EtherCAT Communication

EtherCAT principle of operation EtherCAT Communication | EtherCAT (“Ethernet for Control Automation Technology”) was developed by Beckhoff Automation. All users of this technology automatically become members of the EtherCAT Technology Group (ETG).

How It Works

EtherCAT is based on the summation frame method: The EtherCAT master transmits an Ethernet frame containing data for all nodes on the network. That frame passes through all nodes in sequence. When it arrives at the last node on a trunk, the frame is turned back again. The nodes process the information in the frame as it passes through in one direction. Each node reads out data addressed to it on the fly, and inserts response data back into the frame. In order to support the bandwidth of 100 Mbit/s, special hardware based on ASICs or FPGAs is required for fast processing as data passes through. In effect, the topology of an EtherCAT network always constitutes a logical ring. Even trunks branching out, which can be hooked up to nodes especially designed for such connections, actually only add a two-way junction where the summation frame telegram travels up and back down the branching line.

Structure of an EtherCAT frame

All EtherCAT telegrams with instructions for individual nodes are contained within the payload data area of a frame. Each EtherCAT frame consists of one header and several EtherCAT commands. Each of these comprises its own header, instruction data for a slave, and a working counter. Up to 64 Kbytes configurable address space is available for each slave. Addressing proceeds by auto-increment, i.e. each slave counts up the 16-bit address field. Slaves can also be addressed via distributed station addresses, which are assigned by the master in the start-up phase.

EtherCAT Process Synchronization

Every slave connection provides a real-time clock that is synchronized by the master using a technique similar to IEEE 1588. There are slave devices with and without real-time mechanisms, since these are more demanding on the hardware. Based on the real-time clocks, control signals can be synchronized with high precision. In physical terms, the EtherCAT protocol not only runs on Ethernet, but also on LVDS (Low Voltage Differential Signaling). This standard is used by Beckhoff as an internal bus on the terminals. A PC with a standard Ethernet inter- face is typically used to implement an EtherCAT master. In contrast to other protocols such as POWERLINK or PROFINET, EtherCAT solely extends to Layers 1 through 3 of the seven-layer OSI model. Hence, in order to achieve application functionality comparable to the other systems, an extra protocol layer (CoE, EoE) needs to be super- imposed.