The increasingly connected world is inevitably connecting the factory floors. Human machine interfaces (HMIs), programmable logic controllers (PLCs), motor control and sensors need to be connected in a scalable and efficient way. The industrial Ethernet has gained popularity, becoming more ubiquitous and offering higher speed, increased connection distance, and the ability to connect more nodes. There are many different industrial Ethernet protocols driven by various industrial equipment manufacturers. These protocols include EtherCAT
, PROFINET, EtherNet/IP
, Sercos III and CC-Link, among others. Let's take a quick tour of these protocols.
Industrial Automation Systems
There are four major components in industrial automation including PLC controllers, HMI panels, industrial drives and sensors. The PLC controller is the brain of an industrial automation system; it provides relay control, motion control, industrial input and output process control, distributed system, and networking control. The HMI is the graphical user interface for industrial control. It provides a command input and feedback output interface for controlling the industrial machinery. An HMI is connected through common communication links to other parts of industrial systems. Industrial drives are motor controllers used for controlling optimal motor operation. Sensors are the hands and legs of the industrial automation system that monitor the industrial operation conditions, inspections, measurements, and more, in real time. Communication is the backbone of all the industrial components for efficient automation production systems.
Industrial Ethernet Communication Protocols
Ethernet is becoming ubiquitous and cost effective, with common physical links and increased speed. As such, many industrial communication protocols are moving to Ethernet-based solutions. Ethernet communications with TCP/IP typically are non-deterministic, and reaction time is often around 100 ms. Industrial Ethernet protocols use a modified Media Access Control (MAC) layer to achieve very low latency and deterministic responses. Ethernet also enables a flexible network topology and a flexible number of nodes in the system. Let’s look at some of the popular Industrial Ethernet protocols in detail.
- EtherCAT was originally developed by Beckhoff to enable on-the-fly packet processing and deliver real-time Ethernet to automation applications and that can provide scalable connectivity for entire automation systems, from large PLCs all the way down to the I/O and sensor level. EtherCAT, a protocol optimized for process data, uses standard IEEE 802.3 Ethernet Frames. Each slave node processes its datagram and inserts the new data into the frame while each frame is passing through. The process is handled in hardware so each node introduces minimum processing latency, enabling the fastest possible response time. EtherCAT is the MAC layer protocol and is transparent to any higher level Ethernet protocols such as TCP/IP, UDP, Web server, etc. EtherCAT can connect up to 65,535 nodes in a system, and EtherCAT master can be a standard Ethernet controller, thus simplifying the network configuration. Due to the low latency of each slave node, EtherCAT delivers flexible, low-cost and network-compatible industrial Ethernet solutions.
- EtherNet/IP is an industrial Ethernet protocol originally developed by Rockwell. Unlike EtherCAT, which is MAC-layer protocol, EtherNet/IP is application-layer protocol on top of TCP/IP. EtherNet/IP uses standard Ethernet physical, data link, network and transport layers, while using Common Industrial Protocol (CIP) over TCP/IP. CIP provides a common set of messages and services for industrial automation control systems, and it can be used in multiple physical media. For example, CIP over CAN bus is called DeviceNet, CIP over dedicated network is called ControlNet and CIP over Ethernet is called EtherNet/IP. EtherNet/IP establishes communication from one application node to another through CIP connections over a TCP connection, and multiple CIP connections can be established over one TCP connection. EtherNet/IP uses the standard Ethernet and switches, thus it can have an unlimited number of nodes in a system. This enables one network across many different end points in a factory floor. EtherNet/IP offers complete producer-consumer service and enables very efficient slave peer-to-peer communications. EtherNet/IP is compatible with many standard Internet and Ethernet protocols but has limited real-time and deterministic capabilities.
- PROFINET is widely used industrial Ethernet by major industrial equipment manufactuerers such as Siemens and GE. It has three different classes. PROFINET Class A provides access to a PROFIBUS network through proxy, bridging Ethernet and PROFIBUS with a remote procedure calling on TCP/IP. Its cycle time is around 100ms, and it is mostly used for parameter data and cyclic I/O. The typical application includes infrastructure and building automation. PROFINET Class B, also referred as PROFINET Real-Time (PROFINET RT), introduces a software-based real-time approach and has reduced the cycle time to around 10ms. Class B is typically used in factory automation and process automation. PROFINET Class C (PROFINET IRT), is Isochronous and Real-Time, requiring special hardware to reduce the cycle time to less than 1ms to deliver the sufficient performance on the real-time industrial Ethernet for motion control operations.
- Sercos III is the third generation of Serial Real-time Communication System (Sercos). It combines on-the-fly packet processing for delivering real-time Ethernet and standard TCP/IP communication to deliver low latency industrial Ethernet. Much like EtherCAT, a Sercos III slave processes the packet by extracting and inserting data to the Ethernet frame on-the-fly to achieve low latency. Sercos III separates input and output data into two frames. With cycle times from 31.25 microseconds, it is as fast as EtherCAT and PROFINET IRT. Sercos III supports ring or line topology. One key advantage to using ring topology is communication redundancy. Even if the ring breaks due to failure of one slave, all remaining slaves still get the Sercos III frames with input/output data. Sercos III can have 511 slave nodes in one network and is most used in servo drive controls.
- CC-Link IE is the industrial Ethernet technology of CC-Link, which was originally developed by Mitsubishi. CC-Link IE has two versions: CC-Link IE Control and CC-Link IE Field. CC-Link IE control is intended for controller-to-controller communications and can have 120 nodes per network. CC-Link IE field is intended for I/O communications and motion control, and it can have 254 nodes per network. CC-Link IE leverages the Ethernet data link layer, and its control frames are directly embedded in the Ethernet frame. Only ring topology is supported in CC-Link without switches. This can provide network redundancy, but a limited number of nodes can be supported in a network, and the cycle time is dependent on the number of the nodes in the network.
- Modbus /TCP, an extension of Modbus, was originally developed by Schneider Electric and uses Modbus messaging over TCP/IP on top of Ethernet. Modbus/TCP is simple to implement on the standard Ethernet network, but it does not guarantee real-time and deterministic communications.
To learn more about Industrial Ethernet Communication Protocols, please download below the pdf by Texas Instruments called, An inside look at industrial Ethernet communication protocols, which was the source of information for this document.
 | An inside look at industrial Ethernet communication protocols by Texas Instruments.pdf |