IEC 62410: Real-time Ethernet SERCOS III – Deterministic Motion Control over Industrial Ethernet

1. Introduction to SERCOS III

IEC PAS 62410 defines the specifications for SERCOS III, a real-time Ethernet communication protocol designed for motion control applications. It is the third generation of the SERCOS interface, evolving from IEC 61491. It leverages standard Ethernet hardware (100Base-TX and 100Base-FX) while providing deterministic real-time behaviour for high-performance servo drives.

SERCOS III is classified as Communication Profile CP16/3 under IEC 61784-2, maintaining application-layer compatibility with its predecessors CP16/1 and CP16/2.

2. Network Topologies

2.1 Ring Topology

The ring topology is the primary configuration for SERCOS III networks and provides built-in cable redundancy. Each participant has two bidirectional communication ports (Port 1 and Port 2) that are interchangeable. The master station transmits data in both directions simultaneously: a primary channel (clockwise) and a secondary channel (counter-clockwise). If a cable break occurs, the affected slaves automatically reconfigure and reroute traffic through the remaining path, maintaining continuous operation without any intervention from the controller.

2.2 Line Topology

In a line topology configuration, the last physical slave performs a loopback function. While this simpler topology reduces cabling requirements, it does not provide redundancy against cable breaks. However, unused ports in a line configuration can be repurposed for standard IP communication, offering flexibility for mixed-traffic networks. Line topology is typically used for smaller systems or cost-sensitive applications where redundancy is not a primary concern.

2.2 Synchronisation Classes

3. Communication Mechanisms

3.1 MDT and AT Telegrams

The SERCOS III communication model is based on a master/slave architecture where the master (typically a CNC or motion controller) orchestrates all data exchange. The fundamental communication structure uses two primary real-time telegram types:

Master Data Telegram (MDT) carries command values from master to slaves, including setpoint positions, velocities, torques, and control words. The MDT is broadcast to all slaves, and each slave extracts only its designated data fields from the telegram. This broadcast approach ensures that all slaves receive the same timing reference simultaneously.

Amplifier Telegram (AT) carries feedback data from slaves to master, including actual positions, velocities, status words, and diagnostic information. Each slave inserts its data into the AT at precisely defined time slots, and the complete AT returns to the master containing aggregated data from all slaves.

Each telegram has a service channel (SVC) area for acyclic parameter access and a real-time data field (RTD) area for cyclic process data. The service channel handles configuration, diagnostics, and non-time-critical data exchange without affecting real-time performance.

SERCOS III also supports an IP channel for standard TCP/IP traffic during non-SERCOS communication periods. This enables web server access to drives, file transfers, and remote diagnostics without requiring a separate network infrastructure.

4. Engineering Design Insights

Successful SERCOS III deployment in motion control systems requires attention to several critical factors. Ring topology with automatic redundancy significantly improves system availability and is recommended for all mission-critical applications. The cost difference compared to line topology is minimal (one additional return cable), but the reliability benefit is substantial.

Key engineering considerations include:

• Cable and Connector Quality: Use only shielded CAT5e or better cables with robust industrial connectors (M12 preferred). In ring topology, a single marginal connection compromises the entire network reliability. High-vibration environments require locking connectors with screw flanges.
• Cycle Time Optimization: Match the SERCOS cycle time to the servo drive update rate. For high-performance applications, 31.25 µs or 62.5 µs cycles are typical. For general motion control, 250 µs to 1 ms cycles provide adequate performance with lower CPU load on the master controller.
• Device Addressing: SERCOS III device addresses are independent of physical position in the network. Plan address assignments logically based on machine architecture rather than physical layout, which simplifies maintenance and troubleshooting during commissioning.
• Backward Compatibility: SERCOS III maintains application-layer compatibility with CP16/1 and CP16/2, enabling legacy software migration from earlier SERCOS generations while benefiting from the higher speed and Ethernet-based infrastructure.

5. Frequently Asked Questions