1. Industrial Ethernet
Since the 1980s, in order to meet the needs of information integration and comprehensive automation within factories and enterprises, major companies have designed and developed various fieldbus protocols, 20 of which have been incorporated into the IEC 61158 international standard. However, these bus protocols are not compatible with each other, and the implementation cost of fieldbuses is generally high, and the development difficulty is also relatively large, which has hindered the development of fieldbus technology.
Ethernet, a landmark technology in modern technological development, has achieved success in local area networks (LANs) and wide area networks (WANs), and has been widely adopted at the management and process control levels. Further promotion at the field device level would greatly facilitate the vertical integration of the entire enterprise information system. Therefore, most fieldbuses have begun to expand into the Ethernet domain, merging into Industrial Ethernet technology. Industrial Ethernet inherits the physical layer of standard Ethernet and is compatible with its data link layer, thus enabling the use of cost-effective standard PHY chips and RJ45 connectors, reducing costs and development complexity. Currently, there are many types of Industrial Ethernet; the international standard IEC 61784 includes Modbus, Ethernet/IP, ProfiNet, Tcnet, Vnet/IP, Powerlink, EtherCAT, and Sercos III, among others.
2. Real-time performance and related issues
A key criterion for real-time performance is the determinism of time; that is, after an event occurs, the system reacts within a predictable timeframe.
The real-time requirements of industrial applications can be roughly divided into three levels depending on the application scenario: factory-level control requires a response time of about 1 second, while process-level control generally requires a response time of less than 10 to 100 ms. The highest real-time requirement is for motion control, which requires a response time of less than 1 ms, or even tens of microseconds.
Ethernet's transmission rates can reach 100Mbps or even 1Gbps, sufficient for industrial applications, but it cannot guarantee real-time communication between control devices. This is mainly because the standard Ethernet protocol is based on CSMA/CD (Carrier Sense Multiple Access/Collision Detection) technology. Workstations on the network "listen" to the bus to confirm its idle status. If idle, they begin transmitting data. If two workstations attempt to transmit data simultaneously, a collision occurs. In this case, the access mechanism first ensures the workstation stops transmitting data, and then, according to a predefined random selection algorithm, the workstation attempts to transmit data again. This process repeats until the collision disappears. While this mechanism guarantees secure data transmission, it is a significant obstacle from a deterministic behavior perspective. It allows data transmission time to be arbitrarily delayed, thus preventing real-time communication. To better apply Ethernet technology to industrial control without altering its existing standards, a solution to this problem needs to be found.
3. Real-time solutions for industrial Ethernet
To improve the real-time performance of Ethernet, the communication model must be modified. Currently, there are three possible solutions:
(1) Modify the TCP (UDP)/IP protocol stack to add real-time scheduling to control uncertainties in communication. Typical protocols include Modbus/TCP and Ethernet/IP. The response time of this solution is tens of milliseconds, which is generally called soft real-time industrial Ethernet and is suitable for factory-level control applications with real-time requirements of more than 100ms.
For example, Modbus/TCP does not modify the Modbus protocol itself, but embeds Modbus frames into TCP frames and adopts a connection-oriented approach, requiring a response for each request. This request/response mechanism, in conjunction with the Modbus master/slave mechanism, gives switched Ethernet a high degree of determinism.
(2) Modify the protocols above the data link layer, such as changing the frame structure and optimizing the scheduling method, to ensure real-time performance without changing the hardware solution. Typical protocols include EthernetPowerlink, Profinet, and EPA. The response time of this solution is 1 to 10 milliseconds, and it is generally called hard real-time industrial Ethernet , which is suitable for process control.
For example, EthernetPowerLink introduces the SCNM (Time Slot Communication Network Management) algorithm to ensure real-time communication. SCNM allocates time slots for synchronous and asynchronous data, ensuring that only one device can occupy the network at a time, thus avoiding network conflicts. In communication management, it introduces management nodes (MN) and control nodes (CN), and divides the communication cycle into start phase, synchronous phase, asynchronous phase, and idle phase. This allows each cycle to have a corresponding time domain for transmitting real-time data and standard Ethernet data streams, ensuring both real-time data communication and the transmission of standard Ethernet data.
(3) Modify the data link layer protocol so that the real-time MAC takes over communication control in the real-time channel, avoiding message conflicts and simplifying data processing. Typical protocols include EtherCAT, SERCOSIII, and MECHATROLINKIII. This scheme has precise clock synchronization, a response time of 250μs to 1ms, and jitter of less than 1μs. It is generally called synchronous hard real-time industrial Ethernet and is mainly used in the field of motion control.
For example, the EtherCAT protocol uses a master-slave architecture. The master station is responsible for initiating EtherCAT message frames. Because it uses the standard Ethernet frame header and trailer, the master station can use the standard Ethernet physical layer and data link layer, i.e., the standard network port of a PC. The slave station needs to extract the input data from the data frame according to the address and insert the output data according to the frame format. This work is done by the FMMU module at the data link layer and requires a dedicated MAC layer chip.
4. Summary
The basic approach to improving Ethernet real-time performance is twofold: first, to clearly define the transmission channel and avoid collisions; and second, to reduce processing time and improve response speed. The first point can be implemented at the MAC layer, the TCP/IP layer, or even layers above TCP. However, the closer the implementation is to the application layer, the more steps are involved in reusing standard Ethernet, the more complex the encoding and decoding processes become, and the worse the real-time performance. Implementations closer to the physical layer require abandoning the common Ethernet protocol and even using dedicated MAC chips, which increases development difficulty and cost.
