A protection circuit for a CAN serial interface

Abstract

The present invention discloses a protection circuit for a CAN serial interface, which includes a two-channel digital opto-isolator U1 and a CAN bus transceiver U2; the two-channel digital opto-isolator U1 is respectively connected to power supplies VCC1 and VCC2; the two-channel digital opto-isolator U1 is connected to a chip with UART communication function located outside; the two-channel digital opto-isolator U1 is connected to the CAN bus transceiver U2; the CAN bus transceiver U2 is connected to a CAN serial interface located outside through a circuit protection system; the protection system is used to provide safety protection for the CAN serial interface. The protection circuit for the CAN serial interface disclosed by the present invention can meet the safety protection requirements in multiple aspects such as electrostatic protection and lightning protection for the CAN serial interface, ensure that the CAN serial interface can work normally under harsh outdoor environmental conditions, and improve the user's product usage experience.

Description

Technical Field

[0001] The present invention relates to the technical field of protection circuits, and in particular to a protection circuit for a CAN serial interface. Background Art

[0002] The CAN (Controller Area Network) serial interface has been widely used in technical fields such as instrumentation, industrial control, and mechatronics due to its advantages of strong real-time performance, long transmission distance, strong anti-electromagnetic interference ability, and low cost.

[0003] Among them, for the CAN serial interface circuit and its transmission line in monitoring equipment applied in the railway industry, since they often work in harsh environmental conditions outdoors and under traction power supply, currently, multiple protection requirements such as electrostatic protection and lightning protection are put forward for the CAN serial interface.

[0004] However, there is currently no technology that can solve the above technical problems simultaneously. Summary of the Invention

[0005] The purpose of the present invention is to provide a protection circuit for a CAN serial interface in view of the technical defects existing in the prior art.

[0006] To this end, the present invention provides a protection circuit for a CAN serial interface, including a two-channel digital opto-isolator U1 and a CAN bus transceiver U2;

[0007] The two-channel digital opto-isolator U1 is respectively connected to power supplies VCC1 and VCC2;

[0008] The two-channel digital opto-isolator U1 is connected to a chip with UART communication function located outside;

[0009] The two-channel digital opto-isolator U1 is connected to the CAN bus transceiver U2;

[0010] The CAN bus transceiver U2 is connected to a CAN serial interface located outside through a circuit protection system;

[0011] The protection system is used to provide safety protection for the CAN serial interface.

[0012] As can be seen from the technical solutions provided by the present invention above, compared with the prior art, the present invention provides a protection circuit for a CAN serial interface, which is scientifically designed, can provide multiple safety protection requirements such as electrostatic protection and lightning protection for the CAN serial interface, ensure that the CAN serial interface can work normally under harsh outdoor environmental conditions, improve the user's product usage experience, and has great practical significance. Brief Description of the Drawings

[0013] Figure 1 Schematic diagram of a protection circuit for a CAN serial interface provided by the present invention;

[0014] Figure 2 Equivalent circuit diagram for the common-mode lightning protection test connection of the CAN_H terminal of a CAN serial interface, which is a protection circuit for a CAN serial interface provided by the present invention;

[0015] Figure 3 Equivalent circuit diagram for the differential-mode lightning protection test connection of the CAN_H terminal of a CAN serial interface, which is a protection circuit for a CAN serial interface provided by the present invention;

[0016] In the figure, U1 is a two-channel digital opto-isolator; U2 is a CAN bus transceiver;

[0017] C1 is a capacitor; C2 is a capacitor; C3 is a capacitor; C4 is a capacitor;

[0018] R1 is a resistor; R2 is a resistor; R3 is a resistor; R4 is a resistor;

[0019] D1 is a fast recovery diode; D2 is a fast recovery diode; D3 is a fast recovery diode; D4 is a fast recovery diode;

[0020] D5 is a transient suppression diode;

[0021] G1 is a gas discharge tube; G2 is a gas discharge tube; G3 is a gas discharge tube. Detailed implementation manners

[0022] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0023] In the description of this patent, it should be noted that unless otherwise clearly defined and limited, the terms "installation", "connection", "connection", and "setting" should be understood in a broad sense. For example, it can be fixedly connected and set, or detachably connected and set, or integrally connected and set. For those of ordinary skill in the art, the specific meanings of the above terms in this patent can be understood according to specific situations.

[0024] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality of" means two or more unless otherwise specifically defined.

[0025] See Figures 1 to 3 , the present invention provides a protection circuit for a CAN serial interface, including a two-channel digital opto-isolator U1 and a CAN bus transceiver U2;

[0026] The two-channel digital opto-isolator U1 is respectively connected to power supplies VCC1 and VCC2;

[0027] The two-channel digital opto-isolator U1 is connected to a chip with a universal asynchronous receiver-transmitter (UART) communication function located outside;

[0028] The two-channel digital opto-isolator U1 is connected to the CAN bus transceiver U2;

[0029] The CAN bus transceiver U2 is connected to a CAN serial interface located outside through a circuit protection system;

[0030] The protection system is used to provide safety protection for the CAN serial interface (such as electrostatic protection and lightning protection).

[0031] In the present invention, specifically, the VDD1 port of the two-channel digital opto-isolator U1 is connected to the power supply VCC1;

[0032] The VDD2 port of the two-channel digital opto-isolator U1 is connected to the power supply VCC2;

[0033] The VOA port of the two-channel digital opto-isolator U1 is connected to the data receiving end UART_RXD of a chip with a universal asynchronous receiver-transmitter (UART) communication function located outside;

[0034] The VIB port of the two-channel digital opto-isolator U1 is connected to the data sending end UART_TXD of a chip with a UART communication function located outside;

[0035] The VIA port of the two-channel digital opto-isolator U1 is connected to the RXD port of the CAN bus transceiver U2;

[0036] The VOB port of the two-channel digital opto-isolator U1 is connected to the TXD port of the CAN bus transceiver U2;

[0037] The GND1 port of the two-channel digital opto-isolator U1 is connected to the power supply ground terminal GND1;

[0038] The GND 2 port of the two-channel digital opto-isolator U1 is connected to the power supply ground terminal GND2;

[0039] The S port of the CAN bus transceiver U2 is connected to the power supply ground terminal GND2 through a resistor R1;

[0040] The CANH port of the CAN bus transceiver U2 and the CANL port of the CAN bus transceiver U2 are respectively connected to both ends of the resistor R2;

[0041] The VREF port of the CAN bus transceiver U2 is connected to the power supply ground terminal GND2 through a capacitor C4;

[0042] The GDN port of the CAN bus transceiver U2 is connected to the power supply ground terminal GND2;

[0043] The VCC port of the CAN bus transceiver U2 is connected to the power supply VCC2;

[0044] In specific implementation, the power supply VCC1 is connected to the wire ground terminal GND1 through a capacitor C1;

[0045] The power supply VCC2 is connected to the wire ground terminal GND2 through a capacitor C2;

[0046] In specific implementation, the power supply VCC2 is connected to the wire ground terminal GND2 through a capacitor C3;

[0047] In the present invention, in specific implementation, the protection system includes gas discharge tubes G1 to G3, fast recovery diodes D1 to D4, and transient suppression diode D5;

[0048] Among them, one end of the transient suppression diode D5 is connected to the positive electrodes of the fast recovery diode D1 and the fast recovery diode D3;

[0049] The other end of the transient suppression diode D5 is connected to the negative electrodes of the fast recovery diode D2 and the fast recovery diode D4;

[0050] The negative electrode of the fast recovery diode D1 and the positive electrode of the fast recovery diode D2 are simultaneously connected to the CANH port of the CAN bus transceiver U2;

[0051] The negative electrode of the fast recovery diode D3 and the positive electrode of the fast recovery diode D4 are simultaneously connected to the CANL port of the CAN bus transceiver U2;

[0052] The CANH port of the CAN bus transceiver U2 is connected to the CAN_H port (i.e., the external CAN communication port CAN_H) on the external CAN serial interface through the resistor R3;

[0053] The CANL port of the CAN bus transceiver U2 is connected to the CAN_L port (i.e., the external CAN communication port CAN_L) on the external CAN serial interface through the resistor R4;

[0054] One end of the gas discharge tube G1 is connected to the chassis ground PE, and the other end is respectively connected to one end of the resistor R3, one end of the gas discharge tube G2, and the CAN_H port (i.e., the external CAN communication port CAN_H) on the CAN serial interface;

[0055] One end of the gas discharge tube G3 is connected to the chassis ground PE, and the other end is respectively connected to one end of the resistor R4, the other end of the gas discharge tube G2, and the CAN_L port (i.e., the external CAN communication port CAN_L) on the CAN serial interface.

[0056] In the present invention, it should be noted that the CAN serial interface includes two communication ports, namely the CAN_H port and the CAN_L port. Among them, the CAN_H port refers to the high-level signal line port in the CAN bus, and its voltage range is 2.5V to 5V. The CAN_L port refers to the low-level signal line port in the CAN bus, and its voltage range is 0V to 2.5V.

[0057] The CAN_H port and the CAN_L port (i.e., the external CAN communication port CAN_H and the external CAN communication port CAN_L) on the CAN serial interface, which serve as the external CAN communication ports, can both receive and send, and can be connected to external devices with the same CAN communication function (such as intelligent pressure transmitters). For example, it can be connected to the MPM4784 type intelligent pressure transmitter produced by the external Mike Sensor Company.

[0058] In the present invention, it should be noted that the chip with UART communication function located outside is a mature chip in the prior art, and there are various models. For example, the MYC-Y6ULY2-V2-256N256D-50-I chip produced by Mir Technology Company can be adopted. The chip with UART communication function is used to receive analog or digital signals sent by other external devices and process the signals. Its data sending end UART_TXD can send data to the VIB port of the two-channel digital opto-isolator U1, or its data receiving end UART_RXD can receive the data from the VOA port of the two-channel digital opto-isolator U1.

[0059] In the present invention, specifically, the two-channel digital opto-isolator U1 is a mature electronic component in the prior art. For example, a two-channel digital opto-isolator of the π122M31 model produced by RongPai Semiconductor Company can be used. Its function is opto-isolation, and it can receive a 5V signal or a 3.3V signal.

[0060] It should be noted that for the two-channel digital opto-isolator U1, the VDD1 port is the power supply terminal for VOA and VIB, the VSS1 port is the ground terminal for VOA and VIB, the VDD2 port is the power supply terminal for VIA and VOB, and the VSS2 port is the ground terminal for VIA and VOB. The VOA port is the signal output terminal of channel A, the VIA port is the signal input terminal of channel A, the VOB port is the signal output terminal of channel B, and the VIB port is the signal input terminal of channel B.

[0061] In the present invention, specifically, the CAN bus transceiver U2 is a mature electronic component in the prior art. For example, a CAN bus transceiver of the SIT1050 model produced by XinLite Company can be used. Its function is to convert UART signals and CAN signals into each other.

[0062] It should be noted that for the CAN bus transceiver U2, the VDD port is the power supply terminal; the VSS port is the ground terminal, the TXD port is the data input terminal of the bus transceiver; the RXD port is the data output terminal of the bus transceiver; the CANH port is the high-potential CAN voltage input / output terminal; the CANL port is the low-potential CAN voltage input / output terminal;

[0063] The VREF port is the reference voltage output terminal; the S port is the high-speed mode selection terminal, and a low level represents the high-speed mode.

[0064] In the present invention, it should be noted that the transient suppression diode D5 is a transient suppression diode of the SMBJ6.5CA model produced by STMicroelectronics.

[0065] In the present invention, specifically, the power supply VCC1 can be a 5V or 3.3V DC power supply.

[0066] In the present invention, specifically, the power supply VCC2 is a 5V DC power supply.

[0067] In the present invention, specifically, the capacitors C1, C2, C3, and C4 are all 0.1uF ceramic capacitors.

[0068] In the present invention, specifically, the resistor R1 is a 10KΩ resistor;

[0069] The resistor R2 is a 120Ω resistor;

[0070] The resistors R3 and R4 are both resistors with a resistance value of 1 Ω;

[0071] In the present invention, the fast recovery diodes D1, D2, D3, and D4 are all fast recovery rectifier diodes of model MURS120T3;

[0072] The transient suppression diode D5 is a transient suppression diode of model SMBJ6.5CA;

[0073] The gas discharge tubes G1, G2, and G3 are all gas discharge tubes of model BWF900;

[0074] In the present invention, it should be noted that the function of the two-channel digital opto-isolator U1 is: to isolate the circuit between the chip with UART communication function located outside and the CAN bus transceiver U2, and can be compatible with input 3.3V and 5V signals;

[0075] The function of the CAN bus transceiver U2 is: to convert the UART signal into a CAN signal;

[0076] The function of the capacitor C1 is: to isolate the high-frequency noise in the power supply VCC1;

[0077] The functions of the capacitors C2 and C3 are both to isolate the high-frequency noise in the power supply VCC2;

[0078] The function of the capacitor C4 is: to isolate the high-frequency noise of the reference output voltage of the CAN bus transceiver U2;

[0079] The function of the resistor R1 is: to be used as a pull-down resistor to make the CAN bus transceiver U2 mode selection be the high-speed CAN mode;

[0080] The resistor R2 is a terminal matching resistor, which is used to weaken the signal reflection in the CAN line;

[0081] The resistors R3 and R4 are used to provide the backend resistance value when the gas discharge tube G2 conducts, facilitating the short-circuit of the backend circuit by the gas discharge tube G2;

[0082] The gas discharge tubes G1, G2, and G3 are used to discharge the instantaneous high voltage existing on the CAN_H port and the CAN_L port (i.e., the external CAN communication port CAN_H and the external CAN communication port CAN_L on the CAN serial interface).

[0083] It should be noted that for the protection circuit of the CAN serial interface provided by the present invention, first, the signal is sent from a chip located externally and having UART communication function, then it is optically isolated by a two-channel digital opto-isolator U1 and the UART signal is converted into a CAN signal by a CAN bus transceiver U2, and finally, through electrostatic, electromagnetic compatibility and lightning protection designs, the safe and reliable transceiver of the signal is ensured.

[0084] To understand the technical solution of the present invention more clearly, the working principle of the present invention is described below.

[0085] I. Isolation function.

[0086] The VDD1 and VDD2 ports (i.e., pins) of the two-channel digital opto-isolator U1 are respectively connected to power supplies VCC1 and VCC2 for power supply. The power supplies VCC1 and VCC2 are respectively connected with 0.1uF ceramic capacitors (i.e., capacitors C1 and C2) to the wire ground terminals GND_1 and GND_2, thereby effectively filtering out the high-frequency components in the power supply;

[0087] Since the VDD1 and VDD2 ports (i.e., pins) of the two-channel digital opto-isolator U1 are respectively connected to two independent power supplies (i.e., power supplies VCC1 and VCC2), and the transmission channels are also independent of each other, the mutual interference of the received and transmitted signals of the two-channel digital opto-isolator U1 caused by sharing a power supply is avoided, the isolation of the transmitted signal is realized, and the transmission rate and anti-interference ability of the signal are improved.

[0088] Among them, the power supply VCC1 can be a 3.3V DC power supply or a 5V DC power supply, thus solving the problem of compatibility between 3.3V signals and 5V signals.

[0089] II. Level conversion function (see Table 1).

[0090] 1. Transmission state:

[0091] When the data transmission end UART_TXD of the chip located externally and having UART communication function is a high-level signal, the VIB and VOB ends of the two-channel digital opto-isolator U1 are high level, the TXD end of the CAN bus transceiver U2 is high level, the CANH end of the CAN bus transceiver U2 is high level, and the CANL end is low level.

[0092] When the data transmission end UART_TXD of the externally located chip with UART communication function is at a low-level signal, the VIB and VOB ends of the two-channel digital opto-isolator U1 are at low level, the TXD end of the CAN bus transceiver U2 is at high level, the CANH port of the CAN bus transceiver U2 is 1 / 2 of VCC2, and the CANL port of the CAN bus transceiver U2 is 1 / 2 of VCC2.

[0093] It should be noted that when the CAN communication represents a "1" signal, the CAN_H and CAN_L ports of the CAN bus transceiver U2 are at a voltage of 1 / 2 of VCC2; when the CAN communication represents a "0" signal, the CANH port of the CAN bus transceiver U2 is at high level and the CANL port of the CAN bus transceiver U2 is at low level.

[0094] 2. Receiving state:

[0095] When the CANH end of the CAN bus transceiver U2 is at high level and the CANL end is at low level, the RXD end of the CAN bus transceiver U2 is at high level, then the VIA and VOA ends of the two-channel digital opto-isolator U1 are at high level, and the UART_RXD end (i.e., the signal end RXD of the chip with UART communication function, i.e., the data receiving end RXD) is at high level.

[0096] When the CANH end of the CAN bus transceiver U2 is 1 / 2 of VCC2, the CANL end is 1 / 2 of VCC2, and the RXD end of the CAN bus transceiver U2 is at low level, then the VIA and VOA ends of the two-channel digital opto-isolator U1 are at low level, and the UART_RXD end (i.e., the signal end RXD of the chip with UART communication function) is at low level.

[0097] The UART~CAN input-output truth table is shown in Table 1 (H is high level, L is low level, 0.5V CC is 1 / 2 of the high level voltage equal to VCC2).

[0098] Table 1: Input-output truth table.

[0099]

[0100] III. Electromagnetic compatibility function.

[0101] 1. ESD (electrostatic discharge) protection function;

[0102] According to the provisions of the national standard of the People's Republic of China "GBT24338.5-2018 Rail Transit Electromagnetic Compatibility Part 4: Emission and Immunity of Signal and Communication Equipment": The electrostatic requirements for the CAN interface are ±8 kV for air discharge and ±6 kV for contact discharge. Electrostatic discharge belongs to common-mode interference; the signal amplitude of the CAN interface is 0 to +5 V, and the signal type is bidirectional;

[0103] For the present invention, the CAN bus transceiver U2 specifically uses an existing CAN bus transceiver with the model number SIT1050, and its port withstand voltage is ±40 V.

[0104] For the present invention, the existing transient suppression diode D5 with the model number SMBJ6.5CA has a working voltage of ±6.5 V, air discharge protection of ±30 kV, and contact discharge protection of ±30 kV, which can meet the design requirements. Through the transient suppression diode, that is, D5, the ESD (electrostatic discharge) protection function for the CAN interface can be achieved.

[0105] 2. Electrical fast transient burst protection function;

[0106] According to the provisions of the national standard of the People's Republic of China "GBT24338.5-2018 Rail Transit Electromagnetic Compatibility Part 4: Emission and Immunity of Signal and Communication Equipment": The CAN transmission balanced upper line signal interface requires applying 200 pulse bursts in 1 minute. Each pulse burst consists of 75 pulses with a duration of 15 ms. The peak voltage of each pulse is ±2 kV (open-circuit voltage), the peak / half-peak time is 5 / 50 ns, and the period is 0.2 ms.

[0107] For the present invention, the transient suppression diode D5 specifically uses an existing transient suppression diode with the model number SMBJ6.5CA, and the applied electrical fast pulse burst protection is ±4 kV, which can meet the design requirements.

[0108] 3. Surge protection function.

[0109] According to the provisions of the national standard of the People's Republic of China "GBT24338.5-2018 Rail Transit Electromagnetic Compatibility Part 4: Emission and Immunity of Signal and Communication Equipment": The CAN transmission line requires applying ±2 kV pulses 5 times each to the CANH terminal and CANL terminal of the CAN bus transceiver U2. The peak / half-peak time when open-circuited is 1.2 / 50 μs, the peak / half-peak time when short-circuited is 8 / 20 μs, the output impedance is 42 Ω, and the short-circuit current is approximately 47.62 A.

[0110] For the present invention, the transient voltage suppression diode D5 specifically used is an existing transient voltage suppression diode with the model SMBJ6.5CA, which can withstand a peak current of 226 A (8 / 20 us), meeting the requirements of surge protection.

[0111] IV. Lightning protection function.

[0112] According to the requirements of the severe level 5 of the lightning strike test for communication ports of communication signal electronic equipment specified in the Railway Industry Standard of the People's Republic of China, "TB / T 3498-2018 Test Method for Lightning Strike of Railway Communication Signal Equipment": For the lightning test of the CAN interface of the equipment beside the outdoor rail (i.e., the outdoor train rail), an open-circuit voltage peak of 6 KV should be applied to the ground wire PE (i.e., the chassis ground) of the chassis (i.e., the shell of the equipment applying the present invention's circuit) at the CAN_H port and the CAN_L port (i.e., the external CAN communication port CAN_H and the external CAN communication port CAN_L on the CAN serial interface), and a combined wave of 10 / 700 us - 5 / 320 us with a short-circuit current peak of 150 A; an open-circuit voltage peak of 3 KV should be applied between the CAN_H port and the CAN_L port, and a combined wave of 10 / 700 us - 5 / 320 us with a short-circuit current peak of 75 A.

[0113] In the present invention, the CAN interface utilizes surge protection devices to achieve lightning protection for the interface, thereby realizing the high reliability of the system.

[0114] In the present invention, the lightning common-mode protection system consists of one stage. Gas discharge tubes G1, G2, and G3 are used for surge protection at the input end of the CAN interface (the CAN_H port or the CAN_L port to the ground wire PE of the chassis).

[0115] In the present invention, the lightning differential-mode protection system consists of two stages. Gas discharge tubes G1, G2, and G3 are used for the first-stage surge protection at the input end of the CAN interface (specifically including the CAN_H port and the CAN_L port); resistors R3, R4, fast recovery diodes D1, D2, D3, D4, and transient voltage suppression diode D5 are used for the second-stage surge protection.

[0116] When the lightning surge voltage does not reach the pulse breakdown voltage of the gas discharge tubes G1, G2, and G3, the transient voltage suppression diode D5 and the fast recovery diodes D1, D2, D3, and D4 are activated first to clamp the surge voltage at the CAN bus transceiver interface to a low potential.

[0117] When the lightning surge voltage continues to increase to the pulse breakdown voltage of the gas discharge tubes G1, G2, and G3, the surge current is discharged to the ground through the gas discharge tubes G1, G2, and G3, playing a protective role.

[0118] It should be noted that for the present invention, the functions of the gas discharge tubes G1, G2, and G3 are as follows: they respectively provide primary lightning protection for the CAN_H port and the CAN_L port (i.e., the external CAN communication port CAN_H and the external CAN communication port CAN_L on the CAN serial interface); the fast recovery diodes D1, D2, D3, D4, and the transient suppression diode D5 respectively provide secondary common-mode lightning protection for the CAN_H port and the CAN_L port.

[0119] For the present invention, for the existing gas discharge tubes G1, G2, and G3 of the BWF900 model, the maximum pulse peak current is 5000A, which is greater than the lightning test short-circuit peak current of 150A, and the maximum pulse peak discharge voltage is 600V, which is less than the surge voltage generated by lightning of 6000V, meeting the design requirements; the DC discharge voltage of the gas discharge tubes G1, G2, and G3 is 90V, which is greater than the maximum operating voltage of the circuit of 12V DC, ensuring the normal operation of the circuit and meeting the design requirements.

[0120] For the present invention, regarding common-mode protection, when the surge voltage is greater than 600V, the gas discharge tubes G1 and G3 are broken down, the resistance approaches 0, short-circuiting the current at the back end, and the surge voltage discharges energy through the gas discharge tubes.

[0121] See Figure 2 , Figure 2 In, K is the bus node, SG is the lightning surge generator, the dotted line is the equivalent circuit when the surge generator generates a surge, r is the internal resistance of the 40Ω surge generator, and U is the instantaneous voltage of the surge generator. The lightning protection process is described below by taking the lightning protection of the H end as an example. The H end is the CANH port on the CAN bus transceiver U2. When the surge voltage is less than or equal to 600V, the lightning protection circuit does not start. When the surge voltage exceeds 600V, the gas discharge tube G1 is broken down, the resistor R1 and the subsequent circuit are short-circuited, and the lightning protection is started.

[0122] For the present invention, regarding differential-mode protection, when the surge voltage is less than or equal to 600V, between the CAN_H port and the CAN_L port, a loop is formed through the fast recovery diodes D1, D2, D3, D4, and the transient suppression diode D5 and the chassis ground PE, and the lightning surge voltage is clamped to 11.2V by the transient suppression diode D5, starting the secondary lightning protection;

[0123] When the surge voltage is greater than 600V, the gas discharge tube G2 is broken down, the resistance approaches 0, short-circuiting the fast recovery diodes D1, D2, D3, D4, and the transient suppression diode D5, and the primary lightning protection is started, discharging the 600V - 6000V surge voltage through the primary lightning protection.

[0124] See Figure 3 , Figure 3In it, A and B are bus nodes, H is the CANH port of the CAN bus transceiver U2, L is the CANL port of the CAN bus transceiver U2, SG is a lightning surge generator, and the dotted line shows the equivalent circuit when the surge generator generates a surge. r is the internal resistance of the 40Ω surge generator, and U is the instantaneous voltage of the surge generator. The following takes the lightning protection of the CAN_H terminal (i.e., the H terminal) as an example to illustrate the lightning protection process.

[0125] The maximum instantaneous forward voltage values of the fast recovery diodes D1, D2, D3, and D4 are 0.875V, the breakdown voltage of the transient voltage suppressor diode D5 is 7.22V, and the clamping voltage is 11.2V. When the instantaneous voltage U of the lightning surge generator SG reaches 8.97V, the transient voltage suppressor diode D5 breaks down, and the current flows from the CAN_H point to the H point, then through the B point and A point in sequence to the L point, and finally back to the CAN_L point. When the instantaneous voltage U of the lightning surge generator SG reaches 12.95V, the transient voltage suppressor diode D5 clamps; when the instantaneous voltage U of the lightning surge generator reaches 600V, the gas discharge tube G2 breaks down, the resistance of the gas discharge tube G2 approaches 0, and the short-circuit resistors R3, R4, fast recovery diodes D1, D2, D3, D4, and transient voltage suppressor diode D5, and the surge current flows back to the lightning surge generator through the gas discharge tube G2. Connected in this way, it is ensured that the CAN bus transceiver U2 at the back end will not be damaged due to the instantaneous high voltage borne between its CANH port and CANL port.

[0126] When the instantaneous voltage U of the lightning surge generator SG reaches 8.97V, the transient voltage suppressor diode D5 breaks down, and the voltage at H is 8.97V. When the instantaneous voltage U of the lightning surge generator SG reaches 12.95V, the transient voltage suppressor diode D5 clamps, and the voltage at H is 12.95V;

[0127] When the instantaneous voltage U of the lightning surge generator SG reaches 600V, the gas discharge tube G2 breaks down, the resistance of G2 approaches 0, and the short-circuit resistors R3, R4, fast recovery diodes D1, D2, D3, D4, transient voltage suppressor diode D5, and the subsequent circuit, and the surge current flows into the ground through the gas discharge tube G1. Connected in this way, it is ensured that the CAN bus transceiver U2 at the back end will not be damaged due to the instantaneous high voltage borne by its CANH port.

[0128] Compared with the prior art, the protection circuit of the CAN serial interface provided by the present invention has the following beneficial effects:

[0129] 1. Compared with the prior art, the protection circuit of the CAN serial interface provided by the present invention can be applied in the technical field of railway system monitoring equipment, especially for outdoor equipment, can transmit information accurately, and reduce the cost of replacing equipment;

[0130] 2. For the protection circuit of the CAN serial interface provided by the present invention, its input terminal can be compatible with 3.3V and 5V signals, and has a wide application range.

[0131] 3. For the protection circuit of the CAN serial interface provided by the present invention, its input signals are only TXD and RXD signals (i.e., the data transmission terminal TXD signal and the data reception terminal RXD signal of a chip with UART communication function), without a data direction control signal. The transient suppression diode adopted by the present invention has multiple protection effects such as anti-static, anti-surge, and anti-fast pulse group. The integration degree of the PCB board is high, reducing the cost.

[0132] 4. By applying the present invention, this solution can be adopted for each device with CAN communication design, and the design of specifically using the existing lightning protection module can be abandoned, effectively controlling the cost and reducing the occupied space.

[0133] 5. There is no capacitive device directly connected to the CANH port and the CANL port (i.e., the CANH port and the CANL port of the CAN bus transceiver U2). For example, there is no inductor, capacitor, and transient suppression diode, etc., connected to the CANH port and the CANL port of the CAN bus transceiver U2, and the high-frequency signal transmission is clear.

[0134] After inspection, the protection circuit of the CAN serial interface provided by the present invention is an interface circuit with strong anti-interference ability, high safety, and stable output signal. It can be reliably applied to the monitoring equipment in the railway industry, improving the applicability and reliability of the CAN serial interface, ensuring the use safety of the CAN serial interface, and enabling the CAN serial interface on the monitoring equipment in the railway industry to better meet the use requirements in complex environments such as outdoors.

[0135] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principle of the present invention, several improvements and retouches can be made, and these improvements and retouches should also be regarded as the protection scope of the present invention.

Claims

1. A protection circuit for a CAN serial interface, characterized in that It includes a two-channel digital opto-isolator U1 and a CAN bus transceiver U2; The two-channel digital opto-isolator U1 is respectively connected to power supplies VCC1 and VCC2; The two-channel digital opto-isolator U1 is connected to an external chip with UART communication function; The two-channel digital opto-isolator U1 is connected to the CAN bus transceiver U2; The CAN bus transceiver U2 is connected to an external CAN serial interface through a circuit protection system; The protection system is used to provide safety protection for the CAN serial interface; The VDD1 port of the two-channel digital opto-isolator U1 is connected to the power supply VCC1; The VDD2 port of the two-channel digital opto-isolator U1 is connected to the power supply VCC2; The protection system includes gas discharge tubes G1~G3, fast recovery diodes D1~D4, and a transient suppression diode D5; One end of the transient suppression diode D5 is connected to the positive poles of the fast recovery diode D1 and the fast recovery diode D3; The other end of the transient suppression diode D5 is connected to the negative poles of the fast recovery diode D2 and the fast recovery diode D4; The negative pole of the fast recovery diode D1 and the positive pole of the fast recovery diode D2 are simultaneously connected to the CANH port of the CAN bus transceiver U2; The negative pole of the fast recovery diode D3 and the positive pole of the fast recovery diode D4 are simultaneously connected to the CANL port of the CAN bus transceiver U2; The CANH port of the CAN bus transceiver U2 is connected to the CAN_H port on the external CAN serial interface through a resistor R3; The CANL port of the CAN bus transceiver U2 is connected to the CAN_L port on the external CAN serial interface through a resistor R4; One end of the gas discharge tube G1 is connected to the chassis ground PE, and the other end is respectively connected to one end of the resistor R3, one end of the gas discharge tube G2, and the CAN_H port on the CAN serial interface; One end of the gas discharge tube G3 is connected to the chassis ground PE, and the other end is respectively connected to one end of the resistor R4, the other end of the gas discharge tube G2, and the CAN_L port on the CAN serial interface.

2. The protection circuit of the CAN serial interface according to claim 1, characterized in that The VOA port of the two-channel digital opto-isolator U1 is connected to the data receiving end UART_RXD of an external chip with UART communication function; The VIB port of the two-channel digital opto-isolator U1 is connected to the data sending end UART_TXD of an external chip with UART communication function; The VIA port of the two-channel digital opto-isolator U1 is connected to the RXD port of the CAN bus transceiver U2; The VOB port of the two-channel digital opto-isolator U1 is connected to the TXD port of the CAN bus transceiver U2; The GND1 port of the two-channel digital opto-isolator U1 is connected to the power supply ground terminal GND1; The GND 2 port of the two-channel digital opto-isolator U1 is connected to the power supply ground terminal GND2; The S port of the CAN bus transceiver U2 is connected to the power supply ground terminal GND2 through a resistor R1; The CANH port of the CAN bus transceiver U2 and the CANL port of the CAN bus transceiver U2 are respectively connected to both ends of the resistor R2; The VREF port of the CAN bus transceiver U2 is connected to the power supply ground terminal GND2 through a capacitor C4; The GDN port of the CAN bus transceiver U2 is connected to the power supply ground terminal GND2; The VCC port of the CAN bus transceiver U2 is connected to the power supply VCC2.

3. The protection circuit of the CAN serial interface according to claim 2, wherein The power supply VCC1 is connected to the wire ground terminal GND1 through a capacitor C1; The power supply VCC2 is connected to the wire ground terminal GND2 through a capacitor C2.

4. The protection circuit of the CAN serial interface according to claim 3, characterized in that, The power supply VCC2 is also connected to the wire ground terminal GND2 through a capacitor C3.

5. The protection circuit of the CAN serial interface according to claim 4, characterized in that, The power supply VCC1 is a 5V or 3.3V DC power supply; The power supply VCC2 is a 5V DC power supply; The capacitors C1, C2, C3, and C4 are all 0.1uF ceramic capacitors.

6. The protection circuit of the CAN serial interface according to claim 2, wherein, The resistor R1 is a 10KΩ resistor; The resistor R2 is a 120Ω resistor.

7. The protection circuit of the CAN serial interface according to claim 1, wherein The resistors R3 and R4 are both 1Ω resistors.

8. The protection circuit of the CAN serial interface according to claim 1, characterized in that The fast recovery diodes D1, D2, D3, and D4 are all fast recovery rectifier diodes of model MURS120T3; The transient suppression diode D5 is a transient suppression diode of model SMBJ6.5CA.

9. The protection circuit of the CAN serial interface according to claim 1, wherein The gas discharge tubes G1, G2, and G3 are all gas discharge tubes of model BWF900.

Images