Sercos

Sercos (Serial Real-time Communication System) originated in the late 1980s when the transition from analogue to digital drive systems in industrial machinery necessitated a new method of communication that could fully exploit the capabilities of digital technology. To address this need, the German Machine Tool Builders' Association (VDW) and the German Electrical and Electronics Industry Association (ZVEI) collaborated to develop an open interface specification suitable for digital-drive systems. This collaboration led to the creation of the initial version, Sercos I, introduced in 1990 and provided a standardized digital interface for real-time communication between industrial controls and motion devices over fiber-optic cables. 

As industrial automation needs evolved, so did the Sercos protocol. The mid-1990s saw the introduction of Sercos II, which brought improvements such as increased data transmission rates and greater flexibility in network topology. The protocol's ability to achieve cyclic update rates as low as 62.5 microseconds is crucial for applications demanding high precision and dynamics. By 1999, Sercos II had expanded the supported data rates to include 2, 4, 8, and 16 Mbit/s. The third generation, Sercos III, emerged in 2003, merging the hard-real-time aspects of its predecessors with the Ethernet standard, thus broadening the scope of its application and device support. This version allowed for a single master to control up to 511 slaves, with both master and slave devices equipped with two real-time Ethernet ports. 

Sercos is recognized for its high-speed, real-time communication capabilities between various industrial controls, motion devices, input/output (I/O) units, and other peripheral devices, as well as standard Ethernet nodes. The protocol's design allows for the coordination of complex machinery and systems, such as metal cutting tools, packaging machinery, and robotics, ensuring precise and synchronized operations. In terms of safety, Sercos employs the CIP safety protocol for transmitting safety-oriented data, which is also used for Ethernet/IP and DeviceNet, ensuring a high level of safety in industrial environments. Additionally, Sercos real-time data is sent to IEEE 802.3 in cyclical telegrams with Ethernet protocol type 0x88CD, further emphasizing its technical sophistication. 

Sercos protocol provides collision-free industrial communication through a time-slot mechanism, highly efficient communication protocols with minimal overhead, and extremely low telegram jitter. It also specifies over 700 standardized parameters, ensuring that devices from different manufacturers can be combined without compatibility issues. These attributes contribute to its ability to control numerous axes of motion with high update rates, making it a reliable choice for demanding industrial applications. 

Looking forward, Sercos evolves with ongoing development of new function profiles to meet modern industrial demands while maintaining its renowned standardization and interoperability. The Sercos International association, including manufacturers, users, and research institutions, remains dedicated to advancing and promoting the protocol globally, ensuring its leadership in standardized communication for motion control applications. 

In the Sercos communication architecture, a master-slave model is employed where the master device orchestrates the communication process and coordinates data exchange with the slave devices. With the master device often being a programmable logic controller (PLC), an industrial PC (IPC), or a motion controller equipped with Sercos master functionality. Slave devices in this setup can range from servo drives to sensors and actuators, which respond to the master's commands and provide real-time data crucial for industrial automation application. Widely adopted across a spectrum of industrial domains, Sercos protocol variants include: 

Sercos I/II

In this variant, the master device communicates with multiple slaves over high-speed fiber optic or copper cables. This setup provides deterministic, real-time communication for precise motion control and synchronization in industrial automation applications This protocol supports cyclic data exchange, ensuring synchronous system operation. Sercos I use synchronous serial communication via fiber optic cables, while Sercos II offers enhancements like expanded device support and improved transmission rates. Both variants utilize token passing for communication, where the master controls data exchange by passing tokens to slaves in a predefined sequence. 

Sercos III

Sercos III advances from Sercos I/II by adopting Ethernet as its physical layer, enhancing bandwidth and compatibility with standard Ethernet networks. It caters to both real-time and non-real-time communication needs. Leveraging Ethernet-based communication, it employs standard TCP/IP and UDP/IP protocols for non-real-time data exchange alongside real-time communication. Unlike Sercos I/II, Sercos III utilizes Ethernet's built-in mechanisms for data transmission, eliminating the need for token passing. Notably, Sercos III integrates safety functions directly into the protocol, meeting international safety standards. 

At the heart of this system lies the OSI model, which organizes the various layers involved in Sercos communication. The physical layer of the OSI model is responsible for the physical transmission of data over the network medium. It defines various characteristics, including the types of cables and connectors used, as well as the transmission rates. The data link layer, on the other hand, ensures the reliable transmission of data frames between devices, encompassing frame structuring and error handling. 

Sercos supports different cable types, such as fiber-optic and copper cables, to suit various industrial environments. Fiber-optic cables are particularly suited for long-distance communication due to their high noise immunity, while copper cables offer a more cost-effective solution for shorter distances. Connectors commonly used in Sercos networks include SC and ST connectors for fiber optics, and RJ45 connectors for copper cables. The transmission rates across Sercos variants can range from 2 Mbps to 100 Mbps, facilitating efficient and fast data exchange that is essential for real-time monitoring and control of industrial processes. 

At the data link layer of the OSI-Model, Sercos organizes data into frames that consist of a header, payload, and trailer. The header includes control information such as the frame type and destination address, the payload carries the actual data, and the trailer contains error detection and correction codes to ensure the integrity of the data during transmission. Robust error handling mechanisms, like cyclic redundancy check (CRC), are implemented to detect and correct any errors, thereby maintaining reliable communication within industrial setting. 

The transport layer, or Layer 4, plays a pivotal role in Sercos networks by facilitating end-to-end communication between devices. It provides essential services such as error detection, flow control, and data segmentation, ensuring that data exchange is both reliable and efficient. 

Real-time data transmission is a pivotal aspect of Sercos, as it allows for precise control and synchronization of devices within industrial settings. This ensures that essential data, such as position and velocity commands, are exchanged within predefined time intervals, which is vital for the timeliness of control actions. Deterministic communication is another integral feature of Sercos, guaranteeing that data packets are delivered within known and consistent timeframes. This predictability in communication latency is indispensable for applications that require exact timing, like motion control and synchronization tasks. 

Safety is directly integrated into the Sercos protocol, which complies with international safety standards such as IEC 61508 and SIL (Safety Integrity Level) requirements. By incorporating safety functions directly into the communication protocol, Sercos facilitates the implementation of safety-critical applications while ensuring interoperability with standard automation processes.  

Sercos supports various network topologies, including line, ring and hybrid configurations, catering to diverse industrial automation requirements. 

In the Sercos network, master devices play a pivotal role as central controllers that coordinate communication within the network. They initiate communication cycles, poll slave devices, and synchronize data exchange to ensure seamless operation. Slave devices, on the other hand, are peripheral components such as drives, sensors, and actuators. These devices respond to the master's commands and provide essential real-time data for the network's functioning. 

The physical connectivity between devices in the Sercos network is facilitated by communication interfaces like communication controllers, Embedded Modules or PC cards. Controllers and PLCs with Sercos-compatible communication interfaces are integral for integrating Sercos networks into broader automation systems. These industrial controllers and PLC’s serve as the backbone of network integration. 

On the software side, firmware embedded within hardware devices like master controllers and slave devices is crucial. This firmware is responsible for implementing the Sercos communication protocols and managing the data exchange process. Device drivers are software modules installed on host systems, enabling communication with Sercos devices. These drivers are essential for the configuration, control, and monitoring of Sercos networks from various software applications. 

Configuration tools are software applications or programming environments used for setting up Sercos networks. They play a critical role in assigning network addresses, configuring device parameters, and monitoring network performance. 

Sercos has been pivotal in revolutionizing industrial communication for more than two decades, presenting a plethora of advantages, including:

Sercos stands as a cornerstone technology in the contemporary industrial landscape, offering a diverse array of solutions across various industries. Its versatility finds expression in applications such as: 

Sercos I/II finds applications in various manufacturing processes requiring precise motion control, such as CNC machining, metalworking, and assembly lines. It is commonly used in industries where high-speed, deterministic communication is essential for achieving tight tolerances and optimal productivity. In the oil and gas sector, Sercos I/II is utilized in equipment such as pumps, compressors, and drilling machinery for controlling motion and monitoring critical parameters. Its deterministic communication capabilities are valuable in environments where precise control and synchronization are necessary for safe and efficient operation. 

Sercos III is well-suited for modern manufacturing environments adopting Industry 4.0 principles, where integration with Ethernet-based IT networks and the Industrial Internet of Things (IIoT) is essential. This capability allows multiple machines to be linked, enhancing the modularity of machines or production lines—a significant advantage for manufacturers who may need to reconfigure or expand their operations. The protocol's real-time features are essential for applications that demand high precision and dynamics, making it an important solution for challenging automation task. Its Ethernet-based communication facilitates seamless integration with supervisory control and data acquisition (SCADA) systems, enabling efficient management of critical assets and processes.