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Product Introduction
Motion Control Overview
EtherCAT EtherCAT (Ethernet Control Automation Technology) is a high-performance, Ethernet- based fieldbus industrial network system. The protocol is standardized in IEC 61158 and applies to automation applications that need faster and more efficient communications. Short data update times with precise synchronization make EtherCAT suitable for real-time requirements in automation technology. Functional Principle In EtherCAT network, the Master sends Ethernet frames through all of the slave nodes. The Standard Ethernet packet or frame is no longer received, interpreted, and copied as process data at every node. Instead, slave devices read the data addressed to them and input data are also inserted in the same time while the telegram passes through the device, processing data "on the fly". Typically the entire network can be addressed with just one frame.
EtherCAT supports individual nodes to be connected/disconnected during operation. If one of the slaves in the network is removed, the rest of the network can continue to operate normally. EtherCAT also enables other communication features such as cable redundancy or master redundancy with Hot Standby. Synchronization Distributed clocks (DC) mechanism provides highly precise time synchronization between slaves in an EtherCAT network, which is equivalent to the IEEE 1588 Precision Time Protocol standard. By using distributed clocks, EtherCAT is able to synchronize the time in all local bus devices within a very narrow tolerance range. All EtherCAT slaves are provided with an internal clock (system time/local time). One EtherCAT slave is used as a reference clock and distributes its clock cyclically. Possible misalignment between the reference clock and the clocks of the other slaves are caused when a slave is switched on and the internal free-running register that holds the current time is reset to zero. Unfortunately, this action doesn't happen at the same time, and this result in an initial offset among clocks that has to be compensated. Typically, masters send a broadcast to all other slaves in the system. Having received the message, slaves will latch the value of their internal clock. There are two latch values, one is receiving, and the other is returning back. Thus, the master can read all latched values and calculate the delay for each slave. Delays will be stored into an offset register. In the following, the master will send a message periodically to all other slaves in the EtherCAT network to make the first slave the reference clock and forcing all other slaves to set their internal clock by the calculated offset. Because synchronization between slaves in DCmode is done by internal clocks in hardware, EtherCAT guarantees the time jitter is less than 1us.
Data exchanges are cyclically updated between EtherCAT Masters and Slaves. Data in EtherCAT frames is transported directly within the IEEE 802.3 Ethernet frame using Ether type 0x88a4 and are processed by the EtherCAT slave controller on the fly. Each EtherCAT datagram is a command that consists of a header, data, and a working counter. The datagram header indicates what type of access the master device would like to execute: Read, write, read-write Logical addressing is used for the cyclical exchange of process data. The header and data are used to specify the operation that the slave must perform, and the working counter is updated by the slave to let the master to know that a slave has processed the command. Every EtherCAT datagram ends with a 16-bit working counter (WKC). The WKC counts the number of devices that were successfully addressed by this EtherCAT datagram. EtherCAT datagrams are processed before receiving the complete frame. In the case that the data is invalid, the frame check sum is not valid and the slave will not set data for the local application. Access to a specified slave device through direct addressing Access to multiple slave devices through logical addressing
Diagnosis with Exact Localization
EtherCAT is an ultra-fast I/O system. To reach the best high-speed communication, high communication accuracy is demanded. EtherCAT comprises a wide range of systems with inherent diagnostic features which help detect and locate system errors precisely. Every EtherCAT datagram ends with a 16-bit working counter (WKC) to count the number of devices that were successfully addressed by this EtherCAT datagram. The Master can check the data exchange situation by WKC in the same cycle and the error frame can be detected by analyzing the nodes’ error counters. The slave application will be executed only as the frame is received correctly. The automatic evaluation of the associated error counters enables precise localization of critical network sections. Bit errors during transmission are detected reliably by the analysis of the Cyclic Redundancy Check (CRC) check sum. CRC is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. In addition to error detection and localization protocols, transmission physics and topology of the EtherCAT system allows an individual quality monitoring of every single transmission path.
Topology EtherCAT supports a variety of network topologies, including line, tree, ring, and star. The line and tree topologies are more conducive to fieldbus applications because they require fewer connections and utilize a much simpler and more flexible cabling schema that switches and hubs are not necessary for lines or trees topology. Inexpensive industrial Ethernet cable can be used between two nodes up to 100m apart in 100BASE-TX mode. EtherCAT makes a pure bus or line topology with hundreds of nodes possible without limitations. Up to 65,535 devices can be connected to EtherCAT, so network expansion is almost unlimited.
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