Michael Rocci 28 July 2015

The Fundamental Principle and Architecture of EtherCAT Communication

What Is EtherCAT?

EtherCAT is an industrial real-time communication network that can provide sensing and control for different automated processes. A communication network allows for the transfer of data between two or more machines. The data is transferred using a or medium such as an cable or a control area network. EtherCAT makes use of standard Ethernet technology to allow for the transfer of data over an Ethernet cable between multiple machines. This data transfer is made possible through the use of a specialized EtherCAT protocol. A protocol defines a standard method to exchange data between two or more machines such as two computers. The difference between an EtherCAT communication network and a standard Ethernet network is that EtherCAT has a specific protocol that allows for hard real-time communication.

Real-Time Communication

A real-time communication network performs data transfer tasks with very precise timing and a high degree of reliability that standard Ethernet communication does not support. The real-time communication process has a known maximum time for each of the critical operations the network performs. It is deterministic in that its timing can be guaranteed within a certain margin of error. For example, a real-time communication network will perform a cycle of the data transfer process within one millisecond. No cycle will take longer than one millisecond to occur due to EtherCAT’s protocol guaranteeing this communication deadline. Data transfer processes that need to occur within a certain time period in order to prevent unreliable data transfers benefit from the real-time nature of EtherCAT. EtherCAT is not the only industrial communication network available that provides hard real-time communication that is deterministic, but it provides the fastest, simplest, and most reliable communication architecture for automated processes.

The Master-Slave Model

The EtherCAT communication network uses the master-slave model for communication protocols. One device or process, the master, has unidirectional control over one or more other devices or processes, the slave(s). An example of this type of communication is two identical motors connected to two different drives that are coupled to a common load. One drive is defined as the master and is configured for running in the speed-control mode. The other drive is defined as a slave and is configured for running in torque- control mode. The direction of control is always from the master to the slaves, but the slaves send information back to the master during the communication cycle.

The Data Transfer Process

The EtherCAT master sends a telegram that passes through each node of the network. This telegram contains a data frame that houses the data that is transferred between the master and the slave devices. Each EtherCAT slave device reads the data addressed to it “on the fly”, and inserts its data in the frame as the frame is moving downstream. Therefore, the transfer of data is never halted at a specific device. This allows for a faster and more efficient method of transferring data.

The image below shows EtherCAT data within a telegram. The numbers in the diagram represent the number of bits of information that can be stored within each section of the telegram.

Source: http://www.industrialethernetu.com/courses/402_2.htm

The EtherCAT master is the only node within a segment that is allowed to actively send an EtherCAT frame. All of the other nodes within the communication architecture merely forward frames downstream. This concept prevents unpredictable delays and guarantees EtherCAT’s real-time capabilities.

To better explain this concept, a comparison will be made to a train moving down a track that has multiple stations. Imagine the EtherCAT telegram is a train that leaves its first station, or in other words, the master device. The telegram travels down a track towards its next station, or in other words, the first slave device connected to the master. Once the telegram reaches the first slave device, data enters and leaves the telegram at the device without halting the process. Therefore, the train, or telegram, never stops to transfer data. Data is transferred “on the fly.” This is like people jumping on and off of a train at a station without the train stopping. The communication cycle is completed once the telegram reaches the final slave within the communication architecture and returns to the EtherCAT master device. This process allows for extremely fast times to complete one cycle of the transfer of data within the communication network.

The image below shows an EtherCAT telegram traveling to and from slave devices in an EtherCAT network. The colored blocks in the image are of data that are transferred to and from the slave devices.

Source: http://www.ethercat.org/download/documents/EtherCAT_Introduction_EN.pdf

The Bus Topology

A topology is the physical infrastructure that is set up to allow for the transfer of data between machines. Essentially, it is the manner in which multiple devices are connected to each other using Ethernet cables as the physical data transfer medium. EtherCAT can support many different types of physical topologies both simple and complex, but EtherCAT makes a pure bus topology with hundreds of nodes possible within the network. A bus network is a in which nodes are connected in a daisy chain by a linear sequence of buses. A bus is the data link in a bus network, or the connection of one location to another for the purpose of transmitting and receiving digital information. Within this topology, the master would exist at one end of the chain while the slaves are connected throughout the chain. Once the EtherCAT telegram reaches the last slave node within the topology, the information gets transmitted back to the EtherCAT master device using Ethernet’s full duplex technology. This technology allows for the EtherCAT telegram to return to the EtherCAT master device if the physical topology of the network does not allow for the final EtherCAT slave device to be connected to the EtherCAT master.

The image below shows a standard bus topology with four devices connected to each other. The master device would be the device shown on the left of the image and the final slave device would be on the right side of the image. EtherCAT’s full duplex communication system allows the telegram to be transferred directly back to the EtherCAT master from the final slave device.

Source: http://www.cs.umd.edu/class/fall2001/cmsc411/proj01/pub/figure1.jpg

Full and Half Duplex Communication

In a full duplex system, both parties can communicate with each other simultaneously. For example, during a telephone call, the parties at both ends of a call can speak and be heard by the other party simultaneously. In a half duplex system, only one person could talk at a time. An example of this would be a “push-to-talk” two-way radio. The full- duplex technology allows for the transfer of data from the end of the bus topology back to the master while data is simultaneously being transmitted. It is the ability for information to travel simultaneously that allows for the telegram in an EtherCAT communication network to return to the EtherCAT master device from the final slave device in a bus topology even though the network is not physically constructed as a closed loop.

The image below shows the difference between full and half duplex communication through the use of block diagrams.

Source: http://www.windowsnetworking.com/img/upl/image0041220367802008.jpg

The Complete Communication Cycle

One example of an EtherCAT communication network would be three slave devices connected in a physical bus topology to a master device. The master will transmit a telegram that contains the data that is being transferred. This telegram passes from slave device to slave device, just like a train traveling from station to station. The telegram does not fully halt its transmission at each slave device, but instead, data is transferred while the telegram continues to move past the slave device and onto the next one. Once the telegram reaches the final node in the bus topology, EtherCAT’s full duplex communication feature transmits the telegram back to the master device. This signifies the completion of one cycle of the communication process within the EtherCAT communication network.