Sercos I – The Digital Drive Interface of Past and Future

1987 was the year in which SERCOS’ first digital drive for industrial machines was introduced. This took place under the name Sercos I (the Sercos Interface). The entire thing was developed for the purpose of securing against uncontrolled drive movements and speeds which are not classified as allowable. In order to be able to realize this process, a light conductor is integrated as a physical medium. By means of this light conductor, it is possible to satisfy the requirements for the safety in industrial machines. This is because the desired high immunity against jitter (as periods of electromagnetic interference are called) is achieved. After all, with the use of Sercos I, this value is just < 1μsec. Further, in this area of business, the secure exclusion of HF interference is of key importance. This is done by controlling the power which takes place through pulse width modulation. Furthermore, this is where the locally separated components of the network and the earth connections on the control units and drives play a role. The foundations of Sercos I include the SERCOS real-time protocol and the HDLC basis protocol. The topology of the network is that of a ring, and the synchronization takes place by means of hardware. Each ring has only one master, but up to 254 subscribers. This can be increased through the coupling to another ring. A rate of 2 Mbps or 4 Mbps is achieved with the transmission of real-time data. With this, the cycle time remains configurable and at least 62.5 μs.

Possible Applications

Since the introduction of Sercos I at the end of the 80s and its increasing popularity in the 90s, this digital drive interface served in numerous applications to create higher safety as well as optimized communication within a system. As such, Sercos I can be found in nearly all processing technologies that have CNC. However, with this achievement combining IT and industrial communication, the range of possible applications is much wider. As such, the following areas belong to the application possibilities of the interface:

  • Turning and finishing work
  • Machining centers including HSC
  • Milling work including HSC
  • Crankshaft shape grinding
  • Tool grinding machines
  • Transfer machining lines
  • Cog wheel fine machining
  • Rotary transfer machines
  • Assembly robots
  • Assembly lines
  • Cam form grinding

The Monopoly Position of Sercos I

Sercos I has now become so successful, that a third version has been able to conquer the market. This is because only this digital drive interface satisfies all the requirements for such an application. Among other things, these requirements include:

  • top precision,
  • high speed,
  • minimized upgrade costs,
  • openness, standardization and
  • flexibility.

Only Sercos I and its successors offer these preconditions. As such, the monopoly position of the technology is not surprising. This interface has appealed even to car manufacturers. Essentially, it is found in all fields of automation technology. Additionally, there is also use in modern machine concepts, e.g. those that cover the fields of packaging, textiles, and printing.

The Light Conductor

In this digital drive interface, the light conductors (LWL) can vary in length. In the case of a light conductor made of plastic, a transfer section has a length of 50 m. When classical glass fibers are used, this value is increased fivefold. The exact number (max.) in a drive (the max. value is 254), depends on the required cycle time that is necessary for one communication to pass through. Furthermore, the selected scope of operation data and the data rate itself are decisive. Through the use of several of these types of light conductor rings, the number of drives which are to be found in a control unit can be expanded as desired.

With this digital drive interface, initialization is performed in part by means of a number system, which serves the task of identification. As such, the real data to be transferred in the process are specified in advance. These may be numerical data, including the nominal values and actual values, but also bit lists with I/O instructions.

Advantages of using the Sercos Interface

The internal drive functions of the digital drive interface enable self-monitoring, which by itself already works perfectly. This is done by monitoring the target position values and the actual position values, as well as the drive parameters, which are connected to a forced shut-down which also works in the case of a fault or the failure of the drive processor. Through the logical monitoring of the target values received by the control unit within this drive processor, Sercos I is also capable of completely excluding inadmissible speeds of the axis or run-away caused by faulty or wrongly-transferred target position values. This way, safety redundancy within the control unit is achieved by the monitoring component which acquires the actual values that are reported back. The requirement of immunity against faults is helped by the ring technology. After all, this exhibits the lowest number of optical components and as such, it does not require any complex T-branches. This also offers the opportunity of a forced shut-down.


Sercos II – Immunity against Interference and for more Communication!

The field of IT and automation technology requires a high degree of technical progression. After all, in this field an increase in safety matters as well as a reduction in time and the resulting costs. When processes in complex installations and systems are to be standardized, there are always efforts to find an internationally recognized standard. However, this also presents problems due to the numerous specifications of individual companies and associations. Often, there is no clearly defined uniformity even after years of cooperation between the various groups. Especially during the 90s, the number of applications that could be found on the market increased. This was based on growth within the industry, and with it the need for industrial communication. After all, machines and their components must be able to act with each other and with other components, so that faults can be recognized and corrected and the process can run effectively and efficiently. One standard broke initially through internationally in 1995 under the designation IEC 61491. In 1998, it also established itself in the European segment as EN 61491. This is SERCOS. This acronym stands for “Serial Real-time Communication System”. This designates a digital drive interface that is suited for use in industrial machines. Since the 90s, Sercos (initially as Sercos I (= Interface), then Sercos II and now Sercos III) advanced to form a communication standard that makes it possible to pick up and transfer data in real-time. Because of this, Sercos II is suited to complex motion control applications with high specification designs. In addition to real-time, performance and interference resistance are among the technological characteristics of this drive interface. Furthermore, there are also a large number of products and a wide range of their suppliers.

Specifications of Sercos II

Sercos II was introduced in 1999 and as a matter of course, it includes the SERCOS real-time protocol. The basic protocol is HDLC (“High-level Data Link Control”) which is a standardized network protocol. The physical medium is the light conductor (LWL). This is a cable which is made using plug-in connectors and light conductors. The lines are used to transmit light. In this process, the light is transferred using fibers made of quartz glass and a plastic which is made up of polymer optical fibers. The topology in which the subscribers of the network are arranged, is in the shape of a ring. Synchronization in this system is performed by the respective hardware. When a data transfer is initialized, its speed is in an interval between 2 Mbps and 16 Mbps. This depends on the length of the cable. With Sercos II the cycle time can be configured, and is at least 62.5 μs. The advantage of this digital drive is its short jitter time. This describes the clock jitter that can occur from time to time with the transferring of digital signals. This causes a small fluctuation in the accuracy of the transfer clock. This inaccuracy here is at just < 1μsec, an amount that makes the suitability of the application clear. Each ring of a topology has only one master, but up to 254 subscriber. However the number of rings and the number of subscribers can be increased.

Communication Opportunities of Sercos II

In addition to creating immunity against digital interference, industrial communication is naturally the focus of the Sercos II functions. As such, it is not surprising that here as well, there are many opportunities for communication. The interface basically instructs three different options to realize an interaction between the CNC (Communication Network Control) and the digital drive controller.

  • Transfer of the target position
  • Transfer of target speed
  • Transfer of target torque

Within these three different communication possibilities, the transfer of target positions has proven to be most effective. Accordingly, it represents the optimal solution when it comes to the quick and accurate interaction of applications. This way, within a light conductor ring, up to six individual axes can run in synchronisation every 0.5 ms. With this, they are all provided with target position values (another name for target positions). This provision is cyclic, which means that it always takes place according to a specified order. Sercos II is thus suited to the displaying and inputting of internal drive data, diagnoses and parameters. However, this requires a SERCOS-compatible CNC.

Configuration with Sercos II

The ability to configure the real-time data makes it possible to handle and control any other operation modes independently. However, the exchange of service data is only performed upon instructions by the master. As such, these data are transferred by means of a handshake procedure in portions of 2, 4, 6, or 8 bytes within the “info” service data field. They are them reassembled during the reception process. With Sercos II, the HDLC protocols provided with an NRZI coding are used for this application.


What is a Topology?

Topology is the name for the networking structure of several devices with each other, which enables data exchange of these devices with each other. The special topology of a network determines its reliability: the functionality of the entire system in case of failure of individual components only remains intact with alternate/redundant connections between the components. With this, the physical topology describes the structure of the connections and the logical topology describes the data flow in the connections.

What are flexible topologies of Sercos III?


All Sercos networks are generally composed of at least one master for coordination and a slave for execution of the automation functions. They are arranged simply and clearly in a line or ring topology. This is done without switches or hubs, which leads to enormous cost savings.
With this, each device has at least two Ethernet connections. These are connected via CAT5e Ethernet cables with the other devices.
With this, Sercos III does not use the star topology of the Ethernet.

1. Line Topology

With the line topology (also called linear topology or bus topology), all devices are arranged in series. The master is always at the beginning of the line. Differing from this, in certain cases it also can be located between two lines. The data passes through all slaves, which are connected in series, i.e. one behind the other, and it is directed back from the last device (the so-called loop-back). Independent of the line sequence, all data is guaranteed to reach each participant at least once in a cycle, as all components of the system evaluate the data in two directions.
The advantage of this method is the very easy and cost-efficient networking of all devices with each other, even over long distances (for example with assembly lines).
The disadvantage of this topology is that a failure in one transfer segment is immediately felt in the entire system.
The line topology is used mostly in large production installations and also for connecting production cells.

2. Ring Topology

With additional cables, the network can be connected to a ring. With one line, this works with just one cable between the master and the last slave, with two lines between the two last slaves each. With the ring topology, the master sends data into the ring from both ports, so that these are also evaluated in the ring in two flow directions.
With the flexible topologies of Sercos III, this is not a single ring, but a double ring structure. With this, in each case all components are connected with each other via two connections. Only this interconnection enables a further functionality of the network even with failure of individual connections.
The first advantage of the flexible topologies of Sercos III is the redundant wiring, which enables trouble-free operation and reports faults for repair. Faults can then be repaired without impairment of the system. The network thus always remains available, even in the case of a wire break. Furthermore, each component of the ring topology operates as an amplifier, all components have the same access possibilities, the transmission bandwidth is guaranteed and programming is easy because this is a regular topology.
The disadvantages of the flexible topologies of Sercos III are the relative wiring expenditure, the relatively high diameter of the system and the fact that data transmissions can be intercepted easily.

Ring Redundancy and Hot-plugging

In case of a fault (for example a wire break), the flexible topologies of Sercos III use the ring redundancy. With this, the ability to communicate remains fully intact by quick switching of the transmission path, as the active ring is divided into two lines. With this, the real-time data are transmitted redundant, i.e. on two separate paths (the so-called primary and secondary channel) during normal operation. In case of a fault, the nodes directly on the fault location switch within max. 25 μs from “transfer” to “loop-back”. As the ability to communicate remains even with a wire break, new participants (hot-plugging) or new participant groups (hot-swapping) can also be fitted or connected to the communication network.

3. Other Topologies:

Other possible flexible topologies of Sercos III are hierarchical, cascaded network structures. With this, individual network segments are networked with each other via a ring or line topology.
The advantages of this networking are synchronized network structures coupled in real-time, as well as the ability to generate different cycle times in the individual segments, for example by networking of drives and controls. Furthermore, all participants of the network can communicate with each other in real-time.
The infrastructure component Topo Extension offers new options here for simple wiring of flexible topologies of Sercos III, as it reduces the wiring expenditure considerably. This is done by the device first combining the two required lines of the ring wiring to one cable and then dividing the signals again to two individual cables. In this way, individual components can be connected with a single cable while maintaining the advantages of the flexible topologies of Sercos III.

Installation of the flexible Topologies of Sercos III

The installation of a flexible topology of Sercos III is simple and requires no special network configuration. All devices are connected by patch or cross-over cables with each other. The Ethernet ports of the devices are interchangeable and can also be used to connect standard Ethernet devices (for example notebooks) to a SERCOS III real-time network. In this way, any Ethernet and IP protocols can be used to access devices without influencing the real-time behavior of the SERCOS III network or even having to activate the Sercos III protocol.
The max. number of participants per ring/line for the flexible topologies of Sercos III is 511 participants. Only 254 participants per ring are possible for Sercos I and Sercos II.


What is Sercos III?

Sercos III is a further development of the existing Sercos Interface standard (IEC/EN 61491) based on the standard Ethernet, which is characterized by increased performance and efficiency. Sercos mechanisms, which were already tried and tested, for example the motion control profile specified in the Sercos standard, the telegram structure, and the hardware synchronization, were transferred to the Ethernet standard.

How does Sercos III function?

(1) Basics

To fulfill the requirement of real-time transmission, a collision-free real-time channel is connected in parallel to a non real-time channel. the telegrams defined by Sercos are transmitted with high protocol efficiency in the collision-free real-time channel. By doing this, the best possible performance of Sercos III is obtained even with a high number of users.

This parallel connection also enables transmission of any Ethernet telegrams and IP-based protocols (for example TCP/IP, UDP/IP) in the non real-time channel (also called time slot) without endangering the guaranteed cycle times of the real-time data. This enables, for example, a parameterization without continuous control with a standard notebook and Ethernet interface.
The performance and the efficiency of Sercos III are increased especially by maintaining the advantages of the approach and simultaneous enabling of new features.

This considerably widens the application range of Sercos III:

  • max. possible compatibility with the existing Sercos interface (topology, profile, telegram structures, synchronization) reduces the resulting costs.
  • Hardware costs are reduced.
  • Continuous communication with any Ethernet protocols (for example UDP/IP, TCP/IP) is enabled.
  • Direct cross communication between slaves is possible.
  • Synchronization of multiple controls/machine segments.
  • Safety data can be transmitted without interfering with the operation.
  • Increased fault tolerance, for example in case of an open circuit.

With this, the communication between the components is based on standardized parameter sets for control of device functions. These are independent of the manufacturer, as well as the hardware. When a network is initialized, the configuration specifies the different parameters forming the real-time data of a specific device channel.

By doing this, Sercos III has communication channels and device profiles for all current automation applications that increase the performance and the efficiency of Sercos III:

Real-time Communication Channels

  • M/S (master/slave) device channel: Direct exchange of function data between master and slaves
  • CC (cross communication) device channel: Direct cross-communication between devices, either between controls (C2C) or between drive or peripheral slaves
  • SVC (service channel) channel: Exchange of service data
  • SMP (Sercos messaging protocol) time slot: Transmission of function data of multiple participants by means of a multiplex process
  • Sercos Safety: Exchange of safety data in an MS or a CC device channel. These include, for example, switch-off or acceptance signals or also safe target values.

Universal Communication Channel

  • Standard Ethernet: Connection of any Ethernet device like notebook, webcam, or I/O peripheral device
  • Other Ethernet protocols: Transmission of any Ethernet-based protocols in the Unified Communication Channel (UCC).

Device Profiles

  • Motion profile: Motion and SPS functions for control based on the Sercos motion control parameters
  • Drive profile: Functions for control of drives I/O profile
  • Energy profile

(2) Details

1. Synchronicity

Sercos III refers to the proven hardware synchronization. Each model has a special logic chip (ASIC), which coordinates the system points with each other. This makes additional expensive hubs and switches unnecessary, and the network adjusts itself mostly independently. Synchronicity thus becomes a basic characteristic of each Sercos system.
This synchronicity and the data transfer rate of 100 Mbps (Fast Ethernet) in full-duplex mode results in a min. cycle time of 31.25 microseconds and thus a maximum efficiency and performance of Sercos III.

2. Flexibility

As production processes in most cases require high flexibility, the requirements for the flexibility of the network are also high. Normally the individual components (slaves) are controlled by a control (master), which again communicate with each other via a common network segment.
However, the ability of one component to directly inform another one by cross-communication increases the performance and efficiency of Sercos III. The direct data exchange reduces the load of the central control and reduces the data traffic in the network. The decentralized control of complex production installations is based on the so-called C2C (control-to-control) cross-communication.

3. Safety

The technique of cross-communication between individual nodes not only contributes to flexibility, efficiency, and performance of Sercos III, but also increases the safety, as this makes it possible to establish a Sercos III network with a ring structure. Thus, for example, a redundant signal path can be used in the case of anopen circuit. This is not the case with the classic Ethernet. The data transfer is also safer: Sercos offers a certified security log, the Sercos Safety, which meets the requirements of the safety standard IEC 61508 up to the safety integrity level 3. Sercos Safety is safeguarded against errors like repetition, loss, insertion, wrong sequence, corruption, delay, and confusion of secure data with standard data.

4. Reliability

The combination of performance and efficiency of Sercos III, the flexible application, and checked security meets all requirements ofa modern, continuous automation network. It offers suitability for daily use through the classic Sercos protocol and security for the future by the real-time Ethernet solution.


Since October 2007, all three Sercos generations belong to the international standards IEC 61784-2 (Digital Data Communication in Control Technology) and IEC 61158 (Fieldbus for Industrial Control Systems).
First of all, Sercos III offers not only impressive performance data, but also performs complex automation tasks with high efficiency. Thus, for example, up to 330 drives with 4 bytes I/O data and each 8 digital I/E can communicate with each other in a cycle of one millisecond.
Thanks to the high efficiency and performance of Sercos III, for the foreseeable future there will be no applications that would require a higher speed of the network.
For users as well as for developers, efficiency and performance of Sercos III mean sustainability and planning security for investments in existing and new machine systems.


Summary of Sercos

The latest Sercos is the third and thus the newest model of the Sercos Interface Automation bus series, marketed first in 1985. Since then, the Sercos series has developed into a worldwide standard in automation technology, based on good quality as well as low interference susceptibility of the product innovation. In 1995, Sercos Interface was named as worldwide standard for communication for Motion Control. Today, Sercos has more than 3 million installed Sercos nodes in approximately 350,000 applications.

Innovations in the third generation of the Sercos Series

As the newest and most innovative model of the Sercos series, Sercos III is leading the way in real-time communication in the automation technology. Among other factors, the number of new functions and the high efficiency and performance are distinguishing features of the automation bus.
The following gives a short overview of the new developments:

  • Standard Ethernet is currently being used for transfer.
  • Faster operation because of a higher bandwidth
  • Better synchronization by line delay compensation
  • New communication mechanisms and device profiles for versatile application
  • Cross traffic and other innovative opportunities for communication
  • Quick-acting media redundancy guarantees a higher machine availability.
  • Simplified implementation

The Advantages of Sercos III in Detail

Versatile application

  • Any number of automation devices can communicate via one and the same network, which is based on the Ethernet technology for automation technology.
  • The safety technology of the network is not impaired by any participant – no matter whether it is considered as safe or not.
  • Protocol data can be transferred at the same time as real-time data without affecting the data.

Higher Transfer and Operation Speed

  • Sercos III operates with 100 Mbps, which corresponds to the speed of the Fast Ethernet.
  • Short transfer times, because of the extremely efficient and fast processing of the received telegrams, even while passing through the devices. Furthermore, additional network components (like for example switches) can be dispensed with.
  • Any participants can communicate with each other and directly by cross traffic.


  • The multiplexing process uses the bandwidth optimally.
  • Any cycle time for the communication can be selected, and it can vary between 31.25 microseconds and 65 milliseconds.


  • Synchronization can be realized down to the sub-microsecond range.

Consistent Communication

  • Protocols of the office communication can be transferred even during real-time communication of Sercos III without influencing or impairing each other.
  • Redundancy and synchronization can be realized at the same time.
  • Sercos-III-enabled devices can communicate directly by standard Ethernet via MAC or IP addresses, so that a tunnel becomes unnecessary. Examples of such communications are e-mail and Web services. These occur in the Unified Communication Channel (UCC).
  • Diagnosis and start of operation are possible via standardized interfaces.


  • CIP Safety on Sercos guarantees secure communication with Sercos III by routing even beyond the network boundaries. This eliminates an additional safety bus that normally would assume this function. CIP Safety On Sercos transfers the data in the same way as normal standard data, namely on the same medium. With this, the functionality of the safety protocols lies in the terminal devices. This enables simultaneous operation of standard and safety devices in the Sercos Interface network. This safety is guaranteed for all communication levels and channels.

Always available by variable Topology

  • With the ring topology, communication is maintained even in the case of a node failure or a wire break.
  • Sercos III requires only max. 25 microseconds for recognition of a node failure, so that, at worst, the data of the last cycle is lost before data transfer can be continued without impairment.


  • Control can be centralized as well as decentralized.
  • There are no limits for the architecture of the network, as free arrangement of the participants is possible.
  • Ring topology as well as line topology can be used, as well as hierarchical, synchronized networks coupled in real-time.
  • Hot plugging can be used without impairment of the other functions.

Simplified Configuration and Handling

  • Because of consistent device models and device profiles, the Sercos III devices can be put into operation and can be configured to the respective requirements.
  • The protocol can recognize automatically where which device was connected, which facilitates localization of a fault considerably.
  • The connection sequence of the two ports on the device plays no role.

Cost Savings

  • Additional network components are no longer required (for example switches).
  • The master can be implemented as pure hardware, as hardware and software, or as pure software.
  • The slaves are connected via multi-protocol chips.


  • Sercos III is recognized as the international standard for real-time communication and is standardized as real-time Ethernet protocol Sercos III.

Manufacturer-independent technology

  • All specifications are freely accessible.
  • A membership is not mandatory for use of the Sercos III technology.

Future-oriented technology

  • Sercos products are international standard.
  • The technology is being continuously improved and developed further so as to be able to react to the changed market requirements.
  • Backward compatibility offers a more secure return on investment than conventional products.

Proven on the market through quality

  • Sercos innovations are used in widely differing areas of the automation industry, especially for demanding processes.
  • Sercos products are used and supported worldwide by leading automation manufacturers.