A protection circuit and a protection device applied to an RS485 bus.

Abstract

This invention provides a protection circuit and a protection device for RS485 buses. By providing an independent, external, and plug-and-play protection unit, this invention physically separates the protection functions traditionally integrated into the equipment. In the event of lightning strike damage, this external unit acts as a sacrificial unit, ensuring that the damage occurs only within itself, while protecting the overall circuitry of the downstream outdoor equipment. This eliminates the need for maintenance personnel to perform complex chip-level repairs or disassemble and replace the entire device on-site. They can quickly replace this external module simply by plugging and unplugging it, thus avoiding cumbersome device ID reconfiguration, sensor parameter reset, and installation point information resetting. This significantly reduces the technical requirements for maintenance personnel and fundamentally solves the core technical problem of difficult maintenance of outdoor equipment in the water conservancy industry.

Description

Technical Field

[0001] This invention relates to the technical field of water conservancy communication protection, specifically to a protection circuit and a protection device applied to an RS485 bus. Background Technology

[0002] In outdoor water conservancy scenarios, induced lightning protection refers to protecting equipment from indirect voltage surges caused by lightning strikes through various lightning protection measures. Induced lightning occurs when lightning generates strong electric and magnetic fields in the surrounding environment, resulting in high-voltage pulses through electrostatic or electromagnetic induction. These pulses can enter equipment through cables, communication lines, etc., and cause damage.

[0003] In existing technologies, traditional built-in isolation solutions face the technical problem of not being able to be quickly maintained after damage in outdoor water conservancy scenarios. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned technical deficiencies and provide a protection circuit and a protection device for RS485 bus, so as to solve the technical problem that traditional built-in isolation schemes cannot be quickly maintained after damage in outdoor water conservancy scenarios.

[0005] To achieve the above-mentioned technical objectives, the present invention adopts the following technical solution:

[0006] In a first aspect, the present invention provides a protection circuit used to be connected in series between a bus and a bus device to form an independent protection unit, the protection circuit comprising at least:

[0007] The pre-stage protection module has its input terminal electrically connected to the bus to discharge overvoltage energy;

[0008] An isolation communication module, the input of which is connected to the output of the front-end protection module, and the output of which is connected to the bus device, is used to realize bidirectional opto-isolated transmission of communication signals.

[0009] Furthermore, the protection circuit also includes:

[0010] A buffer module, configured between the pre-protection module and the isolation communication module, is used to absorb and delay overvoltage pulses.

[0011] Furthermore, the buffer module includes:

[0012] A common-mode inductor includes a first main winding and a second main winding. The first main winding is connected in parallel between one line of the bus and ground, and the second main winding is connected in parallel between the other line of the bus and ground, for common-mode suppression and energy buffering.

[0013] A buffer resistor R19 and a buffer capacitor C1 are connected in series to form a branch. One end of the branch is connected to any main winding of the common mode inductor, and the other end is grounded, thus forming a damping absorption circuit.

[0014] Furthermore, the buffer module also includes:

[0015] At least one secondary winding is electromagnetically coupled to at least one of the first and second main windings, and is used to sense an overvoltage condition on the main winding and generate a trigger voltage.

[0016] The protection circuit also includes:

[0017] At least one fast discharge module, the fast discharge module being electrically connected to the secondary winding of the buffer module, the fast discharge module comprising:

[0018] A current-limiting resistor R1, one end of which is electrically connected to the secondary winding;

[0019] The control electrode of the switching transistor Q1 is electrically connected to the other end of the current-limiting resistor R1, its first power electrode is connected to the bus through the bleed resistor R2, and its second power electrode is grounded. When the trigger voltage induced by the secondary winding reaches the turn-on threshold of the switching transistor, the switching transistor turns on, and a low-impedance bleed path is formed between the bus and ground through the bleed resistor R2.

[0020] Furthermore, the protection circuit also includes:

[0021] A fault self-test circuit is configured between the buffer module and the isolation communication module. It includes a voltage divider sampling circuit, the input of which is electrically connected to the output of the buffer module. The voltage divider sampling circuit is configured to output a detection voltage during bus communication, and the level of the detection voltage is used to characterize the operating state of the preceding circuit.

[0022] Furthermore, the front-end protection module includes:

[0023] At least one gas discharge tube, wherein the first electrode of the gas discharge tube is electrically connected to one or the other wire in the bus, and the second electrode of the gas discharge tube is grounded; wherein the gas discharge tube is configured to break down and conduct when the voltage across its terminals exceeds a preset breakdown voltage, thereby discharging the overvoltage energy to ground.

[0024] Furthermore, the isolated communication module includes two isolated communication sub-modules configured on one line and the other line for bidirectional opto-isolation; the isolated communication sub-module includes at least:

[0025] The first optocoupler communication module has its input side connected in a first direction between a first side of the line and a first voltage terminal, and its output side connected to a second side of the line.

[0026] The second optocoupler communication module has its input side connected between the first side of the line and the second voltage terminal in a second direction opposite to the first direction, and its output side connected to the second side of the line; wherein the first optocoupler communication module is configured to transmit signals in the first direction, and the second optocoupler communication module is configured to transmit signals in the second direction opposite to the first direction, thereby realizing bidirectional isolated transmission of signals between the first side and the second side of the line.

[0027] Furthermore, the first optical coupler communication module includes:

[0028] The first optocoupler U5 has a first light-emitting diode on its input side. The anode of the first light-emitting diode is connected to the first power supply VCC through the first bias resistor R11, and the cathode is connected to the first side of the line, which is used to transmit signals to the bus device. The output side of the first optocoupler U5 is a first phototransistor. The collector of the first phototransistor is connected to the second power supply VCC through the first pull-up resistor R9 and the second pull-up resistor R10, and the emitter is grounded.

[0029] The second optical coupler communication module includes:

[0030] The second optocoupler U6 has a second light-emitting diode (LED) on its input side. The anode of the second LED is connected to the third power supply VCC through the second bias resistor R12, and the cathode of the second LED is connected to the second side of the line and between the first pull-up resistor R9 and the second pull-up resistor R10. The second side transmits signals to the bus. The output side of the second optocoupler U6 has a second phototransistor. The collector of the second phototransistor is connected to the first side of the line through the feedback resistor R13, and the emitter is grounded.

[0031] Furthermore, the isolated communication submodule also includes:

[0032] An anti-locking capacitor C3 has one end electrically connected to the cathode of the second light-emitting diode and the other end electrically connected to the first side of the line, the cathode of the first light-emitting diode, and the collector of the second phototransistor through a feedback resistor R13. The anti-locking capacitor C3 is configured to maintain the potential of the cathode of the second light-emitting diode when the voltage level on the first side of the line changes from low to high, utilizing the characteristic that the voltage across its terminals cannot change abruptly. This prevents the second optocoupler U6 from conducting, thus avoiding its output pulling down the voltage level on the first side of the line and causing signal lock-in.

[0033] Secondly, the present invention provides a protection device for RS485 bus, including the protection circuit described above.

[0034] Beneficial effects:

[0035] This invention provides an independent, external, and plug-and-play protection unit, physically separating the protection functions traditionally integrated into the equipment. In the event of lightning strike damage, this external unit sacrifices itself, ensuring that the damage occurs only within its own internal components, while protecting the overall circuitry of the downstream outdoor equipment. This eliminates the need for complex chip-level repairs or complete disassembly and replacement of the entire device on-site. Maintenance personnel can quickly replace this external module simply by plugging and unplugging it, avoiding cumbersome device ID reconfiguration, sensor parameter resets, and resetting of installation point information (altitude, offset correction). This significantly reduces the technical requirements for maintenance personnel and fundamentally solves the core pain points of difficult and costly maintenance of outdoor equipment in the water conservancy industry. Furthermore, even without spare parts, the module can be directly removed and the bus restored, ensuring emergency operation of the equipment in an unprotected state. Attached Figure Description

[0036] Figure 1 This is an architectural diagram of a protection circuit provided in an embodiment of the present invention;

[0037] Figure 2 This is a circuit diagram of a protection circuit provided in an embodiment of the present invention;

[0038] Figure 3 This is a schematic diagram illustrating the buffering effect of the high-power LRC buffer circuit provided in an embodiment of the present invention;

[0039] Figure 4 This is a schematic diagram of the coupling effect of the fast discharge circuit provided in the embodiment of the present invention. Detailed Implementation

[0040] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0041] In outdoor industrial IoT applications such as smart water conservancy and environmental monitoring, the RS-485 bus is widely used to connect field devices such as hydrological sensors, rain gauges, and gate position gauges to data acquisition terminals due to its long transmission distance, strong anti-interference capability, and ability to achieve multi-point communication. These devices are typically deployed in remote areas such as fields, rivers, and reservoirs, and communicate and transmit data through cables that are hundreds of meters long.

[0042] However, such long outdoor cable systems are highly vulnerable to induced lightning strikes. When lightning strikes nearby, it induces instantaneous high-voltage pulses (secondary induced lightning voltage can reach 300–400V) in the cable shield and core wires, which can then propagate along the line into the equipment ports, causing permanent damage to the interface chips. To address this issue, traditional solutions typically employ an "optical coupling isolation" design within the equipment, placing the optocoupler between the microcontroller (MCU) and the RS-485 transceiver chip. This architecture can, to some extent, block overvoltage interference such as lightning strikes from propagating to the downstream MCU and core system, thus providing protection.

[0043] In related technologies, RS485 isolated communication schemes achieve electrical isolation by placing an electrical isolation layer between the MCU (Microcontroller Unit) and the RS485 transceiver chip, thereby protecting the core MCU from electrical interference and damage on the bus side. The system architecture consists of three main parts: the MCU side (control domain), the RS485 transceiver side (bus domain), and the isolation layer (intermediate layer). On the MCU side, the MCU transmits and receives data via UART_TX and UART_RX signals, and controls data transmission and reception via DE signals; the power supply is 3.3V and 5V_1SO, and the ground network is GND_1SO. The RS485 transceiver side includes an RS485 transceiver chip (MAX485 or SP3485) used to convert the MCU's TTL level to RS485 differential signals (485A, 485B) and communicate with other devices via the bus; this part is powered by 5V_2SO, and the ground network is GND_2SO. The isolation layer achieves electrical isolation through three optocouplers (U13, U27, and U28), which are responsible for data transmission, reception, and direction control, respectively. This ensures no electrical contact occurs during signal transmission and avoids interference introduced by the bus. When the MCU controls the transmission direction via the DE signal, the UART_TX signal is transmitted to the DI pin of the RS485 transceiver through optocoupler U13, where it is converted into a differential signal and transmitted to the bus. When receiving data, the MCU pulls the DE signal low, enabling the receiver through optocoupler U28. The differential signal on the RS485 bus is converted to TTL level by the transceiver and transmitted to the MCU's UART_RX pin. The entire process achieves bidirectional data transmission while ensuring electrical isolation through optocouplers.

[0044] However, this traditional built-in isolation solution has significant drawbacks: its protection is limited to the circuitry on the device's motherboard, while the RS-485 interface chip, directly connected to the long cable, remains completely exposed to overvoltage threats. In the event of a lightning strike, the interface chip is highly susceptible to damage. For water conservancy equipment deployed in remote outdoor environments, this means either complete on-site replacement or complete rework. Both replacement and repair present challenges such as high maintenance costs, long downtime, and high levels of expertise—maintenance personnel must reconfigure the network parameters, sensor coefficients, and installation point information (such as altitude correction values) of the new equipment, a complex process prone to errors.

[0045] In other words, the above-mentioned solutions are widely used in the field of industrial automation. When damage occurs, only the damaged chip needs to be replaced to restore normal operation. However, in the water conservancy industry, sensors for water level, pressure, and rainfall are generally deployed outdoors in remote areas far from urban areas. Maintenance personnel lack the environment to replace chips on-site, and must either disassemble the entire device for repair or replace it with a new device. In water conservancy scenarios, replacing devices requires ID configuration, remote server settings, sensor configuration, and other configurations based on installation location information, demanding a high level of professional expertise from maintenance personnel. This is a major pain point in the water conservancy industry: difficult and costly maintenance of outdoor equipment.

[0046] The root cause of this technical problem lies in the high coupling between the protection architecture and the main body of the equipment. As part of the mainboard, damage to the protection circuit (optical coupler) inevitably leads to the entire device being taken out of service. Therefore, a solution that can fundamentally change this situation is urgently needed. This embodiment proposes a protection circuit that, through an external bidirectional isolation circuit, fundamentally protects the overall circuitry of the outdoor equipment. In the event of damage, only the circuit of this solution will be damaged. Furthermore, the self-testing function of this solution allows maintenance personnel to accurately locate the cause of the fault and directly replace it with a new module. The circuit of this solution does not require modification or adjustment of the original topology and can be directly connected to the existing RS485 bus. Even if damage occurs and no spare parts are available on-site, this module can be directly removed, the original bus connection restored, and continued use possible.

[0047] like Figure 1 and Figure 2 As shown, this embodiment provides a protection circuit, which is used to connect in series between a bus and a bus device to form an independent protection unit. The protection circuit includes at least:

[0048] The pre-stage protection module has its input terminal electrically connected to the bus to discharge overvoltage energy.

[0049] In this embodiment, the bus can be an RS485 bus;

[0050] In this embodiment, the bus may be a CAN bus;

[0051] In this embodiment, the bus may be a Profibus-DP bus;

[0052] In this embodiment, the bus may be a Modbus bus;

[0053] In this embodiment, the bus device may be an RS485 gateway;

[0054] In this embodiment, the bus device may be a CAN converter;

[0055] In this embodiment, the bus device may be a Modbus gateway.

[0056] In this embodiment, when used in the field, the protection circuit can be set between the bus and the bus device to form an independent protection unit. The other side of the bus can be connected to a sensor, which can be a sensor in the field of water conservancy, such as a radar water level gauge, a pressure water level gauge, an ultrasonic flow meter, or a water quality parameter sensor, etc.

[0057] In this embodiment, the pre-stage protection module can use a gas discharge tube (GDT) as the main protection element. The working principle of the gas discharge tube is that it is insulated by gas between electrodes. When the voltage reaches a certain threshold, the gas is broken down, forming a conductive path and rapidly dissipating the voltage.

[0058] An isolation communication module, the input of which is connected to the output of the front-end protection module, and the output of which is connected to the bus device, is used to realize bidirectional opto-isolated transmission of communication signals.

[0059] In this embodiment, the isolation communication module provides an electrically isolated channel for bidirectional data transmission between the front-end protection module and the bus device. Its purpose is to ensure the safe transmission of communication signals, avoid interference and damage caused by voltage fluctuations, lightning strikes, and other factors, and protect the bus device from overvoltage and electromagnetic interference.

[0060] This embodiment provides an independent, external, and plug-and-play protection unit, physically separating the protection functions traditionally integrated into the equipment. In the event of lightning strike damage, this external unit acts as a sacrificial unit, ensuring that the damage occurs only within itself, while protecting the overall circuitry of the downstream outdoor equipment. This eliminates the need for maintenance personnel to perform complex chip-level repairs or disassemble and replace the entire device on-site. They can quickly replace this external module simply by plugging and unplugging it, avoiding the cumbersome process of reconfiguring device IDs, resetting sensor parameters, and resetting installation point information (altitude, offset correction). This significantly reduces the technical requirements for maintenance personnel and fundamentally solves the core pain points of difficult and costly maintenance of outdoor equipment in the water conservancy industry. Furthermore, even in the absence of spare parts, the module can be directly removed and the bus restored, ensuring emergency operation of the equipment in an unprotected state.

[0061] In some embodiments, the protection circuit further includes:

[0062] A buffer module, configured between the pre-protection module and the isolation communication module, is used to absorb and delay overvoltage pulses.

[0063] In this embodiment, the added buffer module forms a damped oscillation circuit through its internal inductor, resistor and capacitor network, which can effectively absorb and delay the rising edge of the instantaneous high voltage pulse generated by induced lightning strike, and convert its sharp peak voltage into a waveform with a slightly longer duration and lower amplitude. This buffering process buys critical response time for the subsequent fast discharge module, enabling it to reliably conduct and completely discharge energy, thereby greatly reducing the impact of instantaneous high voltage on the subsequent isolation communication module and improving the reliability and energy handling capacity of the entire protection circuit.

[0064] In some embodiments, the buffer module includes:

[0065] A common-mode inductor includes a first main winding and a second main winding. The first main winding is connected in parallel between line A of the bus and ground, and the second main winding is connected in parallel between line B of the bus and ground, for common-mode suppression and energy buffering.

[0066] A buffer resistor R19 and a buffer capacitor C1 are connected in series to form a branch. One end of the branch is connected to any main winding of the common mode inductor, and the other end is grounded, thus forming a damping absorption circuit.

[0067] In one specific implementation, the buffer module can employ a high-power LRC buffer circuit. This high-power LRC buffer circuit consists of a custom transformer T1 with four windings, a buffer resistor R19 (which can be a high-power resistor, with the other path following the same principle), and a buffer capacitor C1 (which can be a high-voltage CBB capacitor, with the other path following the same principle). The two large-value main windings of T1 are common-mode windings to cancel common-mode interference, with an insulation class of 1000V between the windings. The turns ratio of the other two sets of mutual inductance coil secondary windings to the common-mode main windings is 1:40. Therefore, when the sudden voltage change across the common-mode inductor is 400V, the induced voltage in the secondary winding is 4V. This induced voltage is used to drive the subsequent fast discharge circuit. Besides the custom transformer T1, the LRC buffer circuit also includes resistor R19 and capacitor C1. The instantaneous voltage of the secondary induced lightning is high, but due to its short duration, the energy is relatively low. Simulations were performed using 100µs and 400V parameters, and the results are as follows... Figure 3 As shown, the 400V AC signal from the secondary induced lightning (labeled 1 in the figure) charges C1 through T1 and R19. At this time, the instantaneous current is extremely large, and the instantaneous current will form a reverse voltage across the inductor that opposes the change in current, storing the instantaneous electrical energy in the form of a magnetic field. At this time, the voltage across the capacitor is the difference between the induced lightning voltage and the inductor reverse voltage, as shown. Figure 3 The voltage of capacitor C1 (labeled 2 in the diagram) is limited to 100V. Once the capacitor is fully charged, the instantaneous current disappears, and the inductor's current-blocking characteristic releases the previously stored magnetic field. At this point, the buffer circuit composed of LRC has achieved its buffering function, and the fast discharge circuit is activated during the inductor's release process, further releasing electrical energy.

[0068] This embodiment utilizes a unique design that connects buffer resistor R19 and buffer capacitor C1 in series and grounded, working in conjunction with a common-mode inductor connected in parallel between the bus and ground to form a highly efficient damping absorption circuit. This circuit can convert high-energy, high-frequency instantaneous overvoltage pulses generated by induced lightning strikes into heat dissipation and broaden the time domain, thereby significantly reducing the peak value and steepness of the pulse. This not only effectively protects the downstream isolation communication module from high-voltage impacts, but more importantly, it ensures that the upstream protection module GDT and fast discharge module can be reliably triggered within their action thresholds. This provides a key guarantee for the graded and coordinated energy discharge of the entire protection circuit, greatly enhancing the reliability and service life of the protection unit.

[0069] In some embodiments, the buffer module further includes:

[0070] At least one secondary winding is electromagnetically coupled to at least one of the first and second main windings, and is used to sense an overvoltage condition on the main winding and generate a trigger voltage.

[0071] The protection circuit also includes:

[0072] At least one fast discharge module, the fast discharge module being electrically connected to the secondary winding of the buffer module, the fast discharge module comprising:

[0073] A current-limiting resistor R1, one end of which is electrically connected to the secondary winding;

[0074] The control electrode of the switching transistor Q1 is electrically connected to the other end of the current-limiting resistor R1, its first power electrode is connected to the bus through the bleed resistor R2, and its second power electrode is grounded. When the trigger voltage induced by the secondary winding reaches the turn-on threshold of the switching transistor, the switching transistor turns on, and a low-impedance bleed path is formed between the bus and ground through the bleed resistor R2.

[0075] In this embodiment, the switching transistor Q1 can be a high-power transistor. Its control electrode is the base, its first power electrode is the collector, and its second power electrode is the emitter.

[0076] In this embodiment, the switch Q1 can be a metal-oxide-semiconductor field-effect transistor (MOSFET), with its control electrode being the gate, its first power electrode being the drain, and its second power electrode being the source.

[0077] In this embodiment, the switching transistor Q1 can be an insulated gate bipolar transistor (IGBT), with its control electrode being the gate, its first power electrode being the collector, and its second power electrode being the emitter.

[0078] In one specific implementation, the fast discharge module can employ a fast discharge circuit, which includes: a current-limiting resistor R1 (base current-limiting resistor, the other path is the same), a switching transistor Q1 (high-power transistor, the other path is the same), and a discharge resistor R2 (collector current-limiting resistor, the other path is the same). Figure 4 As shown, the voltage across the main coil of T1 (labeled 1 in the figure) reaches 400V, while the coupling voltage of the secondary winding (labeled 2 in the figure) drops to 16V. The voltage signal coupled from the secondary winding of T1 turns on Q1 through R1, instantly forming a path with an impedance of approximately 1Ω between the bus and ground, releasing the energy of the induced lightning. The transistor has a fast turn-on speed and a high base withstand voltage, making it very suitable for the application scenario of this invention.

[0079] This embodiment creatively utilizes the principle of electromagnetic coupling to convert the overvoltage state on the main winding into a trigger signal by setting a fast discharge module after the secondary winding of the buffer module. This triggers the high-power switching transistor Q1 to conduct rapidly, and a controllable low-impedance discharge path is established between the bus and ground through the discharge resistor R2. This design can actively and quickly guide the high-energy pulse generated by induced lightning strikes to the ground wire for discharge, effectively avoiding the problem that traditional passive protection components (TVS tubes) are prone to burnout due to insufficient energy absorption. It significantly reduces the impact of residual voltage on the subsequent precision isolation communication circuit, thus providing more reliable and thorough protection for the back-end equipment under extreme overvoltage conditions.

[0080] In some embodiments, the protection circuit further includes:

[0081] A fault self-test circuit is configured between the buffer module and the isolation communication module. It includes a voltage divider sampling circuit, the input of which is electrically connected to the output of the buffer module. The voltage divider sampling circuit is configured to output a detection voltage during bus communication, and the level of the detection voltage is used to characterize the operating state of the preceding circuit.

[0082] In this embodiment, the damage self-test circuit may include a symmetrical voltage divider sampling network. Specifically, the first voltage divider branch includes resistors R5 and R6 connected in series, and one end of the branch is connected to line A of the bus (i.e., the output terminal of the buffer module).

[0083] The second voltage divider branch is formed by resistors R7 and R8 connected in series, and one end of this branch is connected to the other line B of the bus (i.e., the other output terminal of the buffer module).

[0084] Resistor R6 is connected to resistor R7, and sampling point TP is located at the series connection point of resistor R5 and resistor R6 in the first voltage divider branch. Sampling point TP is configured to output the detection voltage.

[0085] In one specific implementation, the fault self-test circuit consists of a voltage divider composed of a series of resistors (R5, R6, R7, R8). When the circuit is communicating normally, the "TP" signal will receive a voltage divider value, approximately 5 / 6 of the differential signal. When the preceding stage GDT or TVS transistor burns out, the differential bus will exhibit a short circuit during communication, and the "TP" voltage divider value will be close to 0. This phenomenon can be used to determine whether a fault has occurred in the preceding stage circuit.

[0086] This embodiment adds a damage self-test circuit and connects its voltage divider sampling point after the buffer module. It can use the inherent differential voltage signal during RS485 bus communication as a detection source in real time to output a detection voltage directly related to the impedance state of the front-end protection circuit (e.g., GDT, TVS, and buffer network). This design allows maintenance personnel to quickly and accurately determine whether the front-end protection module has failed due to lightning strikes and short-circuited (e.g., the detection voltage drops significantly to near zero) simply by measuring the voltage level of this detection point, without the need for specialized testing instruments or disassembly equipment. This enables precise fault location and predictive maintenance, greatly shortens system fault diagnosis time, reduces the high maintenance costs caused by blindly replacing equipment or rework, and significantly improves the maintainability and operational efficiency of outdoor unattended equipment.

[0087] In some embodiments, the front-end protection module includes:

[0088] At least one gas discharge tube, wherein the first electrode of the gas discharge tube is electrically connected to line A or line B in the bus, and the second electrode of the gas discharge tube is grounded; wherein the gas discharge tube is configured to break down and conduct when the voltage across its terminals exceeds a preset breakdown voltage, thereby discharging the overvoltage energy to ground.

[0089] In this embodiment, two gas discharge tubes can be provided, one of which is electrically connected to line A in the bus, and the other is electrically connected to line B in the bus.

[0090] In this embodiment, the front-end protection module may include two independent and symmetrical gas discharge tube (GDT) sub-modules, respectively used to protect the A and B lines of the bus. Specifically, the first electrode of the first gas discharge tube U1 is connected to one line (A) of the bus, and its second electrode is grounded; the first electrode of the second gas discharge tube U3 is connected to the other line (B) of the bus, and its second electrode is grounded. The breakdown voltage of the gas discharge tubes U1 and U3 can be set to 600V. When the voltage across its electrodes exceeds this preset breakdown voltage due to induced lightning strikes, the gas medium inside the tube will be ionized and broken down, thereby instantly forming a low-impedance conductive path between the bus and ground, rapidly dissipating the fatal overvoltage energy to the ground. This module, as the first barrier of the protection circuit, ensures that subsequent buffer modules and isolation communication modules are protected from ultra-high voltage threats through this irreversible sacrificial conduction method.

[0091] In one specific implementation, when a cable is struck by lightning, the cable, acting as a conductor, generates an induced electromotive force. This signal has a high voltage amplitude (typically reaching several hundred volts), high frequency characteristics (complex frequency components, with higher frequency components having lower energy), and low energy, which can cause significant damage to the circuit.

[0092] The pre-stage protection module can be a pre-stage discharge circuit, which uses a gas-discharge tube (GDT). Its manufacturing process involves filling an inert gas into a ceramic shell and extending electrodes at both ends. When a voltage is applied between the electrodes, a non-uniform electric field is formed. As the voltage increases, the electric field strength exceeds the gas insulation strength, causing gap discharge breakdown and forming a conductive path. At this point, the voltage between the electrodes remains at a low residual voltage (typically 20-50V), protecting the subsequent electronic equipment. After several breakdowns, the internal contacts of the GDT may melt and connect, resulting in a short circuit and affecting circuit function. Therefore, in this invention, the pre-stage discharge circuit only protects against excessively strong induced lightning strikes (>400V) exceeding design specifications. Induced voltages less than 400V enter the high-power LRC buffer circuit.

[0093] This embodiment uses a gas discharge tube (GDT) as the core protection component in the front-end protection module. Utilizing its high energy tolerance and low residual voltage after breakdown, it can withstand the initial high-current, high-energy lightning strikes for the entire protection circuit. When the induced lightning voltage exceeds the GDT's breakdown threshold, the GDT rapidly breaks down, forming a low-impedance path that directly discharges most of the energy to ground. This clamps the voltage across the subsequent circuitry within a safe range, effectively protecting the precision electronic components from damage. Furthermore, this proactive sacrificial design ensures that lightning damage is strictly controlled within the quickly replaceable external protection unit, providing a reliable and easily maintained first line of defense for outdoor equipment.

[0094] In some embodiments, the isolated communication module includes two isolated communication sub-modules configured on line A and line B for bidirectional opto-isolation; the isolated communication sub-module includes at least:

[0095] The first optocoupler communication module has its input side connected in a first direction between a first side of the line and a first voltage terminal, and its output side connected to a second side of the line.

[0096] The second optocoupler communication module has its input side connected between the first side of the line and the second voltage terminal in a second direction opposite to the first direction, and its output side connected to the second side of the line; wherein the first optocoupler communication module is configured to transmit signals in the first direction, and the second optocoupler communication module is configured to transmit signals in the second direction opposite to the first direction, thereby realizing bidirectional isolated transmission of signals between the first side and the second side of the line.

[0097] This embodiment creatively utilizes an isolated communication submodule consisting of two optocouplers connected in reverse parallel, symmetrically arranged on each signal line of the bus. The first optocoupler communication module transmits low-level signals from one side to the other, while the second optocoupler communication module transmits high-level signals in the opposite direction. This achieves truly bidirectional, lossless transmission of differential signals across the isolation barrier without relying on a single complex interface chip. This design not only provides reliable electrical isolation up to several kilovolts, effectively blocking ground loops and common-mode interference, but more importantly, its architecture, composed entirely of discrete optocouplers, isolates the isolation function from the internal device and integrates it into the external protection unit. This strictly limits lightning damage to this module, ensuring that the communication interface and all configuration parameters of the back-end host device remain intact. This enables a minimally invasive maintenance method that requires no on-site operation by professional personnel and no reconfiguration of any parameters in the event of interface damage, completely solving the core pain points of high maintenance costs and excessively high technical barriers caused by lightning strikes to outdoor water conservancy equipment.

[0098] In one embodiment, the first optical coupler communication module includes:

[0099] The first optocoupler U5 has a first light-emitting diode on its input side. The anode of the first light-emitting diode is connected to the first power supply VCC through the first bias resistor R11, and the cathode is connected to the first side of the line, which is used to transmit signals to the bus device. The output side of the first optocoupler U5 is a first phototransistor. The collector of the first phototransistor is connected to the second power supply VCC through the first pull-up resistor R9 and the second pull-up resistor R10, and the emitter is grounded.

[0100] The second optical coupler communication module includes:

[0101] The second optocoupler U6 has a second light-emitting diode (LED) on its input side. The anode of the second LED is connected to the third power supply VCC through the second bias resistor R12, and the cathode of the second LED is connected to the second side of the line and between the first pull-up resistor R9 and the second pull-up resistor R10. The second side transmits signals to the bus. The output side of the second optocoupler U6 has a second phototransistor. The collector of the second phototransistor is connected to the first side of the line through the feedback resistor R13, and the emitter is grounded.

[0102] This embodiment employs a precision topology with the first optocoupler U5 and the second optocoupler U6 reverse-biased at the input and cross-coupled at the output. It utilizes the forward conduction of the first optocoupler to transmit low-level signals and innovatively connects the cathode of the second optocoupler to the intermediate node of the voltage divider resistors R9 / R10 on the output side of the first optocoupler. Simultaneously, the output of the second optocoupler is fed back to the signal source via feedback resistor R13, constructing a fully isolated communication channel capable of lossless and bidirectional transmission of differential signals. This design not only ensures the integrity and anti-interference capability of RS-485 bus signals during long-distance transmission but also provides the necessary circuit foundation for subsequent anti-locking functions through its unique level conversion and feedback mechanism. Ultimately, it achieves a dual improvement in communication reliability and system maintainability under complex lightning strike environments.

[0103] In some embodiments, the isolated communication submodule further includes:

[0104] An anti-locking capacitor C3 has one end electrically connected to the cathode of the second light-emitting diode and the other end electrically connected to the first side of the line, the cathode of the first light-emitting diode, and the collector of the second phototransistor through a feedback resistor R13. The anti-locking capacitor C3 is configured to maintain the potential of the cathode of the second light-emitting diode when the voltage level on the first side of the line changes from low to high, utilizing the characteristic that the voltage across its terminals cannot change abruptly. This prevents the second optocoupler U6 from conducting, thus avoiding its output pulling down the voltage level on the first side A of the line and causing signal lock-in.

[0105] This embodiment creatively solves the signal lock-up risk in the bidirectional optocoupler isolation circuit by introducing an anti-locking capacitor C3. When a critical transition from low to high occurs on the bus side, this capacitor utilizes the physical characteristic that its voltage cannot change abruptly to instantaneously maintain the potential of the cathode on the input side of the second optocoupler U6, effectively preventing misleading conduction caused by changes in the state of the preceding circuit. This design ensures that the second optocoupler operates normally only when a high-level signal is required, completely avoiding the positive feedback lock-up loop formed by its output terminal accidentally pulling the bus side level low through the feedback resistor R13. This significantly improves the stability and reliability of the communication link, ensuring that the protection unit can continuously provide uninterrupted bidirectional isolated communication function in harsh lightning strike environments, while maintaining the maintenance advantages of plug-and-play and quick replacement.

[0106] In one specific implementation, the isolated communication module can be a voltage-limiting isolation circuit, which includes four fast optical coupling isolators to achieve dual-channel differential bidirectional communication; the voltage-limiting function uses a traditional TVS diode solution. Figure 2As can be seen, the two optocouplers U5 and U6 are reversed, one positive and one negative. When the "A" terminal changes from high to low (high level indicates bus idle state, low level indicates valid data), the internal LED of U5 conducts, causing the internal transistor of U5 to receive light and conduct, i.e., pins 3 and 4 conduct. Pin 3 is at ground level. At this time, due to the voltage division effect of R10 and R9, the left side of R10 is at a low level, and the signal is transmitted through optical isolation. Conversely, when the "AA" signal is low, the internal diode of U6 conducts, and pins 3 and 4 of U6 are at a low level. The originally pulled-up signal "A" is pulled low, and the signal is correctly received at a low level, thus achieving communication. The "B" and "BB" signals work on the same principle. However, the system has a vulnerability: when signal A is low, the left side of R10 becomes low, meaning pin 2 of U6 is low. At this time, the diode of U6 conducts, pulling down pins 3 and 4 of U6, causing signal A to be pulled low and resulting in a lock-up. Therefore, C3 is added between signal A and pin 1 of U6. When signal A transitions from high to low, since the voltage across the capacitor does not change abruptly, pin 1 of U6 also instantly becomes low, and the diode of U6 does not conduct, thus preventing a lock-up. Lock-up only occurs when the power supply charges C3 through R12 to a level sufficient to turn on the diode of U6. By designing the values ​​of C3 and R12, the lock-up problem can be avoided. These values ​​are related to the communication baud rate.

[0107] Specifically, terminals A and B are the two communication lines of the water conservancy telemetry terminal. Correspondingly, terminals AA and BB are connected to the sensor, which can be a water level monitoring sensor deployed in the water, etc. Understandably, in water conservancy communication scenarios, the distance from the sensor to the water conservancy telemetry terminal is often quite far (tens of meters, or even hundreds of meters), making it susceptible to damage from induced lightning.

[0108] The RS-485 bus standard specifies the electrical characteristics of the bus interface, namely the definition of two logic states: a positive voltage difference between A and B, between +2V and +6V, represents one logic state; a negative voltage difference between -2V and -6V represents the other logic state; digital signals use differential transmission, which can effectively reduce interference from noise signals.

[0109] In U5, VCC is connected to Pin 1 (the anode of the LED) via R11. Pin 2 of U5 is the cathode of the LED, connected to terminal A of the signal line. Pin 4 of U5 is connected to VCC via R10 and R9. Pin 3 of U5 is directly grounded.

[0110] Therefore, when terminal A is given a high level, the LED is not conducting and will not emit light. Consequently, the transistor inside U5 does not receive light and will not conduct either; the phototransistor is in an off state. Therefore, in this case, AA is also given a high level.

[0111] When terminal A is given a low level, the LED is turned on (because a circuit is formed), so it will emit light, and the phototransistor is in a conducting state.

[0112] When the phototransistor is in the ON state (Pin4 and Pin3 of U5 are ON), because Pin3 is grounded, the right side of R10, i.e., Pin4 of U5, is at a low level. Therefore, VCC-R9-R10-phototransistor-ground can form a loop. Because the voltage division ratio of R9 and R10 is 10:1, AA also outputs a low level.

[0113] The above process shows that the high and low level signals at end A can be transmitted to end AA. It should be noted that ends A and B are two communication lines of the water conservancy telemetry terminal, while ends AA and BB are connected to the sensor. Therefore, the typical process described above can be a query request sent by the data acquisition terminal to the sensor (that is, the right side sends a query request to the left side, and the left side responds to the query request by providing a specific query value).

[0114] Correspondingly, when the sensor returns a query value, terminal AA returns a high level, so pin 2 of U6 is high. Since pin 1 of U6 is connected to VCC, the LED is not conducting and does not emit light in this state, so the phototransistor of U6 is also not conducting. Therefore, pins 3 and 4 of U6 are also not conducting. When pins 3 and 4 are not conducting, and because terminal A is connected to VCC through capacitor C3, terminal A is high. Through this process, the high level returned by terminal AA is transmitted to terminal A.

[0115] It should be noted that when terminal A is low, a circuit is formed between VCC-R11-U5's Pin1, U5's Pin2, and terminal A. At this time, the phototransistor of U5 is turned on, and pins 4 and 3 of U5 are also connected. Because R9 is a high-value resistor and R10 is a low-value resistor, they form a voltage divider circuit. Therefore, the voltage levels on both the left and right sides of R10 are low. Since the left side of R10 is connected to U6's Pin2, in this state, when terminal A is low, U6's Pin2 is also low.

[0116] When pin 2 of U6 is also low, a circuit is formed between VCC, R12, pin 1 of U6, and pin 2 of U6. This causes the phototransistor of U6 to conduct, meaning pins 4 and 3 of U6 are connected. Because pin 3 of U6 is grounded, pin 4 of U6 is low, and through R13, terminal A becomes low, resulting in signal latch-up. Even if terminal A is pulled high again externally, the low level of pin 4 of U6 pulls the signal back low through R13, preventing further signal transmission. To solve this problem, capacitor C3 is introduced. Repeating the above process, when terminal A is low, pin 2 of U6 is also low, and pins 3 and 4 of U6 are connected and both are low. When terminal A is high, since the voltage across the capacitor cannot change abruptly, the right side of C3, i.e., terminal A, suddenly rises to a high level. Consequently, the left side of C3 also becomes high, meaning Pin 2 of U6 is high. At this time, Pin 1 and Pin 2 of U6 are at the same level, and the internal diode of the optocoupler is not conducting. Therefore, Pin 4 and Pin 3 of U6 are not conducting, and terminal A is no longer forcibly pulled low. At this time, Pin 1 and Pin 2 of U5 are both high, and the internal diode is not conducting. Therefore, Pin 3 and Pin 4 of U5 are not conducting, and the right side of R10, i.e., Pin 2 of U6, is high. The signal at terminal A is transmitted normally, and the self-locking is released. The above description explains the entire transmission process. The value of C3 depends on the time it takes for Pin 3 and Pin 4 of U5 to change from conducting to non-conducting after the signal at terminal A becomes high. As long as the left side of C3 remains high within this time, signal self-locking will not occur.

[0117] Understandably, electricity is extremely destructive and generates heat effects; its intense electromagnetic radiation can easily trigger various chain reactions. Lightning current amplitudes can reach hundreds of kA, and induced lightning voltages can reach hundreds of kV or even higher, enough to destroy various equipment, melt metal, and cause fires and explosions. A lightning strike can lead to serious problems such as data loss, traffic disruptions, and medical accidents. Currently, lightning protection systems for electronic equipment typically consist of lightning rods, grounding grids, surge arresters for power and signal lines, lightning protection strips, and lightning protection networks. Lightning rods primarily protect against direct lightning strikes, surge arresters prevent lightning from entering electronic equipment through power and communication cables, and lightning protection strips prevent direct lightning strikes from damaging equipment and computer rooms. However, there is a lack of effective measures to protect the entire equipment system from induced lightning. Induced lightning occurs when lightning strikes near a conductor and indirectly affects the electronic system through electrostatic induction and electromagnetic induction. Its amplitude often reaches several thousand volts. A common protection method is to add air discharge tubes (GDTs) to the circuit for self-damaging protection.

[0118] Besides direct lightning strikes and induced lightning, there is also a secondary induced lightning strike, which is equally destructive. When lightning strikes, the voltage generated by induced lightning on the cable shield can reach 300kV to 400kV. Using the near-field induction formula, it can be calculated that the voltage induced on the core wire is about 1 / 1000, or 300 to 400V. This voltage level still causes significant damage to computer networks and electronic equipment.

[0119] Most existing technologies for protecting against induced lightning are passive, such as using GDTs and TVS diodes in a high-low voltage configuration for amplitude and voltage limiting; or utilizing LC circuit networks to reduce and delay induced lightning strikes within a specific frequency range. In summary, this embodiment proposes a protection circuit with overvoltage protection and delay capabilities, and active discharge capability, for secondary induced lightning strikes.

[0120] The protection circuit provided in this embodiment can protect against excessively high induced voltages (pre-discharge circuit), mitigate the impact of induced lightning (LRC buffer circuit), quickly discharge destructive induced energy (fast discharge circuit), realize RS485 plug-in optical isolation communication (voltage limiting isolation circuit), and perform circuit self-testing (damage self-test circuit). In summary, the protection circuit of this invention provides a relatively complete lightning protection solution for existing outdoor water conservancy equipment. It is plug-and-play, provides isolated communication, and performs damage self-testing, greatly improving equipment safety and reducing maintenance costs.

[0121] This embodiment provides a protection device for RS485 bus, including the protection circuit provided in the above embodiment.

[0122] In this embodiment, the protection device may include a housing, and the protection circuit may be configured within the housing.

[0123] In this embodiment, the protection device can be designed as a plug-and-play module that can be directly connected to the existing RS485 bus communication system. It is easy to install and does not require complex modifications to the original equipment circuitry.

[0124] Specifically, the protection device may include a separate housing containing a printed circuit board (PCB). All electronic components of the protection circuit are arranged and soldered onto this PCB, forming a complete, functionally integrated protection unit. One end of the housing has a first external communication interface for connecting to an RS485 bus from a field sensor. The other end of the housing has a second pair of device interfaces for connecting to the protected data acquisition equipment or a water conservancy telemetry terminal. The first external communication interface and the second pair of device interfaces are electrically connected through the protection circuit on the internal PCB. Preferably, the interfaces use standardized terminal blocks or waterproof aviation plugs to adapt to harsh outdoor environments and simplify the installation process.

[0125] The protection device can be designed as a standalone, plug-and-play modular product. Its installation method is as follows: disconnect the main cable originally connected to the RS485 interface of the data acquisition device and connect it to the first external communication interface of this device; then, use a new short cable to connect the second pair of device interfaces of this device to the RS485 interface of the data acquisition device, thus completing the installation. The entire process requires no modification to the internal circuitry, firmware, or configuration parameters of the original data acquisition device or sensor, achieving true plug-and-play functionality.

[0126] When a lightning strike causes a communication interruption, maintenance personnel can remotely or on-site determine the fault location through the output of the damaged self-test circuit. If the device itself is confirmed to be damaged, the cables at both ends can be directly disconnected, and a functional module of the same type can be used to quickly restore system operation. This eliminates the need to replace or repair expensive data acquisition equipment and completely avoids the complex operation of reconfiguring equipment parameters, greatly reducing maintenance costs and technical barriers. Through its external, modular design, this device strictly confines lightning damage to its internal components. In the event of damage, maintenance personnel do not need to perform on-site chip-level repairs as in industrial automation scenarios. It also completely avoids the need for complete disassembly and repair or replacement of the entire equipment due to the inability to perform on-site repairs. More importantly, it completely avoids the complex configuration process (including device ID reassignment, remote server registration, sensor parameter calibration, and installation point altitude and offset correction) that is required after replacing the equipment and demands a high level of professional expertise from maintenance personnel. Maintenance personnel only need to bring this independent module to the site and perform simple plug-and-play replacements to immediately restore system operation, thereby greatly reducing the technical threshold, time cost, and labor cost of outdoor equipment maintenance in the water conservancy industry and fundamentally solving its core pain points.

[0127] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0128] Optionally, specific examples in this embodiment can refer to the examples described in the above embodiments, and will not be repeated here.

[0129] The sequence numbers of the embodiments in this application are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0130] In the above embodiments of this application, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0131] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A protection circuit, characterized in that, The protection circuit is used to connect in series between the bus and the bus devices to form an independent protection unit, and the protection circuit includes at least: The pre-stage protection module has its input terminal electrically connected to the bus to discharge overvoltage energy; An isolation communication module, the input of which is connected to the output of the front-end protection module, and the output of which is connected to the bus device to realize bidirectional opto-isolated transmission of communication signals; The protection circuit also includes: A buffer module, configured between the pre-protection module and the isolation communication module, is used to absorb and delay overvoltage pulses; The buffer module includes: A common-mode inductor includes a first main winding and a second main winding. The first main winding is connected in series between one line of the bus and a buffer resistor (R19), and the second main winding is connected in series between the other line of the bus and a buffer resistor (R20) for common-mode rejection and energy buffering. A buffer resistor (R19) and a buffer capacitor (C1) are connected in series to form a branch. One end of the branch is connected to any main winding of the common mode inductor, and the other end is grounded, thereby forming a damping absorption circuit. The buffer module also includes: At least one secondary winding is electromagnetically coupled to at least one of the first and second main windings, and is used to sense an overvoltage condition on the main winding and generate a trigger voltage. The protection circuit also includes: At least one fast discharge module, the fast discharge module being electrically connected to the secondary winding of the buffer module, the fast discharge module comprising: A current-limiting resistor (R1) is electrically connected at one end to the secondary winding; The control electrode of the switching transistor (Q1) is electrically connected to the other end of the current-limiting resistor (R1), its first power electrode is connected to the bus through the bleed resistor (R2), and its second power electrode is grounded. When the trigger voltage induced by the secondary winding reaches the turn-on threshold of the switching transistor, the switching transistor turns on, and a low-impedance bleed path is formed between the bus and ground through the bleed resistor (R2).

2. The protection circuit according to claim 1, characterized in that, The protection circuit also includes: A fault self-test circuit is configured between the buffer module and the isolation communication module. It includes a voltage divider sampling circuit, the input of which is electrically connected to the output of the buffer module. The voltage divider sampling circuit is configured to output a detection voltage during bus communication, and the level of the detection voltage is used to characterize the operating state of the preceding circuit.

3. The protection circuit according to claim 1, characterized in that, The front-end protection module includes: At least one gas discharge tube, wherein the first electrode of the gas discharge tube is electrically connected to one or the other wire in the bus, and the second electrode of the gas discharge tube is grounded; wherein the gas discharge tube is configured to break down and conduct when the voltage across its terminals exceeds a preset breakdown voltage, thereby discharging the overvoltage energy to ground.

4. The protection circuit according to claim 1, characterized in that, The isolation communication module includes two isolation communication sub-modules, which are configured on one line and the other line for bidirectional opto-isolation. The isolated communication submodule includes at least: The first optocoupler communication module has its input side connected in a first direction between a first side of the line and a first voltage terminal, and its output side connected to a second side of the line. The second optocoupler communication module has its input side connected between the first side of the line and the second voltage terminal in a second direction opposite to the first direction, and its output side connected to the second side of the line; wherein the first optocoupler communication module is configured to transmit signals in the first direction, and the second optocoupler communication module is configured to transmit signals in the second direction opposite to the first direction, thereby realizing bidirectional isolated transmission of signals between the first side and the second side of the line.

5. The protection circuit according to claim 4, characterized in that, The first optical coupler communication module includes: The first optocoupler (U5) has a first light-emitting diode (LED) on its input side. The anode of the first LED is connected to a first power supply (VCC) through a first bias resistor (R11), and the cathode is connected to a first side of the line, which is used to transmit signals to the bus device. The output side of the first optocoupler (U5) is a first phototransistor. The collector of the first phototransistor is connected to a second power supply (VCC) through a first pull-up resistor (R9) and a second pull-up resistor (R10), and the emitter is grounded. The second optical coupler communication module includes: The second optocoupler (U6) has a second light-emitting diode (LED) on its input side. The anode of the second LED is connected to the third power supply (VCC) through a second bias resistor (R12), and the cathode of the second LED is connected to the second side of the line and between the first pull-up resistor (R9) and the second pull-up resistor (R10). The second side transmits signals to the bus. The output side of the second optocoupler (U6) is a second phototransistor. The collector of the second phototransistor is connected to the first side of the line through a feedback resistor (R13), and the emitter is grounded.

6. The protection circuit according to claim 5, characterized in that, The isolated communication submodule also includes: An anti-locking capacitor (C3) has one end electrically connected to the cathode of the second light-emitting diode and the other end electrically connected to the first side of the line, the cathode of the first light-emitting diode, and the collector of the second phototransistor through a feedback resistor (R13). The anti-locking capacitor (C3) is configured to maintain the potential of the cathode of the second light-emitting diode by utilizing the characteristic that the voltage at its two ends cannot change abruptly when the level of the first side of the line changes from low to high, thereby preventing the second optocoupler (U6) from conducting and avoiding its output side from pulling down the level of the first side of the line and causing signal lock-in.

7. A protection device for an RS485 bus, characterized in that, Includes the protection circuit as described in any one of claims 1-6.

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