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Understanding the Core Features of EtherCAT Fieldbus
by Jerry Leitz, Director of Field Engineering
IntervalZero Posted 07/31/2017
EtherCAT applies Ethernet applications for automation that require short data cycle times with minimal communication delays and reduced hardware costs. The functional goal of EtherCAT is to not target to a single specific node but instead, transmit messages from one node(also known as slave), to the next in the topology. Since messages are transmitted before being processed in each slave, EtherCAT operates at a very high speed and efficiency.
Fundamentals of the EtherCAT fieldbus include:
The EtherCAT master issues a single message with data that passes through all the nodes. As the message is sent downstream and back to the master, each node reads the inputs addressed to it and adds its output to the message. The last node in a segment detects an open port and sends the message back to the master by applying the Ethernet full duplex feature.
The EtherCAT fieldbus is the only node that can issue a message; this principle avoids unpredictable jitter and ensures real-time transmission. A master can be applied to any hardware with an Ethernet port, irrespective of the real-time software in use.
Data is transmitted either directly in the Ethernet frame or put into datagrams. Datagrams are perfect in networks where EtherCAT segments are sent through routers. The physical order of the slaves does not affect the order of data transmission in the network. Communication, broadcast, and multicast between the nodes is also achievable but must be prompted by the EtherCAT master.
EtherCAT technology has fast and short cycles since the slaves are not involved in the processing of data packets. The EtherCAT master handles all data communication processes. This feature makes EtherCAT a high-performance technology. In addition, short cycle times are made possible by maximum bandwidth utilization, since each node and date do not need separate frames.
EtherCAT can be shaped in many types of topologies; for example star, line or bus. This technology enables the creation of a field bus using Ethernet hardware where a combination of topologies is possible resulting in efficient and flexible programming of EtherCAT networks. Also, there is no need for additional switches since during wiring, lines are combined with branches and the ports required to create the branches are directly connected to input or output modules.
EtherCAT systems use a distributed clock mechanism for synchronization that gives it a low jitter without applying any special hardware. The process starts by the master sending a message to all the nodes. Upon receiving the message, all the nodes attach a time stamp twice; first when receiving the message as it is sent through the network and second when the frame returns through the node. The master then calculates the delay for each node each time a message is sent. This process is carried out repeatedly to reduce jitter and average out values. Finally, the master sends a final broadcast on the network clock, making the first node the reference clock to adjust the other nodes.
In simple terms, an EtherCAT fieldbus is a single large Ethernet system that receives and transmits data with a large number of nodes. Its synchronization abilities make it perfect in situations where synchronizing a large number of drives is necessary. It also lowers cost by reducing the use of switches and routers.