Real-time Ethernet: New SERCOS III features

By Control Engineering Staff November 2, 2006

An industrial Ethernet protocol, SERCOS III, has features new to SERCOS users while maintaining backwards compatibility with previous versions of SERCOS (SErial Realtime COmmunications System, a digital motion control industrial network) on servo and motion-profiles, cyclic and non-cyclic communication, and the synchronization mechanism, according to Ronald Larsen, managing director, SERCOS N.A. The protocol provides for easy migration of existing software drivers, he says.

Compared to previous versions of SERCOS, Larsen explains, SERCOS III reduces costs through use of the Ethernet physical layer without the need for switches or hubs; the use of low-cost FPGA communications controllers; and simpler handling with Cat5e cable. It offers a higher data transmission rate (100 Mbit/sec compared to 16 Mbit/sec for SERCOS II); reduced minimum cycle time (down to 31.25

New features for SERCOS include a profile for synchronization and communication between multiple motion controls, cross communication between slave devices on a ring and line, hardware redundancy for fault tolerance in case of a break in the ring, hot plugging for connection and removal of nodes during operation, safe communication for drive-integrated safety functions, and half the minimum cycle time of the previous SERCOS interface, Larsen continues. In addition, standard (non-real time) Ethernet frames can be transmitted in parallel to the real-time channel without affecting performance and behavior of that channel.

The Controller-to-Controller (C2C) Synchronization and Communication Profile is a profile for interconnecting motion controllers using SERCOS III. It defines mechanisms for data exchange between motion controllers, such as parameter values, axis commands and status values, Larsen explains. It provides for the synchronization of controls on a hard real-time level.

“Ethernet physics enables cross communication between slave devices on a ring or line, and between slaves on different rings/lines via the controller-to-controller communication,” Larsen says. “This occurs in the real-time channel, with all telegrams transparent for each device. It allows for shorter reaction times and mutually fast monitoring in axis groups, such as gantry axes.”

Communication in previous versions of SERCOS is unidirectional, he says. Ethernet enables full-duplex bi-directional signal flow so that line and ring logical signal flow structures can be used. The ring topology provides redundancy so that operation can continue in case of a break in the ring, providing better uptime. During operation, devices can hot plug anywhere in a ring or at the end of a line. In addition, standard Ethernet devices, such as notebook computers, can be connected to non-used SERCOS III ports to communicate with SERCOS III devices via the IP channel (non-real time channel). This is useful for set-up and diagnostics.

SERCOS safety, Larsen says, is a protocol extension for centralized and decentralized safety applications, compatible with the established SERCOS transmission mechanisms. It is based on the producer/consumer model with up to 16,384 single and 1,024 multicast connections. It provides a safety network with up to 15 multicast participants. SERCOS Safety provides a routing mechanism for safety data across SERCOS III networks and for direct safe data communication between slave devices. Thus, the SERCOS interface can be used within safety applications up to SIL 3 (safety integrity level) according to IEC 61508, even with the shortest cycle times, Larsen says. As with prior versions, the new interface enables use of controls and drives from different manufacturers in a system by standardizing data, parameters, commands, and feedbacks exchanged between drives and controls.

An extended device profile for I/O connections is based on the existing SERCOS communication protocol. Modular I/O devices with bus couplers and simple block I/Os will be included. To minimize the costs of the bus interface, especially for simple I/O, an FPGA-based low-cost communication controller is being developed. In small quantities, the price is expected to be under $5, Larsen predicts.

SERCOS North America, founded in 1994, has members that include manufacturers, systems integrators, software developers, and educational institutions with interest in industrial automation. They guide the future of the interface standard within the SERCOS N.A. Technical Working Group.

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— Edited by Mark T. Hoske ,editor in chief , Control Engineering