BECKHOFF EP9128 - Manual (Page 12)

Basic functioin principles
• is returned to the master through each EtherCAT slave via two return lines of the Ethernet cable
without further processing.
At short cycle times in the order of 50 µs at 20,000 Ethernet frames are in transit in the EtherCAT system
every second, plus acyclic organizational frames. The master awaits the return of the sent frames, which
return the device input data to the master, for example. Telegram transfer between slaves is link­based: An
EtherCAT slave will only forward a frame if a 'link' signal to the next device is present. Normally it can be
assumed that the downstream device correctly processes each EtherCAT telegram and returns or process it
at the end.
The crucial factor for forwarding EtherCAT telegrams is that a link signal is reported only from one slave to
the next if both slaves are actually ready for real­time participation in data processing. Specifically, this
means that an EtherCAT slave should not open the respective Ethernet port until it is ready to receive and
forward an Ethernet frame immediately.
A switch or router is usually used for standard Ethernet traffic forwarding. Any collisions or frame losses are
compensated through frame repetition in the higher level protocol layers (e.g. TCP). This mode is generally
not used for EtherCAT due to the short cycle times and the real­time requirement. Some Ethernet devices
such as special switches, for example, report a link to the remote terminal even if they will only be ready for
data processing in a few milliseconds. This behavior is particularly noticeable in media converters from
100Base­TX (copper) to 100Base­Fx (optical fiber), which may report a link to the preceding EtherCAT slave
even if the optical fiber connection is interrupted, depending on the setting on the copper side.
Fast link detection is therefore a central component of each ESC (EtherCAT slave controller, hardware
processing unit for the EtherCAT protocol). According to the EtherCAT specification an ESC can have and
control 1 to 4 ports. Via an open port it can handle outgoing and incoming Ethernet traffic. Fig. 3 shows the
direction of the data flow in a fully configured ESC. In the EtherCAT datagrams the data are only processed
between ports 0 (A) and 3 (D) in the EtherCAT processing unit.
Fig. 7: Direction of data flow in the ESC
EP9128 12 Version 2.1.0