How to Implement EtherCAT Redundancy Technology for Cable Breaks?

The fieldbus technology is a hot topic in the field of automation control, applied to real-time communication between multiple devices. In device connections, if a cable breaks at any point, it will affect communication between devices. Let’s take a look at how EtherCAT redundancy technology can remedy and lock communication in the event of a cable break.

What is Cable Redundancy

EtherCAT fieldbus has a flexible topology, supporting linear, star, and tree cable connections between devices. The linear structure is simple and has the highest transmission efficiency, and this connection method is also used in most field applications, as shown in Figure 1 below.

Figure 1 Linear Structure Topology Diagram

The linear connection method is indeed simple, flexible in wiring, and convenient for the layout and maintenance of field devices. In automated industrial production, equipment usually runs for long periods in different environments, and factors such as cable aging and insufficient installation connections can lead to cable breaks. If one day the cable between IO card 1 and IO card 2 breaks, will the devices behind IO card 1 be unable to operate normally? As shown in Figure 2 below.

Figure 2 Cable Break Example Diagram

No matter what type of wiring method is used, a cable break will affect the normal operation of the equipment. Even traditional CAN, RS485, and other communication devices cannot operate normally. The problem needs to be solved: is there a standard solution that does not require too much additional design cost to address the aforementioned issues? Let’s take a look at the solution provided by the EtherCAT bus and the implementation principles of cable redundancy technology. First, let’s look at its connection method, as shown in Figure 3 below.

Figure 3 Cable Redundancy Wiring Diagram

From the EtherCAT cable redundancy wiring diagram, it can be seen that the OUT terminal of the last slave device is reused to connect back to the master station, which is quite clever, right? It reduces hardware costs and solves the problem, and is indeed favored by everyone. Let’s take a closer look at its data flow. Suppose the cable between IO card 1 and 2 is still broken, its working principle is shown in Figure 4 below.

Figure 4 Cable Redundancy Principle Diagram

After the cable between IO card 1 and 2 is broken, it is still connected to the slave device, but the communication line becomes two branch lines, and the devices can still communicate normally, while the software layer can continue to control the operation. This is the solution for EtherCAT’s cable redundancy, turning a linear structure into a ring structure, achieving link redundancy functionality. The electrical layer device connection has been resolved; let’s continue to see how it is implemented on the software layer.

Implementation of Redundant Master Station

The communication system generally consists of a master station and slave devices, where the master station is usually the control end, and the slave is the execution end. We have learned about the wiring method of EtherCAT’s electrical layer cable redundancy. Now let’s see how the redundant master station is implemented?

1. Slave Operating Principle

EtherCAT slave devices in the link receive the Ethernet frame from the master station, copy their data from the Ethernet frame, write the current data, and forward the new Ethernet frame to the next slave device.

2. Master Operating Principle

The master station, as the control end, actively requests data, and the slave responds. Therefore, the redundancy function is mainly implemented in the protocol stack on the master station side.

In the EtherCAT frame structure, each time data is input from the slave IN terminal, the slave increments the Cnt value by 1. In the data frame returned by the slave, the master station checks the Cnt value. If it differs from the configured value, it is deemed a network exception, and the specific location of the abnormal slave can be identified based on the Cnt. The EtherCAT frame is shown in Figure 5 below.

Figure 5 EtherCAT Frame Capture

When the master station detects a network exception, the protocol stack flexibly changes the data flow direction, turning into two branches for control. At this time, the redundant port and the communication port function the same, while in normal conditions, the redundant port is only responsible for forwarding. The data flow direction is shown in Figure 6 below, with the blue circles representing the sending direction and the green circles representing the receiving direction.

Figure 6 EtherCAT Data Flow Direction Diagram

Hardware Redundancy Technology

In the implementation of the redundancy function mentioned above, it is mainly handled by the protocol stack, which belongs to the application layer. After the protocol stack processes the data, it is sent to the hardware. There is some loss in between, and in applications with high requirements for PDO cycles, such as a 256us communication cycle, quick redundancy response is required; otherwise, too many packets will be lost, failing to meet application needs.

ZLG Zhiyuan Electronics PCIe EtherCAT communication card supports EtherCAT cable redundancy function, which can maintain communication between the master and slave devices even if the cable is physically interrupted at some point. At the same time, the redundancy function is designed using a hardware implementation scheme for quick response and lower packet loss rate, as shown in Figure 7 below.

Figure 7 Hardware Redundancy Example

1. FPGA Break Handling

The PCIe EtherCAT communication card uses FPGA for Ethernet data transmission and reception, providing faster speeds. While the FPGA receives data, it also detects any break in the link with all slave devices. If a slave break is detected, the FPGA continues to send data from the redundant port without going through the protocol stack, still maintaining a complete link. The data flow direction is shown in Figure 8 below.

Figure 8 Hardware Redundancy Data Flow Direction

2. Hardware Redundancy Performance

The FPGA continues to send data from the redundant port without going through the protocol stack process, thereby increasing response speed and reducing data packet loss rate.

The Role of Redundancy Technology and Product Applications

1. The Role of Redundancy Function

• Cost-saving in Design

  EtherCAT cable redundancy uses the OUT port of the last slave device, allowing the redundancy function to be standardized.

• Enhancing Reliability and Stability of Communication Systems

  In the industrial automation industry, it is usually required that devices on the bus operate continuously without stopping production. Redundancy technology can achieve reliability and stability in application systems.

• Fault Diagnosis and Handling

  When a cable break occurs, the control continues to work with two links, and EtherCAT can automatically detect the fault point in the bus system, greatly simplifying system maintenance and improving device maintainability.

2. Product Applications

Figure 9 PCIe EtherCAT Communication Card

Zhiyuan Electronics PCIe EtherCAT Communication Card is a PCI-based EtherCAT bus communication interface card. It uses an advanced FPGA control scheme in the industrial field, achieving extremely high communication speeds and strong real-time performance. The PCIe EtherCAT communication card comes in MiniPCIE, half-card, and full-card designs, compatible with any type of 3.3 V/DC MiniPCIE and PCI slots. The EtherCAT communication card has the following advantages:

• The PCIE communication card integrates a commercially licensed EtherCAT master station solution;
• Supports CoE, FOE, FSOE, hot-swappable slaves, master station hardware redundancy, and other functions;
• Minimum PDO cycle of 125μs, jitter of ±5μs;
• High-speed PCIe interface communication, supporting multiple operating system platforms;
• The PCIe interface offers higher efficiency in expandability and supports multiple platform operating systems.

The selection table is shown in Table 1 below.

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