Industrial Ethernet: The Hunt for the CSMA / CD

The unexpected nature of Ethernet TCP / IP s time ‘has long been a weakness for many industrial network applications, especially those with real-time requirements rugged capabilities. However, due to the overall cost vs. performance benefits has increased over time, industrial users have developed a way to overcome the obvious shortcomings.

In total, there are currently about thirty Industrial Ethernet system used around the world, but these are the most significant North American market are:

Ethernet / IP

Modbus / TCP

EtherCAT ®

Ethernet Powerlink

PROFINET

All protocols developed with the challenges of real-time control thinking (constant rotation time, low latency, low jitter), this particular design preventive measures to avoid collision data, and they approach the solution in different ways.

The a real difference to the standard Ethernet TCP / IP is a further implementation of the Real Time Data Exchange module that manages real-time communication in either place or on top of TCP / IP / UDP.Still, all protocols continue support for Ethernet (IEEE 802.3 layer) and physical data and TCP / IP / UDP, thus providing an interface to networks.

Protocols standard level information such as Ethernet / IP and Modbus / TCP seems most vulnerable when it comes to hard real-time control, for they will inherit all the features that make the Ethernet TCP / IP non-deterministic. While the CSMA / CD edition (random time delay after bus arbitration) can be significantly reduced by using Ethernet switches, protocols still face the additional jitter caused by the UDP and IP. Without careful design considerations, they were questioned for hard real-time control, especially for fast applications.

Ethernet Powerlink motion control circumvents the CSMA / CD problem very elegantly passing through TCP / IP / UDP layer when it comes to real-time communication. Ethernet Powerlink uses a focused and highly deterministic time slice assigned to each node in the network. The result is that only one node sends data on the bus during a dedicated time slice, while all other nodes wait for their turn. This procedure effectively prevents any message collision and thereby eliminate idle random times. Obviously, via TCP / IP / UDP also removes the IP / UDP jitter.

EtherCAT ®, however, came the CSMA / CD problem by removing completely the physical level, which, with all consequences time, requiring the use of dedicated hardware. While the EtherCAT Master ® system only requires common Ethernet network interfaces, each EtherCAT ® slave device requires a special controller. EtherCAT ® still using Ethernet as the physical layer, but the Ethernet message is passed from node to node (daisy-chain), thus eliminating the need for bus arbitration per CSMA / CD.

Profinet, compared to all other protocols introduced here, have a transition through all situations described earlier: PROFINET V1 (CBA) using standard Ethernet TCP / IP, PROFINET V2 (RT) bypasses the TCP / IP / UDP layer, while the PROFINET V3 (IRT) using a dedicated Ethernet controller.