The EtherCAT technology
overcomes the system limitations of other Ethernet solutions: The Ethernet
packet is no longer received, then interpreted and copied as process data at
every connection. Instead, the Ethernet frame is processed on the fly: the
newly developed FMMU (fieldbus memory management unit) in each slave node reads
the data addressed to it, while the telegram is forwarded to the next device.
Similarly, input data is inserted while the telegram passes through. The
telegrams are only delayed by a few nanoseconds.
On the master side, very
inexpensive, commercially available standard network interface cards (NIC) or
any on board Ethernet controller can be as hardware interface. The common
feature of these interfaces is data transfer to the PC via DMA (direct memory
access), i.e. no CPU capacity is taken up for the network access.
Protocol
The
EtherCAT protocol uses an officially assigned EtherType inside the Ethernet
Frame. The use of this EtherType allows transport of control data directly
within the Ethernet frame without redefining the standard Ethernet frame. The
frame may consist of several sub-telegrams, each serving a particular memory
area of the logical process images that can be up to 4 gigabytes in size.
Addressing of the Ethernet terminals can be in any order because the data
sequence is independent of the physical order. Broadcast, Multicast and
communication between slaves are possible.
Transfer
directly in the Ethernet frame is used in cases where EtherCAT components are
operated in the same subnet as the master controller and where the control
software has direct access to the Ethernet controller.
However,
EtherCAT applications are not limited such control systems: EtherCAT UDP packs
the EtherCAT protocol into UDP/IP datagrams. This enables any control unit with
Ethernet protocol stack to address EtherCAT systems. Even communication across
routers into other subnets is possible. In this variant, system performance
obviously depends on the real-time characteristics of the control and its
Ethernet protocol implementation. The response times of the EtherCAT network
itself are hardly restricted at all: The UDP datagram only has to be unpacked
in the first station.
Performance
EtherCAT
reaches new dimensions in network performance. Thanks to FMMU in the slave
nodes and DMA access to the network card in the master, the complete protocol
processing takes place within hardware and is thus independent of the run-time
of protocol stacks, CPU performance or software implementation. The update time
for 1000 distributed I/Os is only 30 µs. Up to 1486 bytes of process data can
be exchanged with a single Ethernet frame - this is equivalent to almost 12000
digital inputs and outputs. The transfer of this data quantity only takes 300
µs.
The communication
with 100 servo axes only takes 100 µs. During this time, all axes are provided
with set values and control data and report their actual position and status.
The distributed clock technique enables the axes to be synchronized with a
deviation of significantly less than 1 microsecond.
The
extremely high performance of the EtherCAT technology enables control concepts
that could not be realized with classic fieldbus systems. For example, the
Ethernet system can now not only deal with velocity control, but also with the
current control of distributed drives. The tremendous bandwidth enables status
information to be transferred with each data item. With EtherCAT, a
communication technology is available that matches the superior computing
capacity of modern Industrial PCs. The bus system is no longer the
"bottleneck" of the control concept. Distributed I/Os are recorded
faster than is possible with most local I/O interfaces.
EtherCAT instead of PCI
The
central PC becomes smaller and more cost effective because additional slots are
not needed for interface cards since the onboard Ethernet port can be used.
With increasing miniaturisation of the PC-components, the physical size of
Industrial PCs is increasingly determined by the number of required slots. The
bandwidth of Fast Ethernet, together with the data width of the EtherCAT
communication hardware enables new directions: Interfaces that are
conventionally located in the IPC are transferred to intelligent interface
terminals at the EtherCAT. Apart from the decentralised I/Os, axes and control
units, complex systems such as fieldbus masters, fast serial interfaces,
gateways and other communication interfaces can be addressed. Ethernet devices
without restriction on protocol variants can be connected via decentralised
"hub terminals". The central IPC becomes smaller and therefore more
cost-effective, one Ethernet interface is sufficient for the complete
communication with the periphery.
Topology
Line, tree or star: EtherCAT supports almost any topology. The bus or line structure known from the field busses thus also becomes available for Ethernet. Particularly useful for system wiring is the combination of line and branches or stubs: The required interfaces exist on the couplers; no additional switches are required. Naturally, the classic switch-based Ethernet star topology can also be used.
Wiring flexibility is further maximized through the choice of different cables. Flexible and inexpensive standard Ethernet patch cables transfer the signals optionally in Ethernet mode (100BASE-TX) or in E-Bus (LVDS) (Low Voltage Differential Signal) signal representation. Plastic potical fiber (POF) can be used in special applications for longer distances. The complete bandwidth of the Ethernet network - such as different fiber optics and copper cables - can be used in combination with switches or media converters.
Fast Ethernet (100BASE-FX) or E-Bus can be selected based on distance requirements. The Fast Ethernet physics enables a cable length of 100 m between devices while the E-Bus line is intended for modular devices. The size of the network is almost unlimited since up to 65535 devices can be connected.
Distributed Clock
Accurate
synchronization is particularly important in cases where widely
distributed processes require simultaneous actions. This may be the
case, for example, in applications where several servo axes carry out
coordinated movements simultaneously. The most powerful approach for
synchronization is the accurate alignment of distributed clocks, as
described in the new IEEE 1588 standard. In contrast to fully
synchronous communication, where synchronization quality suffers
immediately in the event of a communication fault, distributed aligned
clocks have a high degree of tolerance from possible fault-related
delays within the communication system.
With
EtherCAT, the data exchange is completely hardware based on "mother"
and "daughter" clocks. Each clock can simply and accurately determine
the other clocks run-time offset because the communication utilizes a
logical and full-duplex Ethernet physical ring structure. The
distributed clocks are adjusted based on this value, which means that a
very precise network-wide timebase with a jitter of significantly less
then 1 microsecond is available.
However,
high-resolution distributed clocks are not only used for
synchronization, but can also provide accurate information about the
local timing of the data acquisition. For example, controls frequently
calculate velocities from sequentially measured positions. Particularly
with very short sampling times, even a small temporal jitter in the
displacement measurement leads to large step changes in velocity. With
EtherCAT new, expanded data types (timestamp data type, oversampling
data type) are introduced. The local time is linked to the measured
value with a resolution of up to 10 ns, which is made possible by the
large bandwidth offered by Ethernet. The accuracy of a velocity
calculation then no longer depends on the jitter of the communication
system. It is orders of magnitude better than that of measuring
techniques based on jitter-free communication.
Hot Connect
The
Hot Connect function enables parts of the network to be linked and
decoupled or reconfigured "on the fly". Many applications require a
change in I/O configuration during operation. Examples are processing
centres with changing, sensor-equipped tool systems or transfer devices
with intelligent, flexible work piece carriers. The protocol structure
of the EtherCAT system takes account of these changing configurations.
Safety over EtherCAT
Conventionally,
safety functions are realized separately from the automation network, either
via hardware or using dedicated safety bus systems. Safety over EtherCAT
enables safety-related communication and control communication on the same
network. The safety protocol is based on the application layer of EtherCAT,
without influencing the lower layers. It is certified according to IEC 61508
and meets the requirements of Safety Integrated Level (SIL) 3. The data length
is variable, making the protocol equally suitable for safe I/O data and for
safe drive technology. Like other EtherCAT data, the safety data can be routed
without requiring safety routers or gateways. First fully certified products
featuring Safety over EtherCAT are already available.
Openness
The EtherCAT technology is fully Ethernet-compatible and truly open. The protocol tolerates other Ethernet-based services and protocols on the same physical network - usually even with minimum loss of performance. There is no restriction on the type of Ethernet device that can be connected within the EtherCAT segment via a hub terminal. Devices with fieldbus interface are integrated via EtherCAT fieldbus master terminals. The UDP protocol variant can be implemented on each socket interface. The EtherCAT Technology Group ensures that each interested party can implement and use this network. EtherCAT is an international standard.
industrial communications 
ETHERCAT
EtherCAT
