EtherCAT (Ethernet for Control Automation Technology) is the motion-control RT solution from Beckhoff. It can process 1000 I/Os in 30 µs [4], but requires full-duplex. It can use copper or fiber-optic cables. EtherCAT is based on the master/slave principal and can interoperate with normal TCP/IP-based networks and other Ethernet-based solutions such as EIP or PROFInet. It also supports any Ethernet topology, including the bus.

The EtherCAT master processes RT data via dedicated hardware and software (Beckhoff currently use their PC-based TwinCAT OS and TwinCAT Y driver). In the future, further variations will be introduced that will also provide the same guarantees. The master prioritizes EtherCAT frames over normal Ethernet traffic, which is transmitted in gaps. The master controls traffic by initiating all transmissions.

The telegrams are standard Ethernet, and the data field encapsulates the EtherCAT frame (an EtherCAT header and one or more EtherCAT commands). Each command contains a header, data and Working Counter (WC) field. Each Ethernet telegram can contain many EtherCAT commands—realizing a higher bandwidth and more efficient use of the large Ethernet data field size and header (see Figure 2). The standard Ethernet CRC is used to verify message correctness.

Figure 2 — EtherCAT Encapsulation

The EtherCAT master fully controls its slaves. Its commands only elicit responses; slaves do not initiate transmissions. The two EtherCAT communication methods used are "Ether Type" or UDP/IP encapsulation.

The "Ether Type" uses the type field (defined in Ethernet II), which is more commonly known as the length field in IEEE 802.3. The Ether Type implementation does not use IP, thus limiting EtherCAT traffic to the originating subnet. Encapsulating commands using UDP/IP allows EtherCAT frames to traverse subnets, but has drawbacks. The UDP/IP header adds 28 (20: IP, 8: UDP) bytes to the Ethernet frame and undermines RT performance through its non-deterministic stack.

EtherCAT slaves range from intelligent nodes to 2-bit I/O modules and are networked via 100Base-TX, fibre optic cable or E-bus (depending on distance requirements). E-bus is an EtherCAT physical layer for Ethernet offering a LVDS (Low Voltage Differential Signal) scheme. Slaves are hot pluggable in any topology of branches or stubs. Multiple "slave rings" can exist on a single network if connected by a switch, (see Figure 3).

Figure 3 — Sample EtherCAT Implementation

EtherCAT slaves have integrated memory from 2 bits to 64 Kbytes. They appear to the Ethernet as a single device though actually comprising up to 65,535 devices. They are configured in an open-ring topology, with the Ethernet interface at the open end. Masters transmit commands to the MAC address of the first device. When the signal reaches the Ethernet/slave interface, it is converted to E-bus specifications (if E-bus is employed) and forwarded.

A slave receives a telegram, processes it (in hardware) then forwards it to the next slave on the ring. This processing delays the telegram by an order of nanoseconds. The last slave returns the completed telegram, via the ring, to the master. On the return route, each slave amplifies and regenerates the signal. Each slave has two Tx & Rx interfaces, so bi- directional communication occurs without contention.

In each EtherCAT command, the WC increments when a slave processes a command addressed to it, allowing the master to determine if each addressed slave is exchanging data, although correct data is not guaranteed.

The FMMU (Field Memory Management Unit) of each configurable slave converts a logical address to a physical one, and that information is available to the master at initialisation. Thus, each slave needs a special ASIC (Application-Specific Integrated Circuit). On telegram reception, a slave determines if it is addressed, and then passes data to/from the telegram, incurring a delay of some nanoseconds. EtherCAT is also internally synchronized by a distributed clock algorithm (a simplified version of IEEE 1588) although external synchronization is achievable with IEEE 1588.

EtherCAT is a fast RT Ethernet solution and deterministic if not used with UDP/IP or intermediate switches or routers between master and slaves.