A comparison: Sercos and Ethercat
At first glance, Sercos and Ethercat have several characteristics in common. Ethernet IEEE 802.3 is used as a transmission medium and protocol for both of these Industrial Ethernet automation busses. Both use the summation frame procedure for efficient use of bandwidth and both process the real-time protocols during node throughput (on-the-fly) in order to minimize running times. Because of these features, Sercos and Ethercat are among the fastest Industrial Ethernet busses.
However, a closer analysis of the two solutions shows that there are certain significant differences.
Unlike Ethercat, Sercos supports direct cross communication, which enables real-time data exchange between any Sercos devices within one communication cycle, master-slave, slave-to-slave and slave-to-master. Slave-to-slave communication is important for decentralized control architectures, e.g., applications, in which one servo drive is used as a virtual master and other drives must follow it closely. With Ethercat, a direct cross-communication is not possible for all connected slave devices as frames are only processed in one transmission direction. Data for slave-to-slave communication is therefore usually collected, processed and retransmitted by the master, which takes additional time.
In addition, with Sercos any other Ethernet protocol can be transmitted via the identical network infrastructure without impacting the real-time communication and without tunneling. Thus, it is also possible to connect any Ethernet devices to the Sercos network directly without additional hardware. With Ethercat, the protocols from other Ethernet devices are tunneled. However, this requires that the Ethercat system already be initialized and in operation (EoE=Ethernet-over-Ethercat). Additional logic is needed in order to assemble and disassemble the Ethernet frames to be tunneled.
If the Sercos real-time communication is not running, a standard Ethernet device, such as a notebook computer, can still access each node in the system for data exchange, set-up and troubleshooting. This feature is also the basis for the blended infrastructure that allows the coexistence of Sercos, EtherNet/IP and TCP/IP devices in one common network.
Another difference is the synchronization process used. With Sercos, no additional synchronization process is required, since the network-wide synchronization is derived directly from the cyclical real-time protocol. With Ethercat, time synchronization based on distributed clocks is used. However, the synchronization of distributed clocks requires a time synchronization protocol which reduces the bandwidth for user data.
Ethercat uses the device profiles from CANopen, such as the DS402 profile for drives. Alternatively, the Sercos drive profile (SoE) can be used. At first sight this seems to be an advantage. However, as the controller has to handle the different drive profiles, the complexity is increased and the level of interoperability is decreased. Sercos consistently relies on its own device profiles (e.g. for drives, I/Os, encoders) in order to enable the best possible interoperability of devices from different manufacturers.
In order to transmit safety-oriented data, Sercos uses the CIP safety protocol, which is also used for EtherNet/IP and DeviceNet. Ethercat uses its own safety protocol, called FSoE (Fail-Safe-over-Ethercat).
In addition to the technical differences, there are also different regulations regarding the rights to use the respective technologies. Although both real-time Ethernet solutions correspond to international standards IEC 61784/61158, the technologies are licensed differently. The rights to the Sercos technology are owned by the Sercos International user organization, a non-profit organization that has dealt with the development, standardization and marketing of the Sercos technology since 1990. Any company can freely implement a Sercos system, and no one company controls the technology. Sercos has paid membership in order to maintain a vendor-independent organization. However, a membership is not required to implement or use the Sercos technology.
In contrast, all rights to Ethercat are owned by Beckhoff. The Ethercat Technology Group is a non-registered association that owns no rights to the Ethercat technology. The membership in this group is free but mandatory in order to have access to the technical specifications. This is the main reason why the Ethercat Technology Group has a huge number of members.
Ethercat is used in a broad range of application areas, most of them with low-performance requirements. This is the main reason that Ethercat currently has a larger market share than Sercos. Sercos focuses on high production machines and demanding automation applications.
Read the complete study in the member area (available only in German).