How the Systems Work
Diverse Approaches to Real-time Generation
There are three different approaches to building a real-time Ethernet solution
- Based on TCP/IP: Protocols are based on standard TCP/IP layers with real-time mechanisms embedded in the top layer. These solutions usually have a limited performance range.
- Standard Ethernet: Protocols are implemented on top of standard Ethernet layers. These solutions benefit from Ethernet evolution without further investment.
- Modified Ethernet: The standard Ethernet layer, the Ethernet mechanism and infrastructure are modified. These solutions put performance before standard compliance.
One crucial difference of the various Industrial Ethernet systems compared within this publication lies in how they organize data transfer and how they manage to deliver real-time performance. EtherCAT and SERCOS III communicate using a summation frame method: in each cycle, data for all network nodes is sent in one telegram that travels from one node to another along the ring topology of the network, also collecting node responses on the way. In contrast to that, the single telegram procedure used by the other systems works by sending individual telegrams to the nodes, which also respond individually in separate telegrams.
The systems use three different mechanisms for network access and data synchronization:
- A master controls the timing on the network. In POWERLINK environments, the master authorizes individual nodes to send data. In EtherCAT and SERCOS III networks, the transfer of summation frame telegrams follows the master‘s clock.
- PROFINET IRT uses synchronized switches to control communication.
- EtherNet/IP employs CIP Sync to distribute IEEE 1588 compliant time information throughout the network.