BECKHOFF EtherCAT Technology Section I - Manual (Page 68)

Distributed Clocks
I-48 Slave Controller – Technology
Propagation Delay
The propagation delay between Reference Clock and slave clock has to be taken into account when
the System Time is distributed to the slaves.
Offset
The offset between local clock and Reference Clock has two reasons: the propagation delay from the
ESC holding the Reference Clock to the device with the slave clock, and initial differences of the local
times resulting from different times at which the ESCs have been powered up. This offset is
compensated locally in each slave.
The ESC holding the Reference Clock derives the System Time from its local time by adding a local
offset. This offset represents the difference between local time (started at power-up) and master time
(starting on January, 1
st 2000 at 0:00h).
Drift
Since Reference Clock and DC slaves are typically not sourced by the same clock source (e.g., a
quartz), their clock sources are subject to small deviations of the clock periods. The result is that one
clock is running slightly faster than the other one, their local clocks are drifting apart.
ESC Classification regarding DC Support
Three classes of ESCs are distinguished regarding Distributed Clocks support:
1. Slaves supporting System Time/Time Loop Control Unit:
Receive time stamps and System Time/Time Loop Control Unit available; SyncSignal generation,
LatchSignal time stamping, and SyncManager Event Times are optionally supported depending on
application.
2. Slaves supporting only propagation delay measurement:
Mandatory for ESCs with 3 or more ports (topology devices like EK1100 and ET1100). Local clock
and receive time stamps are supported.
3. Slaves without Distributed Clocks support:
Slaves with max. 2 ports do not have to support DC features. Processing/forwarding delay of such
slaves is treated like a “wire delay” by the surrounding DC capable slaves.