# SERCOS III

Protocols · `SERCOS III` · verification: source checked

SERCOS III is an open, Ethernet-based real-time communication protocol for automation devices. It uses Fast Ethernet at 100 Mbps full duplex and summation-frame transmission, with configurable communication cycles from 31.25 μs to 65 ms and synchronization accuracy below 1 μs. It supports ring, line and star/tree topologies, direct cross communication between slaves, and safety functions up to SIL3 with CIP Safety according to IEC 61508.

## Specification

| Property | Value |
|---|---|
| Transports | ethernet_l2 |
| Architecture | master slave |
| Encoding | binary |
| Standard body | IEC |
| Standard ref | IEC 61784, IEC 61158, IEC 61800-7 |

## Connected

- Related: [IEC 61508-1:2010](https://machineschema.com/standard/iec-61508.md)

## Safety

- Safety functions up to SIL3 can be implemented with CIP Safety on Sercos according to IEC 61508.
- Safety-relevant and non-safety-relevant data are transferred over the same cable.
- CIP Safety routing allows devices to communicate safely outside network boundaries.
- In a ring configuration, cable breaks are recognized within 25 μs and data is lost for at most one cycle.

## Study notes

- Communication cycle time is configurable between 31.25 μs and 65 ms.
- Synchronization accuracy is less than 1 μs.
- Process data is consolidated into one summation-frame telegram rather than transmitted in individual telegrams.
- Hot plugging does not affect real-time or synchronization characteristics.
- TCP/IP and other Ethernet-based protocols can share the same cable with real-time data.
- Sercos slaves can communicate directly with one another through cross communication.
- Supported topologies include ring, line, and star/tree structures.
- Ring redundancy is supported.
- Devices and their positions in the topology are recognized automatically.
- Drives can be parameterized over Sercos without a separate configuration device.
- Sercos uses IEEE 802.3 Ethernet physics and copper or fiberglass cables.

## Sources

1. [Technology](https://www.sercos.org/technology/) (accessed 2026-10-25)
2. [Sercos Automation Bus](https://www.sercos.org/) (accessed 2026-10-25)

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