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7 EtherCAT IO interface
7.1 ETHERCAT CoE
EtherCAT is an ethernet-based field bus system, developed by Beckhoff and the EtherCAT Technology
Group (ETG). EtherCAT is an open technology standardized in the international standards IEC 61158 and
IEC 61784 as well as in ISO 15745-4. EtherCAT can provide the same communication mechanisms as are
known from CANopen: object directory, PDO (process data objects) and SDO (service object objects). Even
network management is comparable. For example, EtherCAT can be implement on devices that were previ-
ously equipped with CANopen with minimal effort; large parts of the CANopen firmware are reusable. The
objects can optionally be expand to take account of the larger bandwidth of EtherCAT.
In order to create a user-friendly interface for device operation, different organizations have created various
standards in which the following are defined:
- The device classes that exist (e.g.: class 'rotary encoder', 'analogue input module').
- The parameters that each representative of such a class has (obligatory and optional elements).
- The place where these parameters are to be found and the mechanism with which they are to be changed.
EtherCAT follows the so called CoE standard here: Can-Application-protocol-over-EtherCAT.
The process data objects (PDO) are used for the fast and efficient exchange of real-time data (for example I /
O data, setpoints or actual values). In the EtherCAT telegram, no objects are addressed but the contents of
the process data are sent directly from previously mapped parameters.
7.2 EtherCAT installation
EtherCAT supports almost any topology, Line, tree or star. The bus or line structure known from the
fieldbuses thus also becomes available for Ethernet. Particularly useful for system wiring is the combination
of line and junctions 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.
The permissible cable length between two EtherCAT devices must not exceed 100 meters. This results from
the FastEthernet technology, which mainly for reasons of signal attenuation over the Line length allows a
maximum link length of 5 + 90 + 5 m if lines with appropriate properties.
To connect EtherCAT devices, use only Ethernet (cable + plug) connections at least of category 5 (CAT5)
according to EN 50173 or ISO / IEC 11801. EtherCAT uses four wires of the cable for signal transmission.
EtherCAT uses RJ45 connectors, for example. The contact assignment is the Ethernet standard
(ISO / IEC 8802-3) is compatible.
7.3
EtherCAT access handling
The input and output data of the EtherCAT slave are displayed as CANopen Process Data Objects (PDO).
The process data (PDOs) cyclically transmitted by an EtherCAT slave are the user data. They are expected
or sent to the slave in the application. For this purpose, the EtherCAT Master (Beckhoff TwinCAT) parame-
terizes each EtherCAT slave during the start-up phase. It specifies the process data (size in bits / bytes,
source location, transmission type) from or to the slave would like.
Page 46 of 70
POS-124-U
24.01.2022
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