How Ethernet Changes Sercos

By Ronald Larsen

SERCOS (Serial Realtime COmmunications System) is 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 for high-speed serial communication of standard closed-loop real-time data. More than 1.75 million nodes in motion control applications, in all types of automated machinery, use this bus.

The first generation, Sercos I, introduced in 1987, operated at 2 to 4 Mbit/sec. Sercos II, which operates at speeds of 2, 4, 8, and 16 Mbit/sec replaced it in 1999. Both versions use fiber optics as a transport mechanism. Sercos III, introduced in 2005, offers a new generation of real-time communications using Industrial Ethernet to transmit data at a speed of 100 Mbit/sec. It uses the basic elements of Sercos technology, maintaining backwards compatibility.

The Sercos II interface can connect up to 254 devices to a control using a single ring, and multiple rings may be used in a system. Each ring consists of one master and multiple daisy-chained slaves. The 2005 version connects up to 510 devices. In this system, all servo loops are normally closed in the drive. This reduces the computational load on the motion controller, allowing it to synchronize more motion axes than it otherwise could.

The interface allows the use of controls, drives, I/Os and sensors from different manufacturers by regulating all data, parameters, commands and feedbacks exchanged between controls and slave devices.

Since 1995, the interface has synchronized high-performance multi-axis motion control systems. It allows any manufacturer’s compatible digital control to talk to any other compatible digital servo drive, digital spindle drive, hydraulic system, digital I/O or sensors.

Sercos III
In 2003, the Sercos community began combining the mechanisms and properties of this bus with Ethernet physics. The challenge was to combine its deterministic performance with the low-cost and high bandwidth of the non-deterministic Ethernet.

The decision was to put standard Ethernet TCP/IP (Transmission Control Protocol/Internet Protocol) under control of the motion bus, and use Ethernet hardware. A collision-free real-time channel runs in parallel with an optional non-real-time (NRT) channel. This maintains the deterministic motion control of the Sercos interface, preserves the existing protocol structure, allows links to the existing manufacturing communications infrastructure, provides for the possibility of new features, and lowers hardware costs.

Sercos has hardware synchronization for
deterministic data transfer with extremely low jitter. Synchronization
error is typically below 20 ns with the simultaneity error below 100 ns.

Similarities: Rich parameter set: More than 500 parameters define all aspects of motion and I/O control for a variety of devices including servo drives, spindle drives, steppers, hydraulic drives and I/O modules. The communications parameters are identical in all versions; only the parameters that deal specifically with the transport layer are different. And even then, formats, terminology, and other factors are carried over as much as possible.

Deterministic Real-Time Communication:
Both versions include hardware synchronization for deterministic data transfer with extremely low jitter. Nodes derive the synchronization directly from the received real-time frames. High-level synchronization mechanisms, such as IEEE 1588, are not required. Comprehensive measurements show that the synchronization error is below 20 ns and the simultaneity error is below 100 ns.

Message Structures: Devices communicate and stay in sync by sending each other a series of telegrams, a rigidly defined bit steam carrying data and timing information.

Telegrams are: • Master Synchronization Telegram (MST) – Establishes the timing for the system. • Acknowledge Telegram (AT) – Response by the slave devices to the MST. • Master Data Telegram (MDT) – Provides data records for all devices in the loop.

The standard defines a number of IDNs (idents) for real-time data. For example, in each communication cycle, the master transmits a Master Data Telegram that contains a series of IDNs specifying real-time operating data and drive commands for the addressed devices. A slave responds with an Acknowledge Telegram containing IDNs reporting data such as speed, torque and position measurements to the master, effectively closing the control loop.

Standardization: Both versions are standards, where the property is owned and controlled by an independent consortium. Any company can use the technology. Versions I and II have been defined by IEC standard 61491 since 1995. Version III is defined in IEC PAS 62410 (Publicly Available Specification). All versions will be part of the new IEC standard IEC 61784/61158, planned for 2007.

Conformance Testing:
Version II products are tested for both physical and logical conformance with the standard by an independent testing laboratory. The intent of the test is to ensure the compatibility and interoperability of devices from different vendors in multiple vendor environments. Because version III is new, conformance tests are being developed.

Physical Media & Topology:
One of the most obvious differences is that version II uses fiber optics as its physical layer while version III uses Industrial Ethernet. Both use a ring configuration, with straightforward installation. Patch or crossover cables connect nodes.

The unidirectional nature of fiber optics requires that version II be a single ring. However, in version III the full duplex capability of the physical Ethernet structure is used to achieve redundant data transfer based on a double ring topology (logical, not physical) rather than a single ring. Communications are not interrupted if there is a cable fault or node failure anywhere in the ring. An integrated diagnostic function can identify the defective cable link or node, which can then be repaired without affecting machine availability. A linear topology can also be used in version III. This doesn’t offer the redundancy advantage, but does eliminate one cable for cost savings.


Sercos II connects up to 254 devices to
a control using a single ring; Sercos III connects up to 510 devices. A
control system may have multiple rings, each ring consisting of one
master and multiple daisy-chained slaves.

Transmission Speed & Cycle Times:
Version II operates at 2, 4, 8, and 16 Mbit/sec. Version III operates using 100 Mbit/sec Ethernet.

The version II cycle time is specified in a flexible format of 62.5 microseconds, 125, 250, 500 µs, and then multiples of 1 millisecond. Version III cuts the minimum achievable cycle time in half from 62.5 µs to 31.25 µs.

The amount and type of data contained in a cycle is variable. This flexibility lets engineers change 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 permits a higher density of drives per ring. The actual transfer rate that can be achieved depends on the number of drives in a ring and the processing load. The higher Ethernet bandwidth supports a sufficient number of slaves even at the extremely short cycle times.

Operating Modes: Both versions use intelligent drives, where axis-dependent control functions, such as loop closures, interpolation and registration, are placed in the drives, not in the motion controller. 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.

Version II is generally used only in these distributed drive applications, while version III also supports centralized signal processing where only the current loop is closed in the drive, and torque and position loops closed in the control. Thus, the control loops for multiple axes can be implemented in central electronics.

Communications Hardware: Version II uses the SERCON816, an ASIC that operates at 2, 4, 8, and 16 Mbit/sec as a communications controller in both master and slave. It automatically handles most communications functions and reduces the load on the host processor.

For version III, a software core (Sercos III IP) has been developed. With this, manufacturers combine the hardware and their own logic components in one common Field Programmable Gate Array (FPGA), which includes all hardware functions, such as timing, synchronization and processing of cyclic and non-cyclic data on the basis of two integrated Ethernet MACs. Software cores are available for the Xilinx Spartan-3 FPGA platform and the Altera Cyclone II FPGA family.

In addition, Hilscher GmbH has integrated the Sercos core into the netX family; a group of ARM926 based System on Chip network communication controllers. The netX controllers support a number of different industrial Ethernet protocols. Thus, it is possible to implement control and drive devices that can be adjusted to the respective Ethernet protocol by using the appropriate driver software.

New features in version III
Optional NRT Channel (Non-Real-Time): To use Ethernet in hard real-time environments, version III adds a collision-free real-time channel that runs in parallel with an optional non-real-time channel. Telegrams (Ethertype 0x88CD) are transferred on the collision free channel, and the real-time data is processed on the fly as it passes through the nodes. The non-real-time channel can carry Ethernet messages and IP-based protocols including TCP/IP and UDP/IP. The communications cycles and sharing of the 100 Mbit/s bandwidth between real-time and non-real-time channels can be modified to meet the needs of the individual application.

Standard Ethernet communication is possible both during real-time operation and also without any active version III real-time communication. Standard Ethernet devices, such as off-the-shelf PCs, can be connected directly to a slave without special hardware.

Hot Plugging:
Version III allows hot plugging of devices anywhere in a ring or at the end of a line during operation. In addition, standard Ethernet devices, such as notebook computers, can be connected to non-used version III ports to communicate with devices via the NRT channel. This is useful for set-up and diagnostics.

Profile for Controller-to-Controller Communication:
The Controller-to-Controller (C2C) Synchronization and Communication Profile interconnects motion controllers. It defines mechanisms for data exchange between motion controllers, e.g., parameter values, axis commands and status values. It synchronizes controls on a hard real-time level. The specification uses innovative version III features, such as hardware redundancy, hot-plugging and cross communication.

Cross Communication Between Slave Devices: Direct communication between the slaves is not possible with the fiber optic version. However, Ethernet physics enables cross communication between slave devices on a ring or line, and between slaves on different rings and lines through controller-to-controller communication. This occurs in the real-time channel, with all telegrams transparent for each device. This configuration allows shorter reaction times and mutually fast monitoring in axis groups, such as gantry axes.


Sercos III adds a collision-free
real-time channel that runs in paraller with an optional non-real-time
channel. Sercos III uses this channel in Ethernet environments.

Sercos safety features are based on a producer-consumer model supporting single and multi-cast connections. The features route data specific to safety across version III networks and directly to slave devices. Thus, this interface can be used within safety applications up to SIL 3 (Safety Integrity Level) according to IEC61508, even with the shortest cycle times. Version III provides one network for both standard and safe communication. Safety-relevant data are transferred along with real-time data and other standard Ethernet protocols on a single physical medium, without the need for a network between motion control and safety.

In November 2006, ODVA and Sercos International (SI) announced that SI will adopt the CIP Safety specification as its functional safety protocol. ODVA will extend this specification to include safety profiles for Sercos devices. SI will develop the network adaptation to utilize CIP Safety. SI and ODVA jointly will develop and establish conformance testing for devices implementing these protocols.

Inputs and outputs: Version III has an extended device I/O profile, specifying profiles for decentralized I/O modules as well as block and modular I/Os. It also supports hybrid devices that combine several functions in one single device, such as a two-axis controller with I/O and master capability.

Noise Immunity: The fiber optic version is immune to noise. Today’s Industrial Ethernet provides adequate noise immunity for most applications, at a lower cost than fiber optics.

A fiber-optic implementation of version III is planned for those high-noise-critical applications that still require it. This area of Industrial Ethernet is still being defined. For example, small-size, low-cost Ethernet connectors suitable for use in motion control applications are not yet standard.

Higher Performance/Reduced Costs: Compared to previous versions, version III reduces costs through use of the Ethernet physical layer without the need for switches or hubs, the use of low-cost FPGA communications controllers (available from multiple vendors), and simpler handling with CAT5e cable and RJ45 connectors. It offers a higher data transmission rate (100 Mbit/sec compared to 16 Mbit/sec for version II), reduced minimum cycle time (down to 31.25 us compared to 61.5 us for version II) and support of standard Ethernet frames. Direct controller-to-controller and slave-to-slave communications increase the flexibility and possibilities of the interface.

With its protocol extensions and safety features, Controller-to-Controller profile and I/O profile, Sercos III is well suited to take advantage of trends and has transformed itself from a specialized drive interface to a universally applicable real-time Ethernet system.


Sercos International (SI) will adopt
the ODVA CIP Safety specification as its functional safety protocol.
ODVA will extend this specification to include safety profiles for
Sercos devices.

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