Abstract: PROFINET is a new real-time Ethernet standard. This paper introduces the technical characteristics of two types of real-time communication with different performance in PROFINET, and analyzes isochronous real - time communication technology, explaining its improvement over IEEE 1588. Keywords: PROFINET; real-time communication; isochronous 1. Overview PROFINET real-time Ethernet is an Ethernet-based automation standard proposed by PROFIbus International (PI). Starting in April 2004, PI and the Interbus Club collaborated on the development and formulation of the standard. PROFINET constitutes an architecture for component-based distributed automation systems, from the I/O level to the coordination and management level, and can seamlessly integrate PROFIBUS and Interbus fieldbus technologies throughout the system. PROFINET provides minimum performance guarantees for critical tasks and best-effort service for non-critical tasks. 2. PROFINET Real-Time Communication Classification: PROFINET distinguishes between two types of real-time periodic communication with different performance levels: Real-Time (RT) communication, mainly used in factory automation, which does not have time synchronization requirements and generally only requires a response time of 5-10ms; and Isochronous Real-Time (IRT), mainly used in applications with stringent time synchronization requirements, such as motion control and electronic gears. Correspondingly, PROFINET provides two types of real-time communication channels: RT real-time channels and IRT real-time channels. It also includes a standard communication channel, which is a non-real-time communication channel using the TCP/IP protocol, mainly used for device parameterization, configuration, and reading diagnostic data. Real-time channel (RT) is a soft real-time (SRT) scheme that bypasses the TCP/IP layer. To optimize communication, PROFINET RT frames define message priorities according to IEEE 802.1Q/P, with a maximum of seven levels. The status information field in the PROFINET RT frame identifies the status of the device and data (e.g., running, stopped, error). Its communication protocol and frame structure are shown in Figure 1. Figure 1: PROFINET RT communication protocol and frame structure. Real-time channel (IRT) is a hard real-time (HRT) scheme. Its real-time performance is based on a time-triggered protocol built on Fast Ethernet Layer 2, guaranteed by an embedded Switch-ASIC synchronous real-time switching chip. This further reduces the processing time of the communication stack software, making it particularly suitable for high-performance transmission, isochronous transmission of process data, and fast clock-synchronized motion control. Because it is hardware-based, the IEEE 802.1Q VLAN identifier in the RT frame is typically not required in the IRT frame. The PROFINET IRT communication protocol and frame structure are shown in Figure 2. Figure 2 PROFINET IRT communication protocol and its frame structure As can be seen from Figure 1 and Figure 2, PROFINET real-time data frames (including RT and IRT frames) are slightly modified based on the standardized frame format defined by IEEE 802.3, so that the value of its L/T field is >1500. This is a reserved EtherType II, which can be used to uniquely identify PROFINET real-time data frames to distinguish them from other Ethernet frames using the standard IT protocol, and prioritize their transmission. In the Ethernet type identifier of PROFINET, 0x0800 is used to identify IP frames and 0x8892 is used to identify PROFINET real-time frames. The application identifier (frame-ID) field in the frame identifies the transmission of the received data, that is, it identifies periodic transmission and non-periodic transmission (alarms and events). According to the data provided by the PROFIBUS International Organization [1], PROFINET in IRT communication mode is used in synchronous motion control applications, and its performance is 100 times better than the current fieldbus solution. This hardware-based synchronous real-time (IRT) communication solution can maintain a sufficiently high time determinism when a large amount of data needs to be transmitted; at the same time, it can alleviate the communication task of the processor on the PROFINET device. Therefore, this paper will analyze it in detail below. 3. Improvement of IEEE 1588 by PROFINET The time synchronization protocol adopted by PROFINET IRT is based on the improved IEEE 1588[2]. The basic function of IEEE 1588 is to keep the most accurate clock (reference clock) in the distributed network synchronized with other clocks. It defines a precision time protocol PTP (Precision Time Protocol) for sub-microsecond synchronization of clocks in sensors, actuators and other terminal devices in standard Ethernet or other distributed bus systems using multicast technology. An IEEE 1588 precision clock (PTP) system includes multiple nodes, each of which can be considered to represent a clock. The clocks are connected to each other via the network. IEEE 1588 divides the clocks in the entire network into two types: ordinary clock (OC) and boundary clock (BC). A clock with only one PTP communication port is an ordinary clock, and a clock with more than one PTP communication port is a boundary clock. Each PTP port provides independent PTP communication. Boundary clocks (BCs) are typically used in network devices such as bridges (switches) and routers to subnette and prevent large latency jitter. Regular clocks are usually used on nodes. With temperature changes and the passage of time, the clock frequencies of sending and receiving nodes will deviate, causing drifts. Therefore, PTP requires a closed-loop control for compensation, as shown in the following PI-loop example. Here, y[k] is the controlled variable, x[k] is the deviation variable, and k represents the synchronization loop. K[sub]R[/sub] and K[sub]n[/sub] are control parameters. T is the sampling time, which is equal to the transmission interval between PTP synchronization messages. Each PTP slave clock and each slave clock port of the BC must contain similar closed-loop control; the design of the closed-loop control directly affects the accuracy of time synchronization. As shown in the upper part of Figure 3, the clock oscillator in the BC will be adjusted according to a certain function relationship with reference to the PTP Slave. The adjusted clock will become the PTP Master of the next network segment. This process is repeated until the destination node, Time Client, is reached. In this way, when there are multiple bridges linked in a bus topology, this method actually creates a cascade of control loops, which can lead to instability and make it impossible for IEEE 1588 to meet the stringent synchronization requirements[3]. [align=center] Figure 3 Comparison of IEEE 1588 Boundary Clock and PROFINET Bypass Clock[/align] Compared with the "Boundary Clock" of IEEE 1588, PROFINET has made a correction, which is called the Bypass Clock (BpC)[5]. The key problem of time synchronization is the unpredictable delay in network devices such as bridges, which causes time jitter. If a method for calculating the delay in a bridge can be found, it can be compensated. As shown in the lower half of Figure 3, PROFINET's BpC is based on this idea and compensates for the delay by performing necessary operations and processing on PTP messages. The specific processing operations may involve trade secrets and have not yet been publicly reported. The basic idea is as follows: 1) Assuming a PTP bridge receives a Sync message on port s, the receive timestamp T[sub]rx,s[/sub] will be generated. When other ports j of the BpC start transmitting the Sync message downstream, the send timestamp T[sub]tx,j[/sub] will be generated. Thus, the required clock correction value can be obtained: T[sub]tx,j[/sub] - T[sub]rx,s[/sub]. 2) As shown in Figure 4, Ldi represents the delay caused by the transmission distance, and bdi represents the delay within the bridge. The obtained bridge-wide delay bdi and the current segment transmission delay Ldi are added to the Sync message to be forwarded. This allows the destination node to obtain the precise delay experienced by the message. The cumulative delay from the PTP master clock to the PTP slave clock is: Using this method, the bridge can be viewed as a network component with a constant delay, thus avoiding the cascading of control loops. Figure 4. Delay Overlay 5. Conclusion: PROFINET will greatly improve the communication bottlenecks in the development of existing automation technologies. Simultaneously, it will shift automation technology from a focus on control tasks to a focus on highly integrated and optimized information collection, analysis, and processing tasks, making control task fulfillment a lower-level requirement for future automation platforms. This paper hopes that a detailed analysis of PROFINET real-time communication will contribute to the future development of China's own industrial Ethernet standards. References [1] PNO. Profibus User Organization. www.profibus.com, 2004. [2] IEEE. IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems. IEEE, New York, 2002. ANSI/IEEE Std 1588-2002 [3] M. Mueller and K. Weber. Impact of Switch Cascading on Time Accuracy. In Workshop on IEEE-1588, Standard for a Precision Clock Synchronization Protocol for Networked Measurements and Control Systems. National Institute of Standards and Technology (NIST), September 2003. [4] Zhang Yan, Sun Hexu, Lin Tao. Application of IEEE1588 in Real-Time Industrial Ethernet. Microcomputer Information, 2005, 21(9-1):19-21 [5] J. Jasperneite, E. Elsayed. Investigations on a Distributed Time-triggered Ethernet Realtime Protocol used by PROFINET. In the 3rd International Workshop on Real-Time Networks, Catania, Sicily, Italy, July 2004. (Innovation of this paper: PROFINET is a new real-time Ethernet standard. This paper introduces the technical characteristics of two different types of real-time communication in PROFINET, and provides a detailed analysis of isochronous synchronous real-time communication technology, explaining its improvement over IEEE 1588.) About the authors: Peng Jie (1976-), male, from Ji'an, Jiangxi Province, PhD from the School of Optoelectronic Engineering, Shanghai University of Science and Technology, research direction: control networks. Contact: P.O. Box 244, Shanghai University of Science and Technology, forwarded by Ying Qiga, 200093, email: [email protected]. Li Xiuyuan (1966-), male, PhD from the School of Optoelectronic Engineering, Shanghai University of Science and Technology, Associate Professor, research direction: control networks. Ying Qiga (1942-), male, Professor, Doctoral Supervisor, research field: control networks, automation instrumentation.