a different data
> > model.
> > What you have works for now, but it's brittle.  But if you're happy
> > with that caveat, then fine.)
> 
> Can you expand a bit here? If the devices can be moved around I would use
> aliases and if a new device cannot behave like the old one, that would
> require some code rewrite. But I think those concerns are not related to the
> fact I am "autoregistering" the PDO entries.

API-wise, the library is only guaranteeing that the PDOs that you register appear *somewhere* in the domain memory (specifically: at the offset/pointer returned).  (Technically, you only actually need to register one PDO from each SM to guarantee that the entire SM will appear -- registering every PDO is only confirming that the values also appeared in your ecrt_slave_config_pdos call; since you're iterating the same table in both cases it's a little pointless, especially if you're not saving the offsets.)

EtherCAT itself requires that all PDOs within a single SM will always appear "packed" in consecutive bytes, matching the declared SM layout.  It has no particular constraints between different SMs (although some slaves may misbehave if their own SMs are split between different packets, although that is supposed to be legal in general.)  But a particular master is entirely free to order the SMs from different slaves however it likes, or to (for example) insert padding between SMs to ensure that each one aligns on a convenient word/dword boundary, or to group RxPDOs and TxPDOs differently (or overlap them), or to request additional SMs you didn't ask for, although usually the preference is to minimize packet size.

From practical considerations, because ecrt_slave_config_reg_pdo_entry returns an offset (that cannot later be changed without restarting the configuration from scratch) and Etherlab doesn't do any alignment or extra background data (and doesn't overlap unless specifically requested), *with current implementation* you'll always end up with the domain memory being ordered with each slave's SM according to when it was first referred to by ecrt_slave_config_reg_pdo_entry and with no padding between.  Doing something different is only a theoretical possibility.

But it does mean that your process_image_t is tied to having exactly those slaves in exactly that order (registered order, not necessarily network order), making it brittle to changes in your design or wanting to re-use implementation for another project.  My point was that for more flexibility, you could make a per-slave "process_image_t" rather than a global one -- combined with actually using the offset/pointer for the first PDO in each SM, you can make each slave a reusable component that you could repeat as many times as needed for each project, rather than a one-off.  And this would also make you immune to SMs being ordered differently than you originally expected, in some theoretical future change.


Gavin Lambert
Senior Software Developer TOMRA Fresh Food

 


COMPAC SORTING EQUIPMENT LTD | 4 Henderson Pl | Onehunga | Auckland 1061 | New Zealand
Switchboard: +49 2630 96520 | https://www.tomra.com

The information contained in this communication and any attachment is confidential and may be legally privileged. It should only be read by the person(s) to whom it is addressed. If you have received this communication in error, please notify the sender and delete the communication.