What is SERCOS III?
SERCOS III is the third generation of the Serial Real-time Communication System: an open, real-time Industrial Ethernet protocol that synchronises servo drives, I/O and controllers over standard 100 Mbit/s Ethernet with cycle times from 31.25 µs and jitter below 1 µs. The protocol was designed for motion control in industrial automation, such as robotics, CNC machines, printing presses and packaging machines. To connect drives and controllers, SERCOS III combines the deterministic behaviour of the earlier Sercos bus with ordinary Ethernet hardware, and it is laid down in the international standards IEC 61158, IEC 61784 and IEC 61800-7.
🕰️ How did SERCOS come about?
Sercos started in 1987 as a joint project of two German trade associations, the VDW (machine tool builders) and the ZVEI (electrical and electronics industry). Machine builders wanted a single digital interface between CNC controller and drive, so that they no longer had to wire analogue ±10 V speed signals differently for every manufacturer. Early participants included ABB, AEG, AMK, Robert Bosch, Indramat (later part of Bosch Rexroth) and Siemens.
| Generation | Year | Medium | Speed | Characteristic |
|---|---|---|---|---|
| Sercos I | 1991 | Optical fibre (ring) | 2 and 4 Mbit/s | Cycle time from 62.5 µs, service channel for parameters |
| Sercos II | 1999 | Optical fibre (ring) | 2, 4, 8 and 16 Mbit/s | Higher speed, same principle |
| SERCOS III | 2003 (first products 2005) | Ethernet, copper or fibre | 100 Mbit/s full duplex | Cycle time 31.25 µs–65 ms, IP traffic alongside real-time data |
Sercos I and II are defined in IEC 61491 (1995). The technology is managed by Sercos International e.V., a German association with more than 90 member companies worldwide. The association is an IEC partner and works with organisations such as ODVA and the North American machine builders’ association OMAC. According to Sercos International, more than 4 million real-time nodes have been installed.
🔧 How does SERCOS III work technically?
SERCOS III uses the Ethernet physical layer as defined in IEEE 802.3, but controls access to the medium itself. The master sets the cycle; slaves never transmit on their own initiative.
- Two telegram types - the Master Data Telegram (MDT) carries setpoints to the slaves, and the Acknowledge Telegram (AT) collects actual values. Each cycle contains up to four MDTs and four ATs.
- Summation frame - every slave reads and writes its own pre-assigned area of the telegram as it passes through. One frame can therefore serve dozens of axes.
- Own EtherType - real-time frames use EtherType 0x88CD and run on layer 2 of the OSI model. The real-time part needs no IP address; slaves have a Sercos address from 1 to 511.
- Synchronisation - during start-up the master measures the delay (latency) through each node and compensates for it. The sync mark sits in the first MDT, so all axes adopt their setpoint at the same moment with less than 1 µs deviation.
- No switches required - every device has two ports (P1 and P2) and acts as a built-in switch. Ordinary switches in the real-time segment would actually disturb the timing.
- Unified Communication channel (UC) - the time left after the real-time telegrams is free for ordinary Ethernet frames such as TCP/IP and HTTP. With 8 axes and a 250 µs cycle, roughly 85 Mbit/s remains for UC traffic.
- Direct communication - Cross Communication lets slaves exchange data with each other, and Controller-to-Controller (C2C) links several masters, for example in coupled machine modules.
In Sercos, parameters are called IDNs (identification numbers). There are more than 700 standardised IDNs, grouped into profiles for drives, I/O, encoders and energy management. The drive profile is part of IEC 61800-7-204 and is also used by EtherCAT as SoE (Servo profile over EtherCAT).
🔁 Which topologies and redundancy does SERCOS III offer?
SERCOS III supports a line, a ring and, with special hardware, tree or star structures. In a line the last slave folds the telegram back to the master. In a ring two telegrams travel in opposite directions.
| Topology | Cabling | Redundancy | Typical use |
|---|---|---|---|
| Line | One cable fewer than a ring | None | Small machines, cost-sensitive layouts |
| Ring | Return path to the master | Cable break detected within 25 µs, at most one cycle of data lost | Packaging and printing machines, robot cells |
| Tree/star | Via branching modules | Depends on layout | Modular machines |
Because a ring break is handled within a single cycle, the machine keeps running while the fault is reported. This resembles ring redundancy in switched networks, but works much faster: MRP restores a ring in 10 to 500 ms depending on its class, SERCOS III within 25 µs. With hot plug, a technician can also add a device at the end of a running line without disturbing real-time communication.
🔄 How does SERCOS III compare with other protocols?
| Protocol | Managed by | Minimum cycle time | Strength |
|---|---|---|---|
| SERCOS III | Sercos International | 31.25 µs | Motion profile with a long track record, ring without switches, CIP Safety |
| EtherCAT | EtherCAT Technology Group | 12.5 µs | Very large vendor base, on-the-fly processing |
| ProfiNET IRT | PROFIBUS & PROFINET International | 31.25 µs | Integration into the Siemens ecosystem, PROFIdrive |
| IP with CIP Motion | ODVA | Depends on switches and time synchronisation | Standard TCP/UDP network, CIP Safety |
| Powerlink | B&R (EPSG until 2023) | About 100 µs | Open-source stack, time-slot approach |
In practice the choice mostly follows the controller and the drives. SERCOS III is strong in machines with Bosch Rexroth drives and in the PacDrive 3 motion controllers from Schneider Electric, which moved from Sercos II to SERCOS III. In process plants run by a DCS, SERCOS III is uncommon; there Profibus PA, Foundation Fieldbus and Ethernet-APL dominate. For an overview of older buses, see Fieldbuses.
🛡️ How does functional safety work on SERCOS III?
Sercos International and ODVA agreed in late 2006 that Sercos would adopt CIP Safety as its safety protocol. The CIP Safety on Sercos specification followed in 2008 after assessment by TÜV Rheinland and the German BGIA, and in 2013 ODVA extended CIP Safety with safe motion services for EtherNet/IP and SERCOS III. Safety data such as emergency stop, safe standstill and safe speed travels over the same cable as ordinary process data. Because CIP Safety follows the black channel principle, the safe end devices detect errors themselves, which allows use up to SIL 3 according to IEC 61508. No separate safety bus is needed, and safe messages can be routed to DeviceNet or EtherNet/IP.
🔐 How do you secure a SERCOS III network?
The real-time part of SERCOS III has no built-in security such as authentication or encryption. Anyone who can place a frame with EtherType 0x88CD on the segment can, in principle, disrupt telegrams. The UC channel also brings ordinary IP services such as web servers and engineering access right into the drive. A practical approach:
- Keep the real-time segment physically separate - connect it to the machine network only through the master or a UC switch, never directly to the plant LAN.
- Segment according to IEC 62443 - treat each machine as its own zone with one controlled connection upwards. Network segmentation and an industrial firewall limit who can reach the master and UC services.
- Apply Defense in Depth around the controller - the PLC or motion controller acting as master is the most valuable target. Restrict engineering access, use strong passwords and patch the firmware.
- Inventory the nodes - include drives, I/O and masters in your asset inventory; Wireshark recognises SERCOS III frames for diagnostics and checks.
- Mind the upward connections - data for SCADA, MES or the Cloud preferably flows via OPC UA, for which Sercos International created a companion specification together with the OPC Foundation.
Machine builders that supply SERCOS III equipment also have to deal with the Machinery Regulation and the Cyber Resilience Act, which set cybersecurity requirements for machinery and for products with digital elements.
🚀 What is the future of SERCOS III?
The specification is actively maintained: version 1.3 appeared in 2012, and from version 1.3.1 other Industrial Ethernet devices can take part in communication phase 0 with a 20 ms cycle. A Sercos International working group studied how Sercos fits with TSN. At the SPS IPC Drives 2016 trade fair, a demonstrator ran with a TSN soft master, Bosch Rexroth drives and Hirschmann switches, with existing SERCOS III devices participating unchanged. For new devices, ready-made implementations are available as FPGA designs and on chips such as Hilscher netX, TI Sitara AM335x and Renesas RZ/N1, along with an open-source soft master.
❓ Frequently asked questions
What does SERCOS stand for?
SERCOS stands for Serial Real-time Communication System. It originated in 1987 as a digital drive interface developed by the German associations VDW and ZVEI. SERCOS III is the third generation and the first to run on Ethernet.
What is the fastest cycle time of SERCOS III?
SERCOS III supports cycle times from 31.25 µs up to 65 ms, selectable by the user. Synchronisation between nodes stays below 1 µs. The chosen cycle depends on the number of axes and the required dynamics.
Does SERCOS III need switches?
SERCOS III needs no switches in the real-time segment, because every device has two ports and forwards frames itself. Devices are connected in a line or ring with patch cables. To access the UC channel in the middle of a line, you use a dedicated IP or UC switch.
Can SERCOS III carry ordinary TCP/IP traffic?
SERCOS III carries TCP/IP traffic through the Unified Communication channel, in the time left after the real-time telegrams. Web interfaces, parameterisation and diagnostics are therefore reachable over the same cable. This does not disturb the real-time behaviour of the drives.
What is the difference between SERCOS III and EtherCAT?
SERCOS III and EtherCAT are both real-time Ethernet protocols for motion control with cycle times in microseconds. EtherCAT reaches shorter minimum cycles (12.5 µs) and has a larger vendor base. SERCOS III has ring redundancy built in, whereas EtherCAT needs an extra port on the master for it. The SERCOS drive profile is also used within EtherCAT as SoE.
Does SERCOS III support functional safety?
SERCOS III supports functional safety through CIP Safety on Sercos, specified in 2008. Safety data travels over the same cable as process data and is suitable up to SIL 3 according to IEC 61508. A separate safety bus is therefore not required.
📌 In summary
SERCOS III is the open real-time Ethernet protocol from Sercos International for precise, synchronised motion control, with cycle times from 31.25 µs, jitter below 1 µs and ring redundancy within 25 µs. It combines a drive profile proven since 1987 with standard Ethernet, CIP Safety up to SIL 3 and a UC channel for IP traffic. Because the protocol itself has no security features, every SERCOS III network belongs in its own segmented machine zone.