[etherlab-users] DC questions

Gavin Lambert gavinl at compacsort.com
Mon Jun 13 01:21:01 CEST 2016

Just to try to clarify a little: "DC" is actually two separate but related

The first is distributed time synchronisation, which ensures that the whole
network has an approximately synchronised clock.  This works in one of the
two ways that Graeme indicates in #1 below.  Most "infrastructure" slaves
(bus couplers etc) and "smart" slaves (those with CoE) have a DC clock and
support this kind of synchronisation, even if they don't support the next

The second feature is action synced to that clock -- typically a slave can
be set to capture inputs and/or generate outputs or just do its own internal
processing synched to a specific timestamp (and then repeated cyclicly).
This reduces jitter relative to the EtherCAT communications cycle.  Only a
smaller number of devices support this (typically those that require
precisely timed outputs or that provide input timestamps), but this is what
the ecrt_slave_config_dc() function is for -- all the other functions are
for the first feature instead.

The second feature requires the first, but the first can operate without the

(And of Graeme's examples below, 1a is the method used by the dc_user
sample.  There's some sample code for 1b floating around the newsgroup
archive somewhere.)

> -----Original Message-----
> From: etherlab-users [mailto:etherlab-users-bounces at etherlab.org] On
> Of Graeme Foot
> Sent: Monday, 13 June 2016 10:28
> To: Tommaso <furiosi.tommaso at gmail.com>; etherlab-users at etherlab.org
> Subject: Re: [etherlab-users] DC questions
> Hi,
> 1) Distributed clocks can work in a couple of ways, but all ways require a
> DC master which should be the first DC slave in the network.  Note: some
> slaves can act as a DC time master even though they are not fully DC
> a) EtherCAT master is the master clock:
>   - The computer is used as the DC master for the entire system.
>     ecrt_master_application_time() must be called every cycle to tell the
> EtherLab master what the current PC time is.
>   - Call ecrt_master_sync_reference_clock() to tell the slave DC master to
> to the EtherLab masters time.
>   - Call ecrt_master_sync_slave_clocks() to tell all other DC slaves to
sync to the
> slave DC master
> b) Slave DC master is the master clock. What I do is:
>   - Get the slave DC masters time using ecrt_master_reference_clock_time()
> and sync the EtherLab masters cycle and time to it
>   - Call ecrt_master_sync_slave_clocks() to tell all other DC slaves to
sync to the
> slave DC master
>   - Call ecrt_master_application_time() with the next cycles master time
> Note: With option b you need to adjust your masters PC's time by the drift
> from the slave DC master time and adjust your realtime cycle to suit.  I
do this
> by having a wrapper around the time calls to rt_get_time() and use
> rt_sleep_until() (I use RTAI) rather than using a fixed periodic cycle.
> Option b is the best, because option a has way too much jitter and the
> find it very hard to synchronise.  Note: option b is the default option
used via
> TwinCAT.
> 2) Yes, call ecrt_master_sync_slave_clocks() every cycle (and
> ecrt_master_application_time() and ecrt_master_reference_clock_time()).
> Just before the ecrt_master_send() call to reduce jitter.
> 3) No, only call ecrt_slave_config_dc() on slaves that support DC and are
> to be used with DC.
> Regards,
> Graeme.
> -----Original Message-----
> From: etherlab-users [mailto:etherlab-users-bounces at etherlab.org] On
> Of Tommaso
> Sent: Friday, 10 June 2016 7:14 p.m.
> To: etherlab-users at etherlab.org
> Subject: [etherlab-users] DC questions
> Good morning,
> In the master documentation is reported, in DC section, that the reference
> clock is the one of the first slave that supports this functionality. This
> can be synchronized with the master clock.
> My questions, based on the 'dc_user' example, are the following:
> 1 - calling cyclically the function 'ecrt_master_sync_reference_clock()'
> I have that the reference clock is the one of the master? Only for this
case is
> useful to call the function 'ecrt_master_application_time()' or I have to
call it
> every time I want to use the DC?
> 2 - the function 'ecrt_master_sync_slave_clocks()' is used for the
> synchronization of all the slave clocks for every reference clock? If I
want to
> exploit the DC functionality I have always to call it cyclically?
> 3 - it makes sense to call 'ecrt_slave_config_dc()' even for slaves which
do not
> support the DC functionality, like the EL2004?
> Thank you for your help.
> Best regards,
> Tommaso

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