Abstract: By physically isolating the 485 bus, the common impedance coupling area is reduced, thus lowering the degree of mutual interference between nodes. In the harsh electromagnetic environment of substations, this 485 HUB star bus can prevent the complete paralysis of the 485 bus caused by faults such as excessive static induced voltage of one node, strong transient pulse voltage, and differential short circuit of signal lines. Keywords: 485 bus; common impedance coupling; 485 Hub star bus; reliability I. Overview Some substations use RS-485 buses that support long-distance communication between multiple nodes as the communication network for local monitoring systems. However, due to the physical structure of the 485 bus, the problem of mutual interference between nodes needs to be solved during communication in the strong electromagnetic environment of substations to improve system reliability. II. Impact of Mutual Interference Between Nodes on the Bus All lower-level machines on the 485 bus share a single signal channel. This physical structure increases the common impedance coupling between nodes, causing mutual interference and thus significantly reducing system reliability. First, the transformers, control room and transmission lines of the substation will induce interference voltage on the nearby communication lines through radiation. When we measured a node A on the 485 bus, we found that: The measurement results show that the electromagnetic coupling between nodes has a great influence on the amplitude and frequency of the induced voltage. Since the communication line is a twisted pair, the induced voltage on the two communication lines can be considered as a common-mode voltage with the same amplitude and phase. However, the twist of the twisted pair cannot be completely consistent, and the pitch is not equal, so differential-mode voltage will appear between the lines. According to the cumulative probability distribution diagram and probability formula of the common-mode voltage provided in reference [1], the probability of the common-mode voltage falling into the differential-mode voltage is calculated as, and the 485 driver/receiver will operate when the voltage is received. Therefore, the mutual interference between nodes increases the common-mode voltage value and the proportion of differential-mode signals, thereby increasing the probability of system malfunction. Second, the transformer switch of the substation will generate a transient electromagnetic field when it operates. Figure 1 and Figure 2 show the transient voltage and current waveforms induced in a control signal line when the isolating switch of a 500kV substation is opened. [align=center]Figure 1 Transient Voltage in Control Lines Figure 2 Transient Current in Control Lines[/align] It can be seen that the instantaneous induced voltage/current on the signal lines near the transformer or connected to it is much higher than the threshold voltage (12V)/current (250mA) of the 485 driver/receiver. Therefore, without a good voltage limiting device, the 485 driver/receiver of the node will be burned out. In a more dangerous situation, other nodes on the bus will inevitably induce high voltage of the same amplitude through the coupling of the common loop, affecting all nodes on the bus. Finally, due to natural wear, improper construction, or malicious damage, the shielding layer of the outdoor communication cable in the substation may short-circuit between two communication lines, causing the bus to remain in a logic 0 state. According to the RS-485 protocol, the lower-level machine interprets this as a new data start bit and attempts to read the subsequent data bits. Since there will never be a stop bit, this will produce a frame error result, and therefore no node will request the bus, and the network will be paralyzed. III. Solution Although the stability of the RS-485 bus can be improved by enhancing the substation's working environment, reducing electromagnetic pollution, adding voltage limiting devices, and using short-circuit bias, the entire bus system will be inoperable without fundamentally isolating faulty nodes. Therefore, only by improving the bus's own reliability can the system's stable operation be guaranteed. Based on this, the 485 HUB star bus proposed in this paper completely isolates nodes physically and improves anti-interference capabilities and reduces mutual interference through software. 3.1 Physical Isolation In the 485 HUB, the same number of 485 drivers/receivers as the lower-level machines are used, each corresponding to one of the nodes, independently completing the task of sending/receiving data packets for its own node. First, the microcontroller on the HUB intercepts the data packet sent by the upper-level machine, confirms the address, and controls the 485 driver in the HUB corresponding to this address node to output the enable terminal. Then, the intercepted data packet is sent back to the driver unchanged. After transmission, the driver is automatically converted into a receiver, waiting for feedback information from the lower-level machine. In the HUB, the enable pins of other 485 drivers/receivers are not controlled and do not change their state; only the selected nodes communicate independently. Structural diagram: 3.2 Software Design The program incorporates statements for multiple judgments, automatic verification, system operation status monitoring, and automatic recovery in case of faults. If differential-mode interference causes bit errors or garbled characters, the program can directly block them after identification. To prevent interference from causing the program to malfunction and enter an infinite loop, a Max813L watchdog chip is used to monitor the program's operation and the microcontroller's power supply. Software flowchart: 3.3 Data Isolation The purpose of data isolation is to isolate the interference source and the susceptible parts from the circuit, ensuring that the field actuators maintain signal communication with the host control unit but do not have direct electrical communication. In the 485 HUB, a high-speed optocoupler 6N137 is added to the feedback information loop to block various interference pulses mixed in with the input switching quantities from the output loop. After using the 485 HUB, the measurements at node A in the same location were taken again, and the results are shown in the table below: 1. The measurements show that interference between nodes in the 485 HUB is significantly reduced, and physical isolation ensures the independence of the nodes. 2. Tests have verified that even if a node in the 485 HUB experiences a short-circuit fault, other nodes can still operate normally. 3. Due to the actual conditions of the substation, the induced voltage at the HUB terminal was not captured during the transformer switch operation in this test. However, through laboratory environment simulation, when a transient pulse voltage higher than the driver threshold appears on the communication line of a certain node, other nodes are not affected. Therefore, the 485 HUB star bus ensures that when a node experiences strong electromagnetic interference or a short-circuit problem, it will not affect other nodes on the bus, improving MTBCF, reducing MTTR, and improving system reliability. IV. Conclusion In terms of its physical structure, this 485 HUB star topology differs from the traditional 485 bus. Each 485 driver/receiver does not implement a one-to-many bus function; instead, it achieves the 485 HUB function through a point-to-point star connection. It can be said that the improved system reliability in this solution comes at the cost of increasing the number of 485 drivers/receivers. However, the 485 HUB has a simple structure, low environmental requirements, and its high reliability and stability are particularly suitable for remote control systems like those in substations. Therefore, in terms of cost-effectiveness, it surpasses the traditional 485 bus. The RS-485 HUB star topology has already been used in some substation remote control systems, operating stably with improved anti-interference capabilities, meeting field requirements. References: [1] Lu Guizhen, Jiang Kehua, Electromagnetic Compatibility Theory and Technology in Communication Systems [M] Beijing Broadcasting Institute Press, 2000 [2] Jan Axelson (author), Elite Technology (translator), Complete Guide to Serial Ports [M] China Electric Power Press, 2001 [3] Sun Zhusen, Zhang Yufang, Zhang Guangzhou, et al., Research on Electromagnetic Interference and Protection Measures of 500KV Substations (I) High Voltage Technology [J], February 2000