General

In a machine control with distributed components (I/O, drives, several masters) it may be useful for the components to operate with a close time link to each other. The components must therefore have a local “time”, to which the component (e.g. an I/O terminal) has access at all times.

Associated requirements may include:

  1. Several outputs in a control system have to be set simultaneously, irrespective of when the respective station receives the output data.
  2. Drives/axes in a control system must read their axis positions synchronized, irrespective of the topology or cycle time.

Both requirements necessitate a synchronization mechanism between the local times of the components of a control system.

  1. If inputs affect the control system, the (absolute) time must be recorded. This can be helpful for subsequent analysis, if such an analysis is required for determining the sequence of events in functional chains.
    This means that time running in the components must be coupled to a globally valid time, e.g. Greenwich world time or a network clock.
  2. tasks on different controllers should run synchronous and without phase shift.

The terms “close temporal reference” or “simultaneous” can be interpreted depending on requirements: for a “simultaneity” in the 10 ms range a serial communication structure may be adequate, while in other ranges 100 ns or less are required.

General 1:
Simple I/O topology

Fig. Simple I/O topology shows a simple EtherCAT topology consisting of a master, several I/Os and an axis. A local time is to be applied in different components. The tasks:

are discussed below.