Introduction To The SERCOS interfaceâ¢ - Sercos N.A.

I. General Description 

Introduction to the SERCOS interface® 1 

By Ronald Larsen, Managing Director, SERCOS N.A. 

SERCOS is an acronym for SErial Realtime COmmunications System, a digital motion 

control bus that interconnects motion controls, drives, I/Os, sensors and actuators for 

numerically controlled machines and systems. It is an open controller-to-intelligent 

digital device interface, designed for high-speed serial communication of standardized 

closed-loop real-time data over a noise-immune, fiber optic ring (SERCOS I & II) or 

Industrial Ethernet cable (SERCOS III). 

Motion Control 

SERCOS driver 

Real-time OS 

Cyclic position 

command 

(500 µs, 1000 µs 

or 2000 µs) 

As fast as the system 

dynamic requires 

Drive internal interpolation 

Position loop 

(250 µs) 

Velocity loop 

As fast as required to 

smooth external noise 

Current loop 

The SERCOS interface® offers more than 500 standardized parameters that describe the 

interaction of drives and controls in terms independent of any manufacturer. This rich 

instruction set provides for communication of commands, parameters, status, and 

diagnostics, handing many different types of systems. SERCOS also includes features for 

I/O control that often allow a machine builder to dispense with the need for a separate I/O 

bus. 

The SERCOS interface is the only internationally standardized open digital interface for 

motion control and I/O with the performance required to synchronize high-performance 

multi-axis motion control systems. The SERCOS interface was designed to include 

mechanisms to ensure the high level of determinism required when synchronizing 

multiple axes of digital drives, while some other buses attempt to ensure determinism via 

brute speed. The SERCOS interface is defined with a minimum of overhead, in order to 

ensure maximum throughput. 

1 Much of the information in this document was adapted with permission from data published on 

the SERCOS N.A. website, www.sercos.com. 

Introduction to the SERCOS interface -- Sept. 2006 -- Page 1

The SERCOS interface greatly reduces connectivity problems in control systems. It can 

connect up to 254 drives to a control using one fiber optic cable ring or a single daisychained 

Industrial Ethernet cable, and multiple rings may be used in a system. 

Control unit 

Master 1 

Ring 1 

max. 254 drives 

Master 2 

Ring 2 

max. 254 drives 

Slave Slave Slave 

Single 

Drive 

Group of 

Drives 

Single 

Drive 

Slave 

Single 

Drive 

Slave 

Group of 

Drives 

A comparable traditional analog servo system with 8 axes of motion may require over 

100 wires between the drive and control. This reduces system cost, eliminates many types 

of noise problems and helps machine designers get motion control systems up and 

running quickly. 

The SERCOS interface was created in the 1980s by a group of European machine tool 

builders and control/drive vendors to specify a digital open interface that would ease the 

transition from analog to digital drive technology. It was originally intended to be a drive 

interface for advanced machine tool applications, but today is in wide use in all servocontrolled 

automation disciplines. The SERCOS interface standard covers control topics 

that are common to all servo applications, as well as topics that are industry specific. It is 

estimated that the majority of SERCOS interface axes in the field today are on 

applications such as web-fed printing presses; packaging, converting & food processing 

machines; assembly, handling and robots; semiconductor processing equipment; 

metalforming and welding machines; textile processing; fiber winding; simulators and 

test equipment; plastics machines and many other types of special machines. 

II. SERCOS interface is an International Standard 

The SERCOS interface is a set of standard specifications that may be incorporated into 

any company’s products, with each control and drive maintaining its own functions and 

features. Because it is an international IEC standard, it allows any manufacturer’s 

SERCOS interface-compatible digital control to talk to any other SERCOS interfacecompatible 

digital servo drive, digital spindle drive, hydraulic system, digital I/O or 

sensors over a well-defined fiber optic link or standard Ethernet technology. Controls and 

drives conforming to the standard comply with a standard medium for transmission, 

topology, connection techniques, signal levels, message (telegram) structures, timing and 

data formats. 


III. Three Generations of the SERCOS interface 

There have been three generations of the SERCOS interface: 

• The first generation – SERCOS I -- operated at 2 and 4 Mbit/sec, using the 

SERCON410B ASIC and fiber optic transmitters/receivers and cabling. The 

SERCON410B is no longer produced. 

• The second (current) generation – SERCOS II -- operates at 2/4/8/16 Mbit/sec, using 

the SERCON816 ASIC and fiber optics. The SERCON816 is downward compatible 

with the SERCON410B. 

• The new third generation – SERCOS III -- operates at up to 100 Mbit/sec utilizing a 

Field Programmable Gate Array (FPGA) or a General Purpose Communication 

Controller (GPCC) and is based on standard Industrial Ethernet hardware. SERCOS 

III maintains backward compatibility with previous versions in regard to profiles, 

synchronization, message structures and the standardized parameter set that describes 

all aspects of real-time motion and I/O control. 

More than 50 control manufacturers and 30 drive manufacturers worldwide offer first and 

second generation SERCOS interface products, with hundreds of thousands of systems 

installed. Several manufacturers offer SERCOS interface I/O stations plus hardware and 

software development tools. The first SERCOS III products were introduced in late 2005 

and 2006, with many more in development. 

IV. Functions of the SERCOS interface 

• Exchanges data between control and drives, transmitting command and actual 

values with extremely short cycle times. 

• Guarantees exact synchronization for precise coordinated moves with as many 

axes as required. 

• Supports four operating modes: torque, velocity, position control and block 

mode. The transmission of nominal position has proven to be the best solution for 

fast, highly precise applications. 

• Includes a service channel for non-cyclic data transmission, such as internal 

parameters, data and diagnostics. A complete set of drive parameters can be 

downloaded and uploaded for storage via the service channel. 

• Enables the use of controls and drives from different manufacturers in a system, 

by standardizing all data, parameters, commands and feedbacks exchanged 

between drives and controls. 

V. Advantages 

The SERCOS interface allows drive and control manufacturers to create intelligent 

digital drives with vastly improved capabilities and flexibility. A single drive can be 

designed to handle multiple motors, such as permanent magnet servomotors, high 

horsepower induction servomotors (vector drives) and linear motors, with the 

configuration set-up parametrically. 


Types of products using the SERCOS interface include: 

• AC and DC servo positioning drives 

• Spindle and main drives 

• Stepper indexer/drives 

• Vector drives 

• Linear motor drives 

• Inverters/variable frequency drives 

• Multi-axis motion controllers 

• I/O modules 

• PLC interfaces 

• Numerical controls 

• Electrohydraulic valves and positioning systems 

The SERCOS interface provides the flexibility of configuring multi-vendor control 

systems with plug and play interoperability. Designers are not limited to products from 

one manufacturer, but are free to choose the best-in-breed to solve their individual motion 

and I/O control problems. 

The SERCOS interface reduces system cost, eliminates many types of noise problems 

and helps machine designers get motion control systems up and running quickly. 

Even more importantly, the SERCOS interface has facilitated great advances in machine 

productivity. End users have realized incredible timesaving advantages with SERCOS on 

their machines -- faster delivery, faster installation, faster setup, faster product 

changeover, and faster production speeds. 

VI. SERCOS III 

Industrial Ethernet has become the de facto standard for manufacturing information 

networking, and the market is requesting Ethernet connectivity for servo drives. The 

problem is that Industrial Ethernet is characterized by high bandwidth and low hardware 

costs, but is not deterministic. 

The SERCOS interface, on the other hand, is optimized for high-speed deterministic 

motion control, which is required for the exact synchronization of multiple drives. 

SERCOS also defines a protocol structure and includes an ample variety of profile 

definitions for control for most motion and I/O devices. It has been successfully used on 

over 300,000 applications. SERCOS III combines the proven mechanisms and properties 

of the original SERCOS interface with the high speed and low cost of the Industrial 

Ethernet physics, thus creating an advanced version of the most successful open motion 

control communications interface. 

Many technologies have been added to the Ethernet stack simply by adding a new 

protocol. But rather than put motion under TCP/IP (transmission control protocol- 

/Internet protocol), resulting in less determinism and the added expense of switching 

devices, the SERCOS solution was to put the standard Ethernet TCP/IP under control of 


the motion bus, and use Ethernet hardware with its lower cost twisted-pair copper cable. 

(However, Ethernet fiber optics will be supported by SERCOS III.) 

This maintains the deterministic motion control of SERCOS, allows links to the existing 

manufacturing communications infrastructure, provides for the possibility of new features 

and lowers hardware costs. And noise immunity will not be a major problem, as 

Ethernet’s twisted pair cable is generally extremely noise immune because it uses a 

differential driver and receiver. 

SERCOS III offers the following advantages: 

• Protection of investment due to high compatibility with previous SERCOS 

interface (topology, profiles, telegram structures, synchronization) 

• Reduction of hardware costs for a SERCOS III interface connection down to the 

level of an analog interface 

• Integration of IP protocols 

• Cross communication between slaves 

• Synchronization of several motion controls 

• A double ring structure with regard to signal flow 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 

• Half the minimum cycle time of the previous SERCOS interface (31.25 μs vs 62.5 

μs) 

VII. How the SERCOS interface Works 

A. Topology 

Components in a generation I and II SERCOS interface-based motion control system are 

connected via fiber optic rings using a master/slave configuration. A typical system may 

include several rings, with up to 254 devices per ring. A SERCOS interface master 

controls each ring, assigning timeslots to ensure deterministic and collision-free access 

for all slaves. 


SERCOS I and II Topology 

SERCOS III communicates over industrial Ethernet and offers both a line and ring 

logical structure within the cable. The double ring structure offers the possibility of 

redundant data transfer. With SERCOS III, in case of a break at any point in the ring, the 

protocol automatically switches over to the line structure, so that the communication and 

manufacturing will continue while the integrated diagnostics tool signals the break, which 

can be repaired without interfering with the plants’ performance. 

SERCOS III doesn’t use the star topology of the standard Ethernet. No hubs or switches 

are needed. 

Ring structure 

(double ring) 

M1 

Master 

M2 

Line structure 

(single ring) 

M1 

Master 

M2 

Slave 

1 

Slave 

2 

Slave 

n 

Slave 

1 

Slave 

2 

Slave 

n 

With hardware redundancy 

SERCOS III Topologies 

Without hardware redundancy 


B. Communications Structure 

In order to ensure strict synchronization of multiple axes and a predictable update time at 

each axis, the SERCOS interface utilizes a master/slave communication structure, where 

the motion control acts as a master, the drives act as slaves. SERCOS I and II drives are 

only permitted to respond to queries from the motion control, so direct communication 

between slaves isn’t possible. However, this feature is advantageous in some motion 

control applications and the Ethernet physics enable such a slave-to-slave data transfer, 

so SERCOS III supports this feature. 

C. Physical Layer 

With SERCOS I and II one fiber optic ring is used to exchange full 32-bit data between 

controllers, drives, I/O and sensors. This includes commands, status, parameters and 

diagnostics. In SERCOS III, standard industrial Ethernet is used for this purpose. Note 

that Ethernet fiber optics can be used for communication with SERCOS III. 

D. Communications Controllers 

ASIC (SERCOS I & II) or FPGA or GPCC (SERCOS III)-based SERCOS interface 

controllers are normally integrated into master motion controls as well as drives, 

amplifiers and I/O modules. They simplify the task of the designer by automatically 

handling most SERCOS interface communication functions. 

E. Timing 

Timing is critical in serial networks because motion controls cannot accurately 

reconstruct the state of the machine unless everything is precisely synchronized – 

measurements, transmissions and replies. Many of the messages that controls and drives 

send each other are for timing and synchronization. 

In CNC position loop software, the position error is determined in a software routine that 

takes time. The system works because the position error updates are designed to occur at 

predictable points in time. The same logic is applied in the SERCOS interface. Methods 

are specified to keep the jitter on the serial link down to a low level, then an internal 

timing sequence is used to ensure that all drives in a loop act upon their command signal 

at the exact same moment, and all acquire their feedback information at the exact same 

moment. The result is transparent to the user. 

F. SERCOS interface IDNs 

All motion control and drive communication is performed via a set of SERCOS interface 

telegrams, which each have an identification, or “Ident” (IDN) number. All parametric 

data, such as scaling and loop gains, and real-time loop closure information is set up this 

way. This allows the SERCOS interface to standardize the most common interface data. 

Over 500 IDNs are presently used to define a comprehensive set of motion control and 

I/O commands. All SERCOS interface products must incorporate a subset of these, but 

do not necessarily need to include all IDNs. 


The format used for an IDN plus a sample IDN are shown in the following illustration. 

IDN 

Name (abbreviation) 

Function/description 

Length in bytes 

Minimum input value 

Maximum input value 

Scaling/resolution 

Units 

S-00047 

Position command value 

During the position control drive operation mode, the position command values are transferred 

from the control unit to the drive according to the time pattern of the control unit cycle. 

4 Min. ≥ -2 31 

Max. ≤ +2 31 -1 

Scaling type: IDN00076 

Scaling factor: IDN 00077 

Scaling exponent: IDN 00078 

Rotational position resolution: IDN 00079 

G. Cycle Times 

The SERCOS interface cycle time is specified in a flexible format of 62.5 microseconds, 

125, 250, 500, and then multiples of 1 millisecond. SERCOS III cycle times begin at 

31.25 microseconds. The amount & type of data contained in a cycle is also variable. 

This flexibility permits a designer to vary cycle time, content and number of drives to 

achieve a particular project’s requirements. More data can be sent faster to a smaller 

number of drives. Slowing the rate down permits a higher density of drives per ring. For 

example, with generations I and II, 4 to 8 axes are common on a ring in machine tool 

applications where intense communication is required to control a high-speed tool path. 

Up to 40 axes may be controlled on a ring in packaging applications, and up to 100 axes 

may be found in web-fed printing applications, where electronic line shafting is utilized 

to synchronize a number of print cylinders and auxiliary axes. When necessary, multiple 

rings can be employed in an application. 

H. Error Correction/Diagnostics 

The SERCOS interface offers extensive diagnostic reporting and produces detailed 

reports on fault conditions. This allows vendors to incorporate detailed context-sensitive 

troubleshooting procedures in their motion controls. An operator can go to his control 

panel and acquire instant diagnosis of every axis motor and drive at every section of the 

machine. Failures can be immediately pinpointed by the control system and quickly and 

easily be repaired as opposed to hit and miss troubleshooting. 

I. Service and IP Channels 

Generations I and II SERCOS interface include a service channel, which can be used for 

the transfer of communication data as well as parameter or diagnostic data in non-realtime. 

To maintain downward compatibility, SERCOS III also has the service channel. 

SERCOS III also offers an optional IP channel for transfer of real-time and non-real-time 

data via standard Ethernet frames. The service channel and IP channel are configurable. 


VIII. Differences and Similarities with other Communication Busses 

Fieldbus protocols (DeviceNet, Profibus and others) were designed for I/O and are 

intended for low-level devices -- such as sensors, motor starters and inverters -- which are 

mainly turned on and off, or which require an asynchronous analog command value. A 

number of these busses can be used for control of loosely coupled drives – but motion 

control is not their primary function. 

On the other hand, the SERCOS interface was designed for high-speed control of tightly 

coupled motion axes, where deterministic control is of high importance. In a high-speed 

packaging line or multi-axis machine tool, sub-microsecond synchronization accuracy of 

multiple axes in the system is required. 

Although the SERCOS interface was not designed primarily to be a fieldbus, it has the 

ability to decentralize I/O via multiple I/O nodes that connect to the field devices. The 

I/O requirements may be satisfied via the SERCOS interface, negating the need for a 

separate fieldbus. However, in high-power applications, such as flexible machine 

systems, both a fieldbus and SERCOS interface often exist in the same control system. 

The decision of whether to use a fieldbus, SERCOS interface, or both, depends on the 

application. If only loose coupling between axes is required, a series of single-axis 

position drives connected via a fieldbus may be adequate. However, in a synchronized 

motion application, the SERCOS interface is required; and I/O can be handled over the 

SERCOS interface, depending on the requirements and complexity of the system. A 

number of manufacturers offer SERCOS interface I/O systems. 


This figure illustrates a system with communication to a higher-level factory network, a fieldbus 

for I/O and the SERCOS II interface for real-time motion control. 

XI. Conformance Testing for SERCOS interface Products 

Products with the SERCOS interface are tested at the Institute for Control Engineering of 

Machine Tools and Manufacturing Units (ISW) at the University of Stuttgart. The aim of 

the conformance test is to ensure the compatibility and interoperability of SERCOS 

interface devices from different vendors in multiple vendor environments. Testing to the 

SERCOS I & II Conformance Class A and B levels, plus testing for conformance to the 

SERCOS Packaging Profile, are presently available. Conformance tests for SERCOS III 

are now in development. 

Only products that have passed the conformance test are allowed to carry 

the SERCOS Conformance-tested logo. 


X. Ideal for Distributed Multi-axis Control Systems 

The SERCOS interface is a foundation for building distributed multi-axis control systems 

for a myriad of applications. Distributed control improves machine flexibility by moving 

processing power and decision making from the CNC or motion control down into the 

drives and sensors. These devices then become intelligent building blocks that can easily 

be added to a machine or production line without major changes in hardware and 

software. 

The SERCOS interface is well suited for distributed control because it places axisdependent 

control functions, such as loop closures, interpolation and registration, in the 

drives, not in the motion controller. Thus, motion controllers can concentrate on motion 

control profiles and toolpaths independent of the axes. In operation, the control issues a 

position command to the drive, which then closes its own loops and micro-interpolates its 

trajectory, based on previously downloaded parameters. The SERCOS interface also 

integrates input/output functions such as limit switches, pushbuttons and various sensors. 

XI. SERCOS interface Packaging Profile 

Packaging applications have benefited greatly from the adoption of the SERCOS 

interface in controls and drives. Many packaging machinery manufacturers are shipping 

machines today that were designed from the ground up using servo technology, in which 

hundreds of mechanical components have been eliminated, costs have been reduced and 

flexibility has been maximized. 

The OMAC (Open Modular Architecture Controls) Packaging Workgroup published a 

recommendation in November 2001 to use SERCOS interface (IEC 61491) as an open 

drive interface for packaging machines. In this context, the group challenged the 

SERCOS organization to further improve the multi-vendor interoperability of servo 

controls and drives on the basis of a packaging profile. 

The SERCOS organizations responded by creating a packaging profile that defines a 

subset of the SERCOS interface parameters that are applicable to packaging machines. 

The SERCOS interface specifies more than 500 standardized parameters that define the 

interaction between controls and drives in a vendor-independent manner. The SERCOS 

specification supports the interoperability of controls, drives and I/O devices from 

different manufacturers on the basis of an open real-time communication system. 

However, in practice, issues arise which make the idea of “plug and play” more difficult 

to implement. Only a subset of all existing parameters is needed for most applications, as 

some parameters have been designed for very specific use only (i.e., spindle positioning 

in machine tools), or they deal with optional interfaces (e.g., position feedback value of 

an external feedback). Thus, not all vendors implement all parameters in every device 

they produce. 


Another issue is that motion control functions can be integrated centrally in the control as 

well as decentrally in the drives. This depends on the functionality of the servo drives, 

which may vary from manufacturer to manufacturer. Machine builders have different 

philosophies and preferences with regard to control architecture. Thus, depending on the 

drive functions that are used by a control, a different set of SERCOS parameters is 

required. 

To keep this complexity manageable and to ensure the highest interoperability of controls 

and drives, it was decided to implement packaging application-specific profiles as subsets 

to the SERCOS specification. Existing SERCOS function groups and the corresponding 

parameters have been mapped into profiles for three of the six OMAC-defined 

application classes. The profiles consist of mandatory and optional parameters for simple 

servo drives and frequency converters, and also for intelligent drives, which can be 

commanded by any of position, velocity or torque values. Thus, any servo drives/- 

controllers that implement the pack profile will be interoperable. 

It’s important to note that the pack profile doesn't restrict drives/controls 

to only the defined parameters; but it does require that the drive/control 

implement the defined parameters in the defined manner in order insure 

compatibility. 

XII. Synchronization of Motion Controls 

SERCOS III specifies a Controller-to-Controller (C2C) profile for distributed control 

functions in modular machines and systems, providing for controller-to-controller 

synchronization between distributed motion controls. This addresses the increased need 

in manufacturing systems engineering in which individual machine modules not only 

need to be connected mechanically, but also need to be integrated into communication 

networks and coupled in hard real-time. 

For example, consider printing and packaging machines with a modular structure. Often 

each module of the machine has its own PLC or motion control, but the axes in each 

module must work synchronously, such as in four-color (or more) printing. With the C2C 

profile, every module in a machine or entire line can perform a data exchange with every 

other module to insure synchronization, independent of the master or the physical order 

of the modules in the SERCOS ring. 

The specification of communication and synchronization mechanisms between 

distributed motion controls takes into consideration the hardware redundancy and hotplugging 

features of SERCOS III. In addition, the data and commands exchanged 

between controls are defined based on the existing SERCOS parameter model. This 

data includes control and state information, as well as relevant command values and 

actual values. In this context, a convergence with the CIP Motion Profile and the 

development of a SERCOS/CIP-Gateway is planned. 


Typical applications for such a profile are in printing, packaging and processing 

machines; as well as machine tools with special requirements for control systems and 

synchronization, e.g., machine concepts with gantry axes or rotary transfer tables. 

XIII. Keeping the interface Up to Date 

Updates to the SERCOS interface standard are defined by multi-company Technical 

Working Groups (TWGs) in both North America and Europe, which ensure that the latest 

enhancements proposed by SERCOS member companies are incorporated into the 

specification. Updates that have been accepted by the TWGs since official adoption of 

the IEC standard are published by the SERCOS interface member organizations and 

made available to anyone for a modest fee. 

XIV. Support Organizations 

The many SERCOS interface vendors have formed SERCOS interface trade association 

groups in Europe, North America and Japan. These organizations promote the use of the 

digital SERCOS interface, and coordinate and disseminate general and technical 

information. Membership is open to any individual, organization or company interested in 

establishing knowledge of the SERCOS interface, applications, technical implementation 

and/or operation. 

For literature and information on SERCOS interface trade association membership 

contact: 


SERCOS International 

Landhausstrasse 20 

D-70190 Stuttgart, Germany 

Phone: +49 711 / 28 457 50 

Fax: +49 711 / 28 457 55 

Email: info@sercos.de 

www.sercos.de 

SERCOS North America 

405 Loblolly Bay Drive 

Santa Rosa Beach, Florida 32459 

Toll-free Tel: 800/573-7267 

Local Tel & Fax: 850/269-0908 

E-mail: info@sercos.com 

www.sercos.com 

SERCOS Japan 

Katsuyuki Nakata 

Attn. Mr. Shigeru Ueki 

2-7-6 Hirakawa-cho, Chiyoda-ku 

Tokyo, 102-0093, Japan 

Tel: +81-3-5210-4457 ● Fax: +81-3-5210-4458 

E-mail: knakata@eos.ocn.ne.jp 

XV. Sources for More Information 

SERCOS interface Standard (IEC 61491) 

• Available from Global Engineering Documents, a source for technical standards - 

- http//global.his.com. 

• Available from IEC - International Electrotechnical Commission in Switzerland – 

www.iec.ch 

Note that the title of the standard is Electrical Equipment of Industrial Machines – 

Serial Data Link for Real-time Communication Between Controls and Drives. 

Searching for the word SERCOS will not locate the standard. 

Other Documentation and Information 

You can find more detailed technical descriptions, download free documentation and 

purchase the SERCOS specification, profiles and reference manuals on: 

• www.sercos.de 

• www.sercos.com 

###

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