Image Processing Reference
In-Depth Information
21.4.3.2 EtherCAT (Profiles 12/1 and 12/2)
EtherCAT defined by Beckhoff and supported by the EtherCat Technology Group (ETG, see also
www.ethercat.org) uses the Ethernet frames and sends them in a special ring topology [].
Medium access control employs the master/slave principle, where the master node (typically the
control system) sends the Ethernet frames to the slave nodes, which extract data from and insert data
into these frames.
From an Ethernet point of view, an EtherCAT segment is a single Ethernet device, which receives
and sends standard ISO/IEC - Ethernet frames. However, this Ethernet device is not limited to
a single Ethernet controller with a downstream microprocessor, but may consist of a large number of
EtherCAT slave devices. hese devices process the incoming frames directly and extract the relevant
user data, or insert data and transfer the frame to the next EtherCAT slave device. he last EtherCAT
slave device within the segment sends the fully processed frame back, so that it is returned by the
irstslavedevicetothemasterastheresponseframe.
The EtherCAT slave node arrangement represents an open ring bus. The controller is connected
to one of the open ends, either directly to the device, or via Ethernet switches utilizing the full duplex
capabilities of Ethernet, the resulting topology is a physical line (see Figure .). All frames are
relayed from the first node to the next ones. The last node returns the telegram back to the first
node, via the nodes in between.
In order to achieve maximum performance, the Ethernet frames should be processed “on the fly”.
his means that the node processes and relays the message to the next node in the line as the message
is being received, rather than the other (slower) option of waiting until the message is fully received.
If the on the fly method of processing is implemented, the slave node recognizes relevant commands
and executes them accordingly while the frames are passed on to the next node. To realize such
a node, a special application-specific integrated circuit (ASIC) is needed for medium access which
integrates a two-port switch into the actual device.
he nodes have an addressable memory that can be accessed with read or write services, either each
node consecutively or several nodes simultaneously. Several EtherCAT telegrams can be embedded
within an Ethernet frame, each telegram addressing a data section. The EtherCAT telegrams are
either transported directly in the data area of the Ethernet frame or within the data section of an
Send clock cycle
RT phase
Isochronous phase
NRT phase
IO-controller
IO-device 1
IO-device 2
IO-device 3
Time scheduled
communication
Address-based
communication
Time
FIGURE .
PROFINET timing.
EtherCAT TM is the registered trade name of Beckhoff, Verl.
A data section is a set of memory variables (e.g., inputs or outputs).
Search WWH ::




Custom Search