Preface The NS21 product can completely replace Siemens' OLM in implementing Profibus fiber optic fieldbus, and can form various networks such as star, chain, and dual-ring self-healing topologies. Both products can achieve a maximum data transmission rate of 12 Mbps. One of the purposes of using these two devices to form a system communication network is to improve the system's real-time performance. What are the differences in real-time performance between communication networks composed of these two products? What impact will this have on our system design? This is the question this article aims to address. Regarding timing performance parameters , the OLM product manual does not directly describe the timing parameters. The relevant information is excerpted below: 1. Timing parameter calculations for ring network formation (excerpted from Siemens product manual) 2. Minimum values ​​of OLM timing parameters NS21 timing parameters: When there are only two point-to-point devices, the signal delay is 2.8-3.0 µs. For each additional device, the delay increases by 1.6-1.7 µs. During the setup and testing, ring and chain network structures were tested respectively. (The test environment used was a 1-meter single-mode optical patch cord; the transmission delay caused by the optical patch cord is negligible. The above data is extracted from the NS21 product R&D pilot test record.) Analysis From the OLM data above, it can be analyzed that parameter b used for time calculation is approximately three times the transmission time of the electrical signal in the optical path (1KM), which is consistent with the above explanation of three port retransmissions. From the OLM data above, it can be analyzed that parameter c used for time calculation is approximately the delay of the single-end device. If combined with the three-retransmission principle of parameter b, the delay of one retransmission is 8 bits (single byte). It can also be considered that OLM parses the serial asynchronous data transmission mode, uses a fully asynchronous fiber optic interface, and operates in the low-speed signal switching mode of the fiber optic cable. Because the transmission delay is single byte, OLM does not parse the Profibus bus link layer protocol; it only parses the physical layer serial data bit stream format and does not perform data packet integrity analysis on the transmission line. From the above, it can be assumed that during normal communication, the single-ended latency of OLM is 8 bits of the data rate (an additional 4 bits may be added for 12M). Furthermore, its latency is related to the communication rate. At 1.5M, the latency caused by the single-ended device is 4.8µs; at 3M, the single-ended latency is 2.4µs; at 6M, the single-ended latency is 1.2µs; and at 12M, the latency is 1µs. What impact do these timing differences have on practical engineering applications? Let's analyze a medium-sized industrial communication system using a 1.5M communication rate (if a higher rate is used, the timing difference of the bus will have a smaller impact on the system due to the faster bus speed, so we will not analyze the impact of these higher-speed communication networks here). The entire system uses a fiber optic communication network consisting of 20 NS21 (or OLM) units, with a total communication distance of 30KM (which is large enough). The network structure is calculated as a ring network. The time it takes for a master station to send data to query data and receive a reply from a slave station (assuming zero response time for both master and slave stations, only calculating the single-cycle time caused by transmission) is calculated. The longest time results are as follows: NS21: (1.7us/unit * 20 units + 100/30us/KM * 30KM) * 2 (two round trips) = 268us OLM: (15.2us/unit * 20 units + 100/30us/KM * 30KM) * 2 (round trips) = 808us The shortest time results are as follows: NS21: (1.6us/unit * 20 units + 100/30us/KM * 30KM) * 2 (round trips) = 264us OLM: (4.8us/unit * 20 units + 100/30us/KM * 30KM) * 2 (round trips) = 392us If the status update time is 100ms (which should meet the current field-level real-time requirements), and each station uses one query command and one execution command within one update cycle, then a total of 40 commands are used. Therefore, the time used for transmission is: The longest time results are as follows: NS21: 268us * 40 = 10.720ms OLM: 808us * 40 = 32.32ms The shortest time results are as follows: NS21: 264us * 40 = 10.56ms OLM: 392us * 40 = 15.68ms. During normal operation, this accounts for 10-20% of the total time. Both meet the user's real-time requirements. Conclusion Using NS21 products and using OLM products to form an industrial communication system can achieve almost the same real-time performance. Other Information Refer to the Siemens OLM product manual and the NS21 product manual (www.gougunsk.com).