[etherlab-users] Knowing when the packet has finished cycle

[Richard Hacker]

Hi,

I am not sure why you want to go through all this trouble. Of coarse, if your 
calculation is so long that there is no time to exchange IO, you have trouble 
looming anyway!

So what do you want to do with the data if you receive it in the same cycle 
instead of waiting till your next call? For me, there is no point of busily 
waiting till your packet arrives, instead of being relaxed and receiving the 
packet next cycle.

The normal run of a control program is:
calculate; exchange io; wait; calculate; exchange io; wait; etc.
where exchange io means: write output and get new inputs. Master receive and 
domain process simply fetches and processes the data that was transmitted with 
master_send at the end of you pseudo code examples.

Now, exchange io is done in the background by the network card. This means, 
that you could call exchange io right at the start of your cycle and 
subsequently calculate. In this case your calculation and exchange io runs in 
parallel. This is useful when your calculation is long and you have a lot of 
data to exchange, i.e.
exchange io; calculate; wait; exchange io; calculate wait; etc.

The drawback is that your propagation time from input change to output 
reaction is 2 cycles, instead of only 1. That is the price to pay if you have 
lots of data and a long calculation - there is no free lunch!!!

I do not think that you have a problem. Draw your operations on a time line 
and convince yourself that once you are in in the loop, you have max 1 cycle 
delay from input to output. If that is too long for you, then ethercat is not 
your solution. Then you need direct IO like that of microcontrollers and the 
like.

- Richard

On Thursday 13 October 2011 17:12:46 Shahbaz Yousefi wrote:
> Hi,
> 
> I have been working with etherlab recently and got ethercat working up and
> everything is fine.
> 
> There is a delay issue however that I'm concerned about. As seen in the
> examples, the way you read from the network is like this (imagine you are
> interested in reading sensor values):
> 
> while (running)
> {
>   master receive
>   domain process
> 
>   read data
> 
>   domain queue
>   master send
> 
>   rt wait period
> }
> 
> in which case you assume that the task period is long enough to be sure
>  that the packet sent in the bottom of the loop has returned when the loop
>  starts again and so you can receive the data.
> 
> However, I was wondering if it is possible to, instead of taking an upper
> bound of the time, simply check to see whether the data has arrived or not.
> After some research, I got to this code:
> 
> while (running)
> {
>   domain queue
>   master send
> 
>   do
>   {
>     sleep a little
> 
>     master receive
>     domain process
> 
>     ecrt_domain_state(domain, &state);
> 
>   } while (state.wc_state != EC_WC_COMPLETE && !timeout)
> 
>   read data
> 
>   rt wait period
> }
> 
> This way, after sending the packet, you would read the data as soon as they
> arrive.
> 
> The problem with this was that, besides the fact that early calls to
> master_receive (or domain_process) generated a huge amount of warning about
> working counter changing to 0/1 and back to 1/1 again, the kernel started
>  to at some point keep crashing.
> 
> I would like to know, how can I detect when the packet has arrived without
> knowing an upper bound about it and wait blindly by that much?
> 
> Note: This is most useful for me for this reason:
> 
> I may have different threads requesting data from a domain which includes
> different sensors. Each type of sensor produces data at a different rate
>  and I would like to read the data at different rates to. I don't want to
>  (and I don't think is even possible) to have different threads requesting
>  data from the same domain (because they may send the packet while the one
>  for the previous thread hasn't yet arrived). So what I want to is this:
> 
> ethercat coordinator:
> 
> domain_updating = no
> 
> send_request_for_domain
> {
>   if (domain_updating == no)
>   {
>     domain_updating = yes
>     domain queue
>     master send
>   }
>   // else, do nothing, it is being updated!
> }
> 
> receive_from_domain()
> {
>   while (ecrt_domain_data_not_yet_received) // this is the function I need
>     wait
>   domain_updating = no
>   // data available
> }
> 
> and then, each thread that wants something from the domain would look like
> this:
> 
> thread:
>   send_request_for_domain
>   receive_from_domain
>   read data
> 
> This way, if two threads call send_request_for_domain at the same time,
>  only one of them would actually do the
> 
> domain queue
> master send
> 
> and both of them use the result.
> 
> I would appreciate it if you could shed some light on this matter.
> Shahbaz
> 

Mit freundlichem Gruß

Richard Hacker

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Richard Hacker M.Sc. 
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