Bronkhorst EtherCAT - Manual (Page 7)

BRONKHORST ®
2 FUNDAMENTALS OF ETHERCAT
The EtherCAT protocol transports data directly within a standard Ethernet frame without changing its basic structure.
When the master controller and slave devices are on the same subnet, the EtherCAT protocol merely replaces the
Internet Protocol (IP) in the Ethernet frame.
Data is communicated between master and slaves in the form of process data objects (PDOs). Each PDO has an
address to one particular slave or multiple slaves, and this “data and address” combination (plus the working counter
for validation) makes up an EtherCAT telegram. One Ethernet frame can contain multiple telegrams, and multiple
frames may be necessary to hold all the telegrams needed for one control cycle.
Several Device Profiles and Protocols can co-exist side by side
With some real-time protocols, the master controller sends a data packet and must wait for the process data to be
interpreted and copied at every slave node. However, this method of determinism may be difficult to sustain because
the master controller must add and manage a certain amount of processing time and jitter per slave. EtherCAT
technology overcomes these system limitations by processing each Ethernet frame on the fly. For example, suppose
the Ethernet frame is a moving train, and the EtherCAT telegrams are train cars. The bits of PDO data are people in the
cars who can be extracted or inserted by the appropriate slaves. The whole “train” passes through all the slave devices
without stopping, and the end slave sends it back through all the slaves again.
In the same way, when device 1 encounters the Ethernet packet sent by the master, it automatically begins streaming
the packet to device 2, all while reading and writing to the packet with only a few nanoseconds delay. Because the
packet continues passing from slave to slave to slave, it could exist in multiple devices at the same time. What does
this mean practically? Let’s say you have 50 slave devices, and different data is sent to each slave. For non-EtherCAT
implementations, this may mean sending out 50 different packets. For EtherCAT, one long packet that touches all
slaves is sent, and the packet contains 50 devices worth of data. However, if all the slaves need to receive the same
data, one short packet is sent, and the slaves all look at the same part of the packet as it is streaming through,
optimizing the data transfer speed and bandwidth.
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EtherCAT interface 9.17.063