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<p class="MsoNormal"><span lang="EN-GB" style="color:#1F497D">Hello Gavin,<br>
Thank you very much for your explanations. They made the things much more clearly to me!<br>
About the Sync0 phase between the slaves and my master, I forgot to tell that I’m using a shift time of -150 microseconds. My cycle time is 500 microseconds.<br>
As far as I understood the slaves should trigger the data exchange and start their own plc cycle 150 microseconds before my master cycle starts.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="color:#1F497D">When I read the input data at the beginning of my cycle I should get the slave data I/O from 150 microseconds before. Is it right?<br>
In contrast the dc slave reference clock time will be the time when the telegram has passed the reference clock slave (- transmission time). Is it right?<br>
Calling the “queue” commands will just trigger the desired operation for the next “send” command.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="color:#1F497D">So that means I have to compare the time I get from the reference clock with my master application time when I have called the send function. Ok?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="color:#1F497D">I’ll rework my calculation of the application time because I think it’s wrong.<br>
I’m using a servo drive in cyclic synchronous torque mode but very rarely I get a wrong actual position from the slave. This is happened if the jitter for starting my communication cycle is 15 microseconds higher than normal.<br>
Actually the jitter shouldn’t be so important because I request the data triggered by Sync0 150 microseconds before….<o:p></o:p></span></p>
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<br>
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="color:#1F497D">Have a nice weekend!<br>
Matthias<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-GB" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";mso-fareast-language:DE">Von:</span></b><span lang="EN-GB" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";mso-fareast-language:DE"> Gavin Lambert
[mailto:gavinl@compacsort.com] <br>
<b>Gesendet:</b> Freitag, 2. Februar 2018 00:22<br>
<b>An:</b> Matthias Bartsch; etherlab-users@etherlab.org<br>
<b>Betreff:</b> RE: Synchronizing the EtherCAT application time to the DC referen</span><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif";mso-fareast-language:DE">ce clock<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-NZ" style="mso-fareast-language:EN-NZ">Actually you typically shouldn’t have Sync0 simultaneous with your communication cycle; that causes problems. The goal is to get it into a locked phase arrangement.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-NZ" style="mso-fareast-language:EN-NZ"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-NZ" style="mso-fareast-language:EN-NZ">Sync0 is typically when the slave’s actions trigger – it asserts outputs and captures inputs ideally both at that precise instant (though there might be some delay if it needs to do
one before the other). However there is generally some setup time before Sync0 required (so you have to provide the next cycle’s outputs at least this amount of time before Sync0) and some transfer time after Sync0 required (so you have to wait that long
after Sync0 before you can read the inputs). The slave’s documentation should tell you how long each of these times are, and you need to allow a little bit of extra time to cope with jitter on the master’s end and the comms delay of the network itself. If
you have no clue, aiming for somewhere in the middle of your sync cycle is usually a fairly safe bet.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-NZ" style="mso-fareast-language:EN-NZ"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-NZ" style="mso-fareast-language:EN-NZ">The most important aspect of the EtherCAT comms cycle is when you call ecrt_master_send. This is what actually sends (and receives back) the datagrams and transfers all data to and
from the slaves. Your goal is always to make <b>this</b> call happen consistently with as little latency and jitter as possible.
<b>None</b> of the other calls matter in terms of timing.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-NZ" style="mso-fareast-language:EN-NZ"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_master_reference_clock_time retrieves the 32-bit time of the reference slave as of when you called ecrt_master_send, provided that you called ecrt_master_sync_slave_clocks at some previous
point (each cycle).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_master_64bit_reference_clock_time retrieves the 64-bit time of the reference slave as of when you called ecrt_master_send, provided that you called ecrt_master_64bit_reference_clock_time_queue
at some previous point (each cycle).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">(In principle doing both ecrt_master_64bit_reference_clock_time_queue and ecrt_master_sync_slave_clocks on every cycle is a bit wasteful. It would be better if you could just do a 64-bit sync,
but this is not supported at present. If you’re worried about bandwidth then you should call ecrt_master_sync_slave_clocks on every cycle and ecrt_master_64bit_reference_clock_time_queue only occasionally. You cannot omit calling ecrt_master_sync_slave_clocks.)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">For maximum consistency, you should call ecrt_master_application_time immediately prior to ecrt_master_send. You should also call it on each cycle, although in practice it matters most when it’s
doing the slave DC configuration, which will span several cycles across the slaves shortly after activating the master. Also rather than using a monotonically increasing value as it appears you’re using at the moment, I think it’s more typical to use the
actual PC clock time, corrected for the offset between the master clock and the reference clock. I’m not sure which is “better” though.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">(Also note that most things work best when you always use the first DC-capable slave as the reference clock, which is the default. If you’re explicitly designating a clock elsewhere on the network
then there might be complications.)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">If you are trying to synchronise the reference clock to the master (which you are not; it tends to be less accurate), you also have to call ecrt_master_sync_reference_clock between ecrt_master_application_time
and ecrt_master_send. It doesn’t matter when you call ecrt_master_sync_slave_clocks as long as you do it periodically (typically recommended once per cycle, unless you have a really fast cycle time). If you’re using ecrt_master_reference_clock_time then
you have to sync slave clocks at least as often as you ask for the time.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_master_receive can be called at any time after the packets arrive back, which will be shortly after ecrt_master_send (exactly how long depends on your network size) – but it’s most common
to use this time to do a proper idle sleep and process it at the start of the next cycle rather than the end of the previous one. That can cause higher jitter if your processing times aren’t consistent, however (unless you have two smaller sleeps rather than
one large one), so the way you’re doing it isn’t a bad one. You do have to call it before anything else that uses the results of the datagrams, such as the time calls above.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">If you can hook a scope up to a slave’s SYNC0 and SOF pins (if accessible), that will give you the best idea of how your timing cycle looks.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-NZ" style="mso-fareast-language:EN-NZ"> </span><span lang="EN-NZ"><o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="mso-fareast-language:EN-NZ">From:</span></b><span lang="EN-US" style="mso-fareast-language:EN-NZ"> Matthias Bartsch<br>
<b>Sent:</b> Friday, 2 February 2018 05:12<br>
<b>To:</b> <a href="mailto:etherlab-users@etherlab.org">etherlab-users@etherlab.org</a><br>
<b>Subject:</b> [etherlab-users] Synchronizing the EtherCAT application time to the DC reference clock<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-NZ"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:12.0pt">Hello everybody!<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span lang="EN-GB" style="font-size:12.0pt">I‘m using the unofficial patch set from
<a href="https://github.com/ribalda/ethercat">https://github.com/ribalda/ethercat</a> (2017-11-08).<br>
My RTAI communication cycle is synchronized to the DC slave reference clock (average jitter < 1µs). I need to extrapolate the position of servo drives to the beginning of my cycle.<br>
<br>
I’m not sure about the right use of the functions for reading the reference clock.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">I want to start my cycle at the time of the “Sync0” interrupt.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">My questions are: When is the time sampled that I get by calling
<br>
ecrt_master_64bit_reference_clock_time(m_poMaster, &ui64RefClockTime)); ?<br>
<br>
How I have to initialize my application time (first call of “ecrt_master_application_time(…)”)?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">My synchronisation seems to work but I’m not sure about the phase shift between the Sync0 event and the start of my communication cycle.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">My code looks something like this:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">In the first real time cycle I call:<br>
ecrt_master_application_time(m_poMaster, m_ui64AppTime_ns); // with m_ui64AppTime_ns=0<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_master_sync_slave_clocks(m_poMaster);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_domain_queue(m_po_domainInput);<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span lang="EN-GB" style="font-size:12.0pt">ecrt_master_send(m_poMaster);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">// busy wait 25µs for getting the answer<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_master_receive(m_poMaster);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_domain_process(m_po_domainInput);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">//<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span lang="EN-GB" style="font-size:12.0pt">// …. Later sending the output data<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">In the next cycles I do this:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_master_receive(m_poMaster);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_domain_process(m_po_domainOutput);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_master_64bit_reference_clock_time_queue(m_poMaster);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_domain_queue(m_po_domainInput);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_master_send(m_poMaster);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">// busy wait 25µs for getting the answer<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_master_receive(m_poMaster);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_domain_process(m_po_domainInput);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">uint64_t ui64RefClockTime;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_master_64bit_reference_clock_time(m_poMaster, &ui64RefClockTime));<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span lang="EN-GB" style="font-size:12.0pt">m_ui64AppTime_ns += static_cast<uint64_t>((m_io__dNominalCycleTime) * 1e9);<br>
ecrt_master_application_time(m_poMaster, m_ui64AppTime_ns);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">m_out_iDcSysTimeDifferenceMaster = m_ui64AppTime_ns - ui64RefClockTime; // Time drift value<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt">ecrt_master_sync_slave_clocks(m_poMaster);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="font-size:12.0pt;mso-fareast-language:DE">With kind regards<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-GB" style="font-size:12.0pt;mso-fareast-language:DE"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;mso-fareast-language:DE">Matthias Bartsch<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:12.0pt;text-autospace:none"><b><span lang="EN-GB" style="mso-fareast-language:DE">ARADEX AG</span></b><span lang="EN-GB" style="mso-fareast-language:DE">
</span><span lang="EN-US" style="font-family:"Arial","sans-serif";mso-fareast-language:DE"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
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