Real-time

If hard real-time requirements are to be met, then the communication system must be able to guarantee deterministic behavior. This means always being able to exchange the required amount of data within a predefined time and the ability to provide mechanisms to synchronize all participants very precisely.

Today only a few special field buses or other proprietary solutions are suitable for achieving cycle times of less than one millisecond or jitter values in the range of a microsecond. To be able to continue the trend towards using Ethernet for networking in automation systems, special measures were created whereby Ethernet can guarantee the required determinism.

There is a degree of variance in Ethernet propagation times during transmission. A solution to guarantee deterministic system behavior is to have a precise clock in all terminal devices synchronized with all other systems. If actions are referred to such a high precision clock, then the process can be decoupled from the propagation times of the communication.

This issue applies particularly to co-operating systems that must start specific actions simultaneously. An example is several robots that work together on one task, e.g. the transport of heavy objects, something that can only function if the movements of the robots are very precisely matched to each other.