RS485 lightning protection device applied to outdoor remote communication

Abstract

The embodiment of the utility model provides an RS485 lightning protection device applied to outdoor remote communication. The RS485 lightning protection device comprises a first-stage isolation unit, a second-stage isolation unit, a third-stage isolation unit and an RS485 driving unit, through a three-stage lightning protection circuit, large energy of lightning stroke is eliminated through first-stage protection; the secondary protection performs secondary voltage limiting to meet the voltage requirement of the interface chip; and the third stage adopts electrical isolation to eliminate ground voltage difference and ground loop current, so that a lightning protection and isolation circuit for common-mode and high-frequency differential-mode suppression is realized. After three-level protection, the anti-interference capability of the RS485 bus is greatly improved.

Description

Technical Field

[0001] The utility model relates to the field of lightning protection of circuits, in particular to an RS485 lightning protection device applied to outdoor remote communication. Background Art

[0002] The RS485 bus uses differential form for data transmission. The allowable common mode voltage range of the differential input terminal to ground is -7V to +12V. Overvoltage exceeding this range may damage the interface devices, or even connected devices such as microprocessors. The sources of overvoltage transients are usually external lightning, static electricity, and other interference. For example, static electricity discharge generated by human body contacting the pins of the chip can reach a voltage of tens of kilovolts, which can cause the working device to lock up and fail to operate or damage the device. The transient interference caused by lightning induction on the RS485 bus can burn all devices connected to the transmission line in an instant, and power supply interference may also cause device lockup or damage.

[0003] When using the RS485 bus for field communication, twisted pair cables are used as transmission lines. Transmission lines are generally installed outdoors overhead or along cable trenches. During the thunderstorm season, transient interference caused by lightning on the transmission lines often damages devices. In addition, since the network topology of the RS485 bus generally adopts a bus structure with matched terminal resistance, a bus is usually used to connect each node in series. Each node is connected to the same bus. When transients caused by lightning are often easily damaged, the transceivers of multiple nodes on the transmission line are often damaged.

[0004] In the process of implementing the utility model, the applicant found that there are at least the following problems in the prior art:

[0005] The RS485 bus exposed outdoors is susceptible to high-amplitude transient interference generated by lightning and high-power loads, which can cause the RS485 chip to break down and be damaged. Utility Model Content

[0006] The embodiment of the utility model provides an RS485 lightning protection device applied to outdoor remote communication, which solves the problem that the RS485 bus exposed outdoors is easily affected by high-amplitude transient interference generated by lightning and high-power loads, thereby causing breakdown and damage of the RS485 chip.

[0007] To achieve the above-mentioned object, on the one hand, an embodiment of the present invention provides an RS485 lightning protection device for outdoor remote communication, comprising: a first-level isolation unit, a second-level isolation unit, a third-level isolation unit, and an RS485 drive unit;

[0008] The RS485 driver unit is connected with an RS485 bus and a UART bus, and is used for exchanging data between the RS485 bus and the UART bus;

[0009] The third-level isolation unit is connected to the RS485 driver unit via the UART bus, for protecting and isolating the UART interface, and the UART interface is used to exchange data with the RS485 driver unit;

[0010] The RS485 drive unit, the second-stage isolation unit, and the first-stage isolation unit are sequentially connected in series on the RS485 bus;

[0011] The first-stage isolation unit is used to prevent differential-mode and common-mode overvoltage transients from occurring on the RS485 bus;

[0012] The second-stage isolation unit is used to clamp the voltage of the RS485 bus connected to the RS485 drive unit to below 8 volts.

[0013] The above technical solution has the following beneficial effects: by providing a lightning protection and isolation circuit that can achieve common mode and high-frequency differential mode suppression, specifically a three-level lightning protection circuit for RS485 bus, the first level of protection eliminates the large energy of lightning strikes; the second level of protection performs secondary voltage limiting to meet the voltage requirements of the interface chip; the third level uses electrical isolation such as optical coupling isolation or magnetic isolation to eliminate large ground pressure differences and ground loop currents. After three levels of protection, the anti-interference ability of the RS485 bus is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the embodiments of the utility model or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the utility model. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0015] Figure 1 The present invention is a schematic diagram of an RS485 lightning protection device for outdoor remote communication according to one embodiment of the present invention.

[0016] The reference numerals are as follows: 10, first-stage isolation unit; G1, first ceramic gas discharge tube; G2, second ceramic gas discharge tube; G3, third ceramic gas discharge tube; PE, first reference ground; 20, second-stage isolation unit; R11, first resistor; R12, second resistor; D1, first TVS diode; D2, second TVS diode; D3, third TVS diode; GND485, second reference ground; L, common mode inductor; 40, RS485 drive unit; U1, RS485 interface chip; R21, first pull-up resistor; R2, first pull-down resistor; R1, terminal matching resistor; C1, first filter capacitor;

[0017] C2, second filter capacitor; VCC485, first power supply; 30, third-level isolation unit; 31, read isolation module; 311, first amplifier module; 312, first isolation module; 32, direction control isolation module; 321, second amplifier module; 322, second isolation module; 33, write isolation module; 331, third amplifier module; 332, third isolation module;

[0018] R22, second pull-up resistor; R13, third resistor; Q1, PNP transistor; R14, fourth resistor; U2, first isolation chip; R23, third pull-up resistor; R24, fourth pull-up resistor; R25, fifth pull-up resistor; R15, fifth resistor; Q2, first NPN transistor; U3, second isolation chip; R26, sixth pull-up resistor; R27, seventh pull-up resistor; R28, eighth pull-up resistor; R16, sixth resistor; Q3, second NPN transistor; U4, third isolation chip;

[0019] R29, the ninth pull-up resistor; R30, the tenth pull-up resistor; VCC33, the second power supply. DETAILED DESCRIPTION

[0020] The following will be combined with the drawings in the embodiments of the utility model to clearly and completely describe the technical solutions in the embodiments of the utility model. Obviously, the described embodiments are only part of the embodiments of the utility model, not all of the embodiments. Based on the embodiments in the utility model, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the utility model.

[0021] On the one hand, if Figure 1 As shown, an embodiment of the present invention provides an RS485 lightning protection device for outdoor remote communication, comprising: a first-level isolation unit 10, a second-level isolation unit 20, a third-level isolation unit 30, and an RS485 drive unit 40;

[0022] The RS485 driver unit 40 is connected to the RS485 bus and the UART bus, and is used to exchange data between the RS485 bus and the UART bus;

[0023] The third-level isolation unit 30 is connected to the RS485 driver unit 40 via the UART bus, and is used to protect and isolate the UART interface, and the UART interface is used to exchange data with the RS485 driver unit 40;

[0024] The RS485 driving unit 40, the second-stage isolation unit 20, and the first-stage isolation unit 10 are sequentially connected in series on the RS485 bus;

[0025] The first-stage isolation unit 10 is used to prevent differential-mode and common-mode overvoltage transients from occurring on the RS485 bus;

[0026] The second-stage isolation unit 20 is used to clamp the voltage of the RS485 bus connected to the RS485 drive unit to below 8 volts;

[0027] The RS485 bus includes an A line and a B line, and the A line and the B line are differential data lines for communicating with a remote RS485 chip; the UART bus includes a read line, a write line and a direction control line; the read line corresponds to the RXD signal in the USART interface, and the write line corresponds to the TXD signal in the USART interface; the direction control line is used to control the read and write direction of the local RS485 drive unit, which can be provided by the general IO port of the microprocessor (MCU) or by other circuits or chips. The UART interface is the USART interface, which can be the USART interface on the microprocessor (MCU) or the USART interface provided by other circuits or chips.

[0028] In some embodiments, the inventors found that the RS485 bus implements long-distance transmission (more than 1200 meters), and its transmission line is usually exposed to the outdoors. Although the transient interference lasts for a short time, there may be hundreds of thousands of volts of voltage. In addition, when switching high-power inductive loads such as motors, transformers, relays, etc. or during lightning, transient interference with a high amplitude will be generated. If not properly protected, it is very easy to introduce overvoltage due to lightning strikes and other reasons. Since the RS485 transceiver has a low operating voltage (about 5V), its own withstand voltage is very low (-7V to +12V). Once the overvoltage is introduced, it will break down and damage the chip. Therefore, the inventor provides a lightning protection and isolation circuit that can realize common mode and high-frequency differential mode suppression, specifically a three-level lightning protection circuit of the RS485 bus, the first-level protection eliminates the large energy of lightning strikes; the second-level protection performs secondary voltage limiting to meet the voltage requirements of the interface chip; the third level uses electrical isolation such as optical coupling isolation or magnetic isolation to eliminate the earth pressure difference and ground loop current. After three levels of protection, the anti-interference ability of the RS485 bus is greatly improved.

[0029] Furthermore, a common-mode inductor L is connected in series between the second-stage isolation unit 20 and the RS485 driving unit 40 .

[0030] In some embodiments, in the circuit, L is a common-mode inductor, which can attenuate common-mode interference, and can suppress interference inside and outside the board to a certain extent, thereby improving the product's anti-interference capability, and can also reduce the external radiation of the signal line. The typical value of the common-mode inductor impedance is 1000Ω / 100MHz.

[0031] Further, the RS485 bus includes an A line and a B line; the first-level isolation unit 10 includes: a first ceramic gas discharge tube G1, a second ceramic gas discharge tube G2, and a third ceramic gas discharge tube G3;

[0032] The first ceramic gas discharge tube G1 is connected between the A line and the B line;

[0033] One end of the second ceramic gas discharge tube G2 is connected to the first reference ground PE, and the other end of the second ceramic gas discharge tube G2 is connected to the A line;

[0034] One end of the third ceramic gas discharge tube G3 is connected to the first reference ground PE, and the other end of the third ceramic gas discharge tube G3 is connected to the B line.

[0035] In some embodiments, in the first level of protection, the ceramic gas discharge tubes G1, G2, and G3 are mainly used as main protection devices, providing a low-impedance grounding path to prevent overvoltage transients. When the transient voltage reaches the spark discharge voltage of the ceramic gas discharge tube, the discharge tube will switch from the high-impedance closed state to the arc mode. In the arc mode, the discharge tube becomes a virtual short circuit, providing an instantaneous open-circuit current grounding discharge path, and transferring the transient impact current from the protected device to the ground. Among them, the ceramic gas discharge tubes G2 and G3 perform common-mode protection, and the ceramic gas discharge tube G1 performs differential-mode protection. After the first level of protection, the overvoltage is greatly weakened and is generally limited to hundreds of volts or less, generally not exceeding 500V. After the induced overvoltage such as lightning is introduced from the port, the selection of the ceramic gas discharge tubes G1, G2, and G3 first considers their withstand voltage and current capabilities, and is selected according to the protection level.

[0036] Further, the RS485 bus includes an A line and a B line; the second-level isolation unit 20 includes: a first resistor R11, a second resistor R12, a first TVS diode D1, a second TVS diode D2, and a third TVS diode D3;

[0037] The first resistor R11 is connected in series in the B line, and the right end is connected to the first-stage isolation unit;

[0038] The second resistor R12 is connected in series to the A line, and the right end is connected to the first-stage isolation unit;

[0039] The first TVS diode D1 is connected across the left ends of the first resistor R11 and the second resistor R12;

[0040] One end of the second TVS diode D2 is connected to the second reference ground GND485, and the other end of the second TVS diode D2 is connected to the left end of the first resistor R11;

[0041] One end of the third TVS diode D3 is connected to the second reference ground GND485, and the other end of the third TVS diode D3 is connected to the left end of the second resistor R12;

[0042] The left ends of the first resistor R11 and the second resistor R12 are connected to the RS485 driving unit through the common mode inductor L.

[0043] In some embodiments, in the second level protection, after the first level protection, there is still a large amount of energy in the circuit. At this time, resistors R11 and R12 are mainly used for current limiting. R11 and R12 can be ordinary wire-wound resistors, or PTC resistors with good current limiting effect but higher price. The resistance value is generally less than 10 ohms, and the current carrying capacity is about 300mA; in the secondary protection, TVS diodes D1, D2, and D3 are used for the protection of 485 transceiver, among which TVS diode D1 is used for differential mode protection, and TVS diodes D2 and D3 are used for common mode protection. The response speed of TVS is very fast. At this time, the voltage reaching the transceiver is clamped below 8V or even lower (depending on the TVS withstand voltage used). When TVS selects withstand voltage, it is generally slightly higher than the maximum operating voltage of the 485 transceiver chip.

[0044] Further, the RS485 bus includes an A line and a B line; the UART bus includes a read line, a write line and a direction control line; the RS485 drive unit 40 includes: an RS485 interface chip U1, a first pull-up resistor R21, a first pull-down resistor R2, a terminal matching resistor R1, a first filter capacitor C1 and a second filter capacitor C2;

[0045] The RS485 interface chip U1 includes: an RS485 read port, an RS485 write port, an RS485 direction control port, an A output port and a B output port;

[0046] The RS485 read port, RS485 write port, and RS485 direction control port of the RS485 interface chip U1 are respectively connected to the read line, write line, and direction control line; and are used to exchange data with the UART interface via the third-level isolation unit;

[0047] The A output port and the B output port of the RS485 interface chip U1 are connected to the A line and the B line respectively;

[0048] One end of the first pull-up resistor R21 is connected to the A output port of the RS485 interface chip U1, and the other end of the first pull-up resistor R21 is connected to the first power supply VCC485;

[0049] One end of the first pull-down resistor R2 is connected to the B output port of the RS485 interface chip U1, and the other end of the first pull-down resistor R2 is connected to the second reference ground GND485;

[0050] The terminal matching resistor R1 is connected across the A output port and the B output port of the RS485 interface chip U1;

[0051] The first filter capacitor C1 is connected in parallel between the B output port of the RS485 interface chip U1 and the second reference ground GND485;

[0052] The second filter capacitor C2 is connected in parallel between the A output port of the RS485 interface chip U1 and the second reference ground GND485.

[0053] In some embodiments, in the circuit, C1 and C2 are filter capacitors, which mainly provide a low-impedance return path for high-frequency interference signals, which can effectively reduce external common-mode current interference and filter external high-frequency interference. The typical value of the capacitor capacitance is 100pF.

[0054] Further, the UART bus includes a read line, a write line and a direction control line; the UART interface includes: a read port, a write port and a direction control port; the third-level isolation unit 30 includes: a read isolation module 31, a write isolation module 33 and a direction control isolation module 32;

[0055] The read isolation module 31 includes: a first amplification submodule 311 and a first isolation submodule 312;

[0056] The input end of the first amplifying submodule 311 is connected to the read line; the output end of the first amplifying submodule 311 is connected to the input end of the first isolating submodule 312; the output end of the first isolating submodule 312 is connected to the read port of the UART interface;

[0057] The direction control isolation module 32 includes: a second amplification submodule 321 and a second isolation submodule 322;

[0058] The input end of the second amplifying submodule 321 is connected to the direction control port; the output end of the second amplifying submodule 321 is connected to the input end of the second isolating submodule 322; the output end of the second isolating submodule 322 is connected to the direction control line;

[0059] The write isolation module 33 includes: a third amplification submodule 331 and a third isolation submodule 332;

[0060] The input end of the third amplifying submodule 331 is connected to the write port; the output end of the third amplifying submodule 331 is connected to the input end of the third isolating submodule 332; and the output end of the third isolating submodule 332 is connected to the write line.

[0061] Further, the first amplifying submodule 311 includes: a second pull-up resistor R22, a third resistor R13, and a PNP transistor Q1; the first isolating submodule 312 includes: a fourth resistor R14, a first isolating chip U2, a third pull-up resistor R23, and a fourth pull-up resistor R24;

[0062] One end of the second pull-up resistor R22 is connected to the first power supply VCC485, and the other end of the second pull-up resistor R22 is connected to the read line;

[0063] One end of the third resistor R13 is connected to the read line, and the other end of the third resistor R13 is connected to the base of the PNP transistor;

[0064] The emitter of the PNP transistor Q1 is connected to the first power supply VCC485;

[0065] The collector of the PNP transistor Q1 is connected to one end of the fourth resistor R14;

[0066] The other end of the fourth resistor R14 is connected to the signal input end of the first isolation chip U2;

[0067] The enable terminal of the first isolation chip U2 is pulled up to the second power supply VCC33 through the third pull-up resistor R23;

[0068] The signal output end of the first isolation chip U2 is pulled up to the second power supply VCC33 through the fourth pull-up resistor R24;

[0069] The signal output end of the first isolation chip U2 is connected to the read port;

[0070] The second amplifying submodule 321 includes: a fifth pull-up resistor R25, a fifth resistor R15, and a first NPN transistor Q2; the second isolating submodule 322 includes: a second isolating chip U3, a sixth pull-up resistor R26, and a seventh pull-up resistor R27;

[0071] The collector of the first NPN transistor Q2 is pulled up to the second power supply VCC33 through the fifth pull-up resistor R25;

[0072] The direction control port is connected to the base of the first NPN transistor Q2 through the fifth resistor R15;

[0073] The emitter of the first NPN transistor Q2 is connected to the signal input terminal of the second isolation chip U3;

[0074] The signal output end of the second isolation chip U3 is pulled up to the first power supply VCC485 via the seventh pull-up resistor R27;

[0075] The enable terminal of the second isolation chip U3 is pulled up to the first power supply VCC485 through the sixth pull-up resistor R26;

[0076] The signal output end of the second isolation chip U3 is connected to the direction control line;

[0077] The third amplifying submodule 331 includes: an eighth pull-up resistor R28, a sixth resistor R16, and a second NPN transistor Q3; the third isolating submodule 332 includes: a third isolating chip U4, a ninth pull-up resistor R29, and a tenth pull-up resistor R30;

[0078] The collector of the second NPN transistor Q3 is pulled up to the second power supply VCC33 through the eighth pull-up resistor R28;

[0079] The write port is connected to the base of the second NPN transistor Q3 through the sixth resistor R16;

[0080] The emitter of the second NPN transistor Q3 is connected to the signal input terminal of the third isolation chip U4;

[0081] The signal output end of the third isolation chip U4 is pulled up to the first power supply VCC485 via the ninth pull-up resistor R30;

[0082] The enable terminal of the third isolation chip U4 is pulled up to the first power supply VCC485 through the ninth pull-up resistor R29;

[0083] The signal output terminal of the third isolation chip U4 is connected to the write line.

[0084] In some embodiments, in the third level of protection, optocoupler isolation or magnetic isolation is generally used. Optocoupler is the most commonly used isolation device and has a low cost. The rate of the optocoupler must meet the baud rate requirements, and high-speed optocoupler is generally selected for 485 isolation. Compared with optocouplers, magnetic isolation chips have a smaller PCB area, but the cost is also higher; in actual engineering, the corresponding isolation method is selected according to demand. In actual engineering, when the communication nodes of RS485 are far apart, the reference ground PE of each node is connected to the local earth. When there is a large voltage difference between the earth at both ends, the ground potential will be superimposed on the 485 signal line in the form of a common mode voltage, which may exceed the common mode voltage range that the port can withstand, affecting normal communication, and even damaging the back-end circuit. In addition, when the ground plane between RS485 communication nodes that are far apart is connected by a cable, the ground wire will form a ground loop with the earth, couple external common mode noise, generate ground loop current, and cause the entire communication system to fail. The use of an isolation circuit can well suppress the occurrence of the above two situations. In the isolation circuit, the receiving (reading), transmitting (writing) and direction control signals USART_RX (reading), USART_TX (writing), and USART_Con (direction control) of the UART port of the MCU (microprocessor) or other devices are isolated through isolation chips U2, U4, and U3 respectively. The optocoupler and magnetic isolation chips are selected according to the withstand voltage and rate. Transistor Q1 is used to amplify the receiving signal, and transistors Q2 and Q3 are used to amplify the transmitting signal, and also play a role in signal level conversion (because many MCU interfaces are often powered by 3.3V, and isolation devices are usually 5V).

[0085] The above technical scheme of the embodiment of the utility model is described in detail below in conjunction with specific application examples. For technical details not introduced during the implementation process, please refer to the relevant description in the previous text.

[0086] The inventors found that the RS485 bus implements long-distance transmission (more than 1,200 meters), and its transmission lines are usually exposed to the outdoors. Although transient interference lasts for a short time, it may have a voltage of hundreds or thousands of volts. In addition, high-amplitude transient interference will be generated during the switching of high-power inductive loads such as motors, transformers, relays, etc. or during lightning. If not properly protected, it is very easy to introduce overvoltage due to lightning strikes, etc. Since the operating voltage of the RS485 transceiver is relatively low (about 5V), its own withstand voltage is very low (-7V to +12V). Once overvoltage is introduced, it will break down and damage the chip.

[0087] Based on the above findings, the embodiment of the utility model provides an RS485 lightning protection device for outdoor remote communication, specifically a three-level lightning protection device or circuit for RS485 bus, the first-level protection adopts ceramic gas discharge tube for protection, the second-level protection adopts current limiting resistor or PTC (Positive Temperature Coefficient) resistor and TVS (Transient Voltage Suppressor, transient voltage suppression diode), the overvoltage is weakened by the gas discharge tube, and the voltage is limited to hundreds of volts (generally not more than 500 volts); at this time, the current limiting resistor or PTC resistor and TVS tube are used for secondary protection, and after the secondary voltage limiting, the voltage is limited to below 8V (volts); in order to better realize the suppression of common mode voltage, the third level adopts optical coupling isolation or magnetic isolation to eliminate the pressure difference between the two ends of the earth during long-distance signal transmission, and eliminate the loop current formed between the ground wire and the earth. After three levels of protection, the anti-interference ability of the RS485 bus is greatly improved.

[0088] The utility model embodiment proposes a three-level protection circuit diagram of the RS485 bus, such as Figure 1 As shown below, combined Figure 1 The first-stage ceramic gas discharge tubes G1, G2, and G3 are used to discharge large currents; the second-stage resistors R11 and R12 are used for current limiting, TVS (Transient Voltage Suppressor) diodes D1, D2, and D3 are used for clamping voltages, a single TVS diode D1 is used for differential mode protection, and two TVS diodes D2 and D3 are used for common mode protection; the back end is a 485 chip; the third-stage optocoupler isolation is used to prevent ground loop currents and improve common mode capability.

[0089] In the first level of protection, the ceramic gas discharge tubes G1, G2, and G3 are mainly used as the main protection devices, providing a low-impedance grounding path to prevent overvoltage transients. When the transient voltage reaches the spark discharge voltage of the ceramic gas discharge tube, the discharge tube will switch from the high-impedance closed state to the arc mode. In the arc mode, the discharge tube becomes a virtual short circuit, providing an instantaneous open-circuit current grounding discharge path, and transferring the transient impact current from the protected device to the ground. Among them, the ceramic gas discharge tubes G2 and G3 perform common-mode protection, and the ceramic gas discharge tube G1 performs differential-mode protection. After the first level of protection, the overvoltage is greatly weakened and is generally limited to hundreds of volts (generally not more than 500V). After the induced overvoltage such as lightning is introduced from the port, the selection of ceramic gas discharge tubes G1, G2, and G3 first considers their withstand voltage and current capabilities, and is selected according to the protection level.

[0090] In the second level of protection, after the first level of protection, there is still a lot of energy in the line. At this time, resistors R11 and R12 are mainly used for current limiting. R11 and R12 can be ordinary wire-wound resistors or PTC resistors with good current limiting effect but higher price. The resistance value is generally less than 10 ohms and the current resistance is about 300mA. In the second level of protection, TVS diodes D1, D2, and D3 are used to protect the 485 transceiver. Among them, TVS diode D1 is used for differential mode protection, and TVS diodes D2 and D3 are used for common mode protection. The response speed of TVS is very fast. At this time, the voltage reaching the transceiver is clamped below 8V or even lower (depending on the TVS withstand voltage used). When TVS selects withstand voltage, it is generally slightly higher than the maximum operating voltage of the 485 transceiver chip.

[0091] In the third level of protection, optocoupler isolation or magnetic isolation is generally used. Optocoupler is the most commonly used isolation device with low cost. The speed of the optocoupler must meet the baud rate requirements, and high-speed optocoupler is generally selected for 485 isolation. Compared with optocoupler, magnetic isolation chip has a smaller PCB area, but the cost is also higher; in actual engineering, the corresponding isolation method is selected according to the needs.

[0092] In actual projects, when the RS485 communication nodes are far apart, the reference ground PE of each node is connected to the local earth. When there is a large voltage difference between the earths at both ends, the ground potential will be superimposed on the 485 signal line in the form of common mode voltage, which may exceed the common mode voltage range that the port can withstand, affecting normal communication and even damaging the back-end circuit. In addition, when the ground plane between the RS485 communication nodes that are far apart is connected by cable, the ground wire will form a ground loop with the earth, couple external common mode noise, generate ground loop current, and cause the entire communication system to fail. The use of isolation circuits can effectively suppress the occurrence of the above two situations.

[0093] In the isolation circuit, GND is the digital ground of the MCU end. The receiving (reading), transmitting (writing) and direction control signals USART_RX (reading), USART_TX (writing), and USART_Con (direction control) of the UART port of the MCU (microprocessor) or other devices are isolated through isolation chips U2, U4, and U3 respectively. The optocoupler and magnetic isolation chips are selected according to the withstand voltage and rate. Transistor Q1 is used to amplify the received signal, and transistors Q2 and Q3 are used to amplify the transmitted signal, and also play a role in converting the signal level (because many MCU interfaces are often powered by 3.3V, and isolation devices are usually 5V).

[0094] Through the above-mentioned primary, secondary and tertiary protection circuits, the ceramic gas discharge tube can withstand large lightning energy, the TVS clamps the residual voltage of the primary protection to a level that the chip can withstand, the isolation circuit eliminates the long-distance ground loop current and ground voltage difference problems, and the RS485 circuit can achieve good lightning protection.

[0095] In the circuit, L is the common mode inductor. The common mode inductor L can attenuate the common mode interference, and can suppress the interference inside and outside the board to a certain extent, thereby improving the anti-interference ability of the product. At the same time, it can also reduce the external radiation of the signal line. The typical value of the common mode inductor impedance is 1000Ω / 100MHz.

[0096] In the circuit, C1 and C2 are filter capacitors, which mainly provide a low-impedance return path for high-frequency interference signals, which can effectively reduce external common-mode current interference and filter external high-frequency interference. The typical value of the capacitor is 100pF; C3 is a jumper capacitor between the RS485 interface ground (the second reference ground GND485) and the digital ground (the first reference ground PE), with a typical value of 1000pF. The capacitance of C3 needs to be adjusted according to the actual situation;

[0097] In the circuit, R21 and R2 are pull-up and pull-down resistors, which can ensure that the voltage value of the RS 485 bus in the idle state is not within the threshold level (-200mV ~ +200mV), ensuring the normal RS485 differential signal level.

[0098] The embodiments of the present invention have the following technical effects: providing a lightning protection and isolation circuit that can achieve common mode and high-frequency differential mode suppression. It is a three-level lightning protection circuit for RS485 bus. The first level of protection uses a ceramic gas discharge tube to eliminate the large energy of lightning strikes; the second level of protection uses a current limiting resistor or a PTC resistor and a TVS to perform secondary voltage limiting to meet the voltage requirements of the interface chip; the third level uses optical coupling isolation or magnetic isolation to eliminate the earth pressure difference and ground loop current. After three levels of protection, the anti-interference ability of the RS485 bus is greatly improved.

[0099] It should be understood that the specific order or hierarchy of steps in the disclosed process is an example of an exemplary method. Based on design preferences, it should be understood that the specific order or hierarchy of steps in the process can be rearranged without departing from the scope of protection of the present disclosure. The attached method claims present the elements of the various steps in an exemplary order and are not intended to be limited to the specific order or hierarchy described.

[0100] In the above detailed description, various features are grouped together in a single embodiment to simplify the disclosure. This method of disclosure should not be interpreted as reflecting an intention that the embodiments of the claimed subject matter require more features than those clearly stated in each claim. On the contrary, as reflected in the attached claims, the utility model is in a state with less than all the features of the disclosed individual embodiments. Therefore, the attached claims are hereby expressly incorporated into the detailed description, with each claim acting alone as a separate preferred embodiment of the utility model.

[0101] The disclosed embodiments are described above to enable any person skilled in the art to implement or use the present invention. Various modifications of these embodiments are obvious to those skilled in the art, and the general principles defined herein may also be applied to other embodiments without departing from the spirit and scope of the present disclosure. Therefore, the present disclosure is not limited to the embodiments given herein, but is consistent with the broadest scope of the principles and novel features disclosed in the present application.

[0102] The above description includes examples of one or more embodiments. Of course, it is impossible to describe all possible combinations of components or methods for the purpose of describing the above embodiments, but it should be recognized by those skilled in the art that the various embodiments may be further combined and arranged. Therefore, the embodiments described herein are intended to cover all such changes, modifications and variations that fall within the scope of protection of the appended claims. In addition, with respect to the term "comprising" used in the specification or claims, the word is covered in a manner similar to the term "including". In addition, any term "or" used in the specification of the claims is intended to mean "non-exclusive or".

[0103] The specific implementation methods described above further illustrate the purpose, technical solutions and beneficial effects of the utility model in detail. It should be understood that the above description is only the specific implementation method of the utility model and is not used to limit the protection scope of the utility model. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the utility model should be included in the protection scope of the utility model.

Claims

1. An RS485 lightning protection device for outdoor remote communication, characterized in that: include: A first-stage isolation unit (10), a second-stage isolation unit (20), a third-stage isolation unit (30), and an RS485 drive unit (40); The RS485 drive unit (40) is connected to an RS485 bus and a UART bus, and is used to exchange data between the RS485 bus and the UART bus; The third-level isolation unit (30) is connected to the RS485 drive unit (40) via the UART bus, and is used to protect and isolate the UART interface, and the UART interface is used to exchange data with the RS485 drive unit (40); The RS485 drive unit (40), the second-stage isolation unit (20), and the first-stage isolation unit (10) are sequentially connected in series on the RS485 bus; The first-stage isolation unit (10) is used to prevent differential mode and common mode overvoltage transients from occurring on the RS485 bus; The second-stage isolation unit (20) is used to clamp the voltage of the RS485 bus connected to the RS485 drive unit (40) to below 8 volts.

2. The RS485 lightning protection device for outdoor remote communication as claimed in claim 1, characterized in that: A common mode inductor (L) is connected in series between the second-stage isolation unit (20) and the RS485 drive unit (40).

3. The RS485 lightning protection device for outdoor remote communication as claimed in claim 1, characterized in that: The RS485 bus includes an A line and a B line; The first-stage isolation unit (10) comprises: a first ceramic gas discharge tube (G1), a second ceramic gas discharge tube (G2), and a third ceramic gas discharge tube (G3); The first ceramic gas discharge tube (G1) is connected between the A line and the B line; One end of the second ceramic gas discharge tube (G2) is connected to the first reference ground (PE), and the other end of the second ceramic gas discharge tube (G2) is connected to the A line; One end of the third ceramic gas discharge tube (G3) is connected to the first reference ground (PE), and the other end of the third ceramic gas discharge tube (G3) is connected to the B line.

4. The RS485 lightning protection device for outdoor remote communication as claimed in claim 2, characterized in that: The RS485 bus includes an A line and a B line; The second-stage isolation unit (20) comprises: a first resistor (R11), a second resistor (R12), a first TVS diode (D1), a second TVS diode (D2), and a third TVS diode (D3); The first resistor (R11) is connected in series in the B line, and the right end is connected to the first-stage isolation unit; The second resistor (R12) is connected in series in the A line, and the right end is connected to the first-stage isolation unit; The first TVS diode (D1) is connected across the left ends of the first resistor (R11) and the second resistor (R12); One end of the second TVS diode (D2) is connected to the second reference ground (GND485), and the other end of the second TVS diode (D2) is connected to the left end of the first resistor (R11); One end of the third TVS diode (D3) is connected to the second reference ground (GND485), and the other end of the third TVS diode (D3) is connected to the left end of the second resistor (R12); The left ends of the first resistor (R11) and the second resistor (R12) are connected to the RS485 drive unit through the common mode inductor (L).

5. The RS485 lightning protection device for outdoor remote communication as claimed in claim 1, characterized in that: The RS485 bus includes an A line and a B line; the UART bus includes a read line, a write line and a direction control line; The RS485 drive unit (40) comprises: an RS485 interface chip (U1), a first pull-up resistor (R21), a first pull-down resistor (R2), a terminal matching resistor (R1), a first filter capacitor (C1) and a second filter capacitor (C2); The RS485 interface chip (U1) comprises: an RS485 read port, an RS485 write port, an RS485 direction control port, an A output port and a B output port; The RS485 read port, the RS485 write port, and the RS485 direction control port of the RS485 interface chip (U1) are respectively connected to the read line, the write line, and the direction control line; and are used to exchange data with the UART interface via the third-level isolation unit; The A output port and the B output port of the RS485 interface chip (U1) are respectively connected to the A line and the B line; One end of the first pull-up resistor (R21) is connected to the A output port of the RS485 interface chip (U1), and the other end of the first pull-up resistor (R21) is connected to a first power supply (VCC485); One end of the first pull-down resistor (R2) is connected to the B output port of the RS485 interface chip (U1), and the other end of the first pull-down resistor (R2) is connected to the second reference ground (GND485); The terminal matching resistor (R1) is connected across the A output port and the B output port of the RS485 interface chip (U1); The first filter capacitor (C1) is connected in parallel between the B output port of the RS485 interface chip (U1) and the second reference ground (GND485); The second filter capacitor (C2) is connected in parallel between the A output port of the RS485 interface chip (U1) and the second reference ground (GND485).

6. The RS485 lightning protection device for outdoor remote communication according to claim 1, characterized in that: The UART bus includes a read line, a write line and a direction control line; The UART interface comprises: a read port, a write port and a direction control port; the third-level isolation unit (30) comprises: a read isolation module (31), a write isolation module (33) and a direction control isolation module (32); The read isolation module (31) comprises: a first amplification submodule (311) and a first isolation submodule (312); The input end of the first amplifying submodule (311) is connected to the read line; the output end of the first amplifying submodule (311) is connected to the input end of the first isolating submodule (312); and the output end of the first isolating submodule (312) is connected to the read port of the UART interface; The direction control isolation module (32) comprises: a second amplification submodule (321) and a second isolation submodule (322); The input end of the second amplifying submodule (321) is connected to the direction control port; the output end of the second amplifying submodule (321) is connected to the input end of the second isolating submodule (322); and the output end of the second isolating submodule (322) is connected to the direction control line; The write isolation module (33) comprises: a third amplification submodule (331) and a third isolation submodule (332); The input end of the third amplifying submodule (331) is connected to the write port; the output end of the third amplifying submodule (331) is connected to the input end of the third isolating submodule (332); and the output end of the third isolating submodule (332) is connected to the write line.

7. The RS485 lightning protection device for outdoor remote communication as claimed in claim 6, characterized in that: The first amplifying submodule (311) comprises: a second pull-up resistor (R22), a third resistor (R13), and a PNP transistor (Q1); the first isolating submodule (312) comprises: a fourth resistor (R14), a first isolating chip (U2), a third pull-up resistor (R23), and a fourth pull-up resistor (R24); One end of the second pull-up resistor (R22) is connected to the first power supply (VCC485), and the other end of the second pull-up resistor (R22) is connected to the read line; One end of the third resistor (R13) is connected to the read line, and the other end of the third resistor (R13) is connected to the base of the PNP transistor; The emitter of the PNP transistor (Q1) is connected to a first power supply (VCC485); The collector of the PNP transistor (Q1) is connected to one end of the fourth resistor (R14); The other end of the fourth resistor (R14) is connected to the signal input end of the first isolation chip (U2); The enable terminal of the first isolation chip (U2) is pulled up to the second power supply (VCC33) through the third pull-up resistor (R23); The signal output end of the first isolation chip (U2) is pulled up to the second power supply (VCC33) through the fourth pull-up resistor (R24); The signal output end of the first isolation chip (U2) is connected to the read port; The second amplifying submodule (321) comprises: a fifth pull-up resistor (R25), a fifth resistor (R15), and a first NPN transistor (Q2); the second isolating submodule (322) comprises: a second isolating chip (U3), a sixth pull-up resistor (R26), and a seventh pull-up resistor (R27); The collector of the first NPN transistor (Q2) is pulled up to the second power supply (VCC33) through the fifth pull-up resistor (R25); The direction control port is connected to the base of the first NPN transistor (Q2) through the fifth resistor (R15); The emitter of the first NPN transistor (Q2) is connected to the signal input end of the second isolation chip (U3); The signal output end of the second isolation chip (U3) is pulled up to the first power supply (VCC485) via the seventh pull-up resistor (R27); The enable terminal of the second isolation chip (U3) is pulled up to the first power supply (VCC485) through the sixth pull-up resistor (R26); The signal output end of the second isolation chip (U3) is connected to the direction control line; The third amplifying submodule (331) comprises: an eighth pull-up resistor (R28), a sixth resistor (R16), and a second NPN transistor (Q3); the third isolating submodule (332) comprises: a third isolating chip (U4), a ninth pull-up resistor (R29), and a tenth pull-up resistor (R30); The collector of the second NPN transistor (Q3) is pulled up to the second power supply (VCC33) through the eighth pull-up resistor (R28); The write port is connected to the base of the second NPN transistor (Q3) through the sixth resistor (R16); The emitter of the second NPN transistor (Q3) is connected to the signal input end of the third isolation chip (U4); The signal output end of the third isolation chip (U4) is pulled up to the first power supply (VCC485) via the ninth pull-up resistor (R30); The enable terminal of the third isolation chip (U4) is pulled up to the first power supply (VCC485) through the ninth pull-up resistor (R29); The signal output end of the third isolation chip (U4) is connected to the write line.

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