Distributed Clocks - Leuze electronic DDLS 538 S2 Series Original Operating Instructions

EtherCAT
7.5

Distributed Clocks

Overview
Spatially distributed processes are synchronized with respect to time using Distributed Clocks (DC).
The time of the first synchronous EtherCAT slave is used as the reference (master clock).
The reference time is sent cyclically to all other slave clocks. These can then adjust themselves exactly to
the reference time one after the next.
The time information is subject to propagation time delays in the EtherCAT network.
Causes of propagation time delays:
• The signal propagation time on the cable
• The signal propagation time through the participants
• The signal propagation time through infrastructure components such as an optical data transceiver
• Variable propagation time delay due to constantly changing transmission distances when using an opti-
cal data transmission system.
Synchronization via DC
• Each DC slave sends the exact time to the DC master at which it received the telegram.
• The DC master stores these times and allows a relative deviation of 2 µs to the measured propagation
time delay.
• EtherCAT is able to constantly recalculate and compensate for the propagation time delays.
The propagation time delay of the EtherCAT network is remeasured by the DC master every 10 s.
Example:
DC-capable slaves with time information can be networked with one another via an optical data transceiver.
One of the two data transmission devices is stationary, the other is installed, e.g., mobile on a high-bay
storage device.
• At a maximum expected speed of the high-bay storage device of 10 m/s, this will traverse a distance of
100 m within 10 s*.
(*: the propagation time delay is remeasured every 10 s)
• The resulting propagation time difference is approx. 660 ns and is, thus, below the permissible devia-
tion of 2 µs.
NOTICE
When cascading data transmission paths without DC-capable slave between the data transmis-
sion devices, the propagation time difference may increase further, but remains below the per-
missible deviation of 2 µs.
The DDLS 538 ... S2 ... is, thus, suitable for DC-synchronized applications.
Leuze electronic GmbH + Co. KG DDLS 538 ... S2 ... 59