Glossary · System coordination, integration and orchestration
Distributed clock
Also known as: distributed clocks (EtherCAT), DC
German: Verteilte Uhr
In industrial communication, a distributed clock is a mechanism that keeps local clocks in several network devices synchronized to a common reference so that inputs can be sampled and outputs applied at the same instant; EtherCAT, for example, uses distributed clocks to synchronize its devices.
- System integration
In one sentence
Distributed clocks keep the local clocks of networked devices synchronized so inputs and outputs across devices happen at the same instant.
Example
On a printing machine, EtherCAT distributed clocks let all servo drives take over new setpoints at the same instant, keeping the print units in register.
How it applies
- Engineering: Synchronized clocks allow simultaneous sampling and actuation across devices independent of communication delays, which is needed for coordinated motion and time-stamped measurements.
- Commissioning: Check the synchronization status of all devices after start-up and after topology changes. A device that has not locked to the reference clock may still exchange data, but out of sync.
- Documentation: Device and system documentation should state which devices support and use distributed clocks, how synchronization is monitored and what diagnostics appear when it is lost.
Distributed clock vs. logical clock
A distributed clock approximates a common physical time across devices. A Logical clock only counts events to capture their order and has no relation to physical time. Motion control needs the former; ordering of messages can use the latter.