The three main topologies are ring, line, and star. Each has its own benefits and drawbacks, which will become important as they interact with your individual application.

This is the generally preferred topology, as it adds cable redundancy. If the cabling between two devices becomes inoperable, the cable coming from the other direction will continue to complete the network. This topology can only be achieved using Real-Time PXI controllers, as it requires three or more Ethernet ports. Additional ports can be added to a PXI system using the PXI-8231, PXI-8232, or PXI-8234.

This is the easiest topology to implement, as you don’t need to worry about connecting the head of the line to the tail – which requires multiple Ethernet ports, and hence a PXI chassis (only the second port of a CompactRIO can be configured to work on the EtherCAT network). This topology is also referred to as daisy chain, and is the most common used with NI products.

This is the most familiar topology for many new to EtherCAT®, as it resembles a regular Ethernet network using hubs. However, to implement this, you will need a dedicated EtherCAT® junction. Because of this, it is potentially costlier than ring or line. Also, this topology will be marginally slower than the others, as there are more interstitial nodes that must repeat the message between the end nodes (e.g. for an EtherCAT® packet to go from master to slave, it must go through the EtherCAT® junction first, which will introduce a small delay). In fact, EtherCAT® star topology is not like traditional star topologies – it is actually a line topology in which data goes through junction port 1, reaches its end slave and comes back to the junction, and then comes through junction port 2 the same way. This topology is best for systems in locations with physical constraints that make it difficult to implement line or ring.

Note: NI EtherCAT does not officially support the star topology. However, people have been successful in making star networks with NI EtherCAT hardware. We recommend you test with your EtherCAT® junction before committing to a star topology, as we cannot guarantee interoperability with all EtherCAT® junctions.

• Setting Up EtherCAT on NI Programmable Automation Controllers
• NI EtherCAT RIO: Deterministic Expansion for LabVIEW RIO Systems
• Can Switches, Hubs, or Routers Be Used With The NI 9144?
• NI-Industrial Communications for EtherCAT® Help: Hardware Setup (Ring)
• NI-Industrial Communications for EtherCAT® Help: Use Cable Redundancy (Software)