Beckhoff CU2508 Documentation page 17

ESL protocol
The software driver in the controller/control device/IPC forms the counterpart to the CU2508. It works on a
Gbit port in the controller and "packs" the user data into the EtherCAT Switch Link Protocol (ESL) or unpacks
the ESL protocol from the CU2508 and forwards the user data to the application. Therefore, no extra
telegram containing control data is sent to/from the CU2508 for the handling of the user data; instead, the
user data generated by user programs are supplemented by several bytes of control and information data for
the connection between controller and CU2508.
The CU2508 driver is integrated in TwinCAT from version 2.11R2 onwards; pay attention to the
specifications in the Technical data. The ESL protocol is disclosed, see the Description page. In addition, it
has been included in the Wireshark-Installation since version 1.4.2.
EtherCAT systems and CU2508
The CU2508 can be used to operate several FastEthernet EtherCAT systems on one port of the IPC, i.e.
quasi as port-multiplier. Hence the term "port multiplier".
When operating several EtherCAT systems on the ports of a CU2508, temporal effects may occur that may
be relevant for the application. Some explanations are provided below.
The CU2508 essentially supports the following three operation modes. To understand this, basic knowledge
of the EtherCAT operation modes and synchronization methods is helpful.
1. Standard mode: no frame influence, no DistributedClocks
◦ The CU2508 forwards incoming frames via Gbit-ESL to the desired FastEthernet port, also in the
opposite direction. There is no time control for the Ethernet frames.
◦ The EtherCAT slaves of the lower-level systems therefore operate on a frame-triggered basis (also
referred to as FreeRun mode), and the output times are essentially dependent on frame delays/
jitter, for example.
2. Time-controlled sending/time-stamped receiving: with frame influence, no DistributedClocks
◦ The CU2508 forwards incoming frames via Gbit-ESL to the desired FastEthernet port at the
requested time. Time stamping takes place in the opposite direction. In other words, the Ethernet
frames are time-controlled.
◦ Frame-triggered EtherCAT slaves thus operate with "low jitter", and "synchronized" between the
EtherCAT systems.
◦ In order for the frames to be forwarded on a time-controlled basis, buffering in the CU2508 is
required, which may cause considerable delays. For short cycle times the feasibility should be
verified!
◦ This operation mode is not yet supported (as of 2019)
3. DistributedClocks mode, no frame influence
◦ The forwarded EtherCAT frames are subject to temporal influence by the transmitting IPC, the
CU2508 and the EtherCAT slaves.
◦ The ports X1..8 are parameterized as DistributedClocks - ReferenceClocks
◦ Thus, the EtherCAT slaves of the lower-level systems that support DistributedClocks also operate
DC-synchronously. This means that the input/output operations in these slaves can be
synchronized, even at the "same" time between the EtherCAT systems on ports X1..8.
In this case the overall system is essentially independent of frame delays/jitter, as long as these
are not significant enough to impair the DistributedClocks control.
◦ With regard to EtherCAT operation, this method is essentially the most sensible, because
• it offers the best temporal definition for the input/output operations of the EtherCAT devices
• no time buffers are necessary in the CU2508
The following aspects must be taken into account in order to be able to estimate temporal effects in
operation modes 1 and 3:
• Depending on the data content, Ethernet frames have a time length of
◦ X1..8 FastEthernet: 7..128µs, InterFrameGap (IFG) 9.6 µs
◦ X9 Gbit: 0.7..12 µs, IFG 0.96 µs
Real-time Ethernet port multiplier Version: 2.6
Product overview
17