A differential bus system for automotive diagnosis

Abstract

The present invention discloses a differential bus system for vehicle diagnosis, which includes a main control MCU circuit, an adjustable differential bus circuit and an OBD diagnostic interface connected in sequence. The main control MCU circuit outputs or receives square wave pulses. The adjustable differential bus circuit includes a differential bus high-end sending circuit and a differential bus low-end sending circuit. The main control MCU circuit is connected in parallel with the differential bus high-end sending circuit and the differential bus low-end sending circuit. The differential bus high-end sending circuit and the differential bus low-end sending circuit are respectively connected to the OBD diagnostic interface. By utilizing the voltage difference change between the differential bus high-end sending circuit and the differential bus low-end sending circuit, the data stream transmission between the main control MCU circuit and the OBD diagnostic interface is simulated, and finally the function of fault finding or diagnosis of the vehicle is realized. It solves the problem that the existing vehicle diagnostic instrument cannot diagnose vehicles with adjustable differential buses, expands the use scenario of the diagnostic instrument, and improves the versatility of the diagnostic instrument.

Description

Technical Field

[0001] The present invention relates to the technical field of automotive diagnosis, and particularly to a differential bus system for automotive diagnosis. Background Art

[0002] An automotive bus is a channel for data exchange between an automotive ECU and various control units of the vehicle. Currently, common automotive buses mainly include K-line, RS485, RS232, J1850, single-wire CAN, low-speed CAN, high-speed CAN, CANFD, automotive Ethernet, etc. The biggest feature is that it can transmit stable and reliable data signals with the least amount of wiring harnesses. At the same time, with the progress of technology, the transmission speed is getting faster and faster, and the information contained in the bus data stream is also increasing. Due to the diverse forms of buses, usually the bus forms adopted by each automotive manufacturer are different, and for the same automotive manufacturer, different vehicle models may also adopt different bus methods. As the control core of the vehicle, the security of the ECU data must be ensured. Different automotive manufacturers have formulated non-standard bus standards, that is, adjustable differential buses, in order to protect the automotive ECU data and prevent the ECU data from being illegally rewritten or read.

[0003] However, this method of setting an adjustable differential bus in a vehicle, although it prevents the ECU data from being illegally rewritten or read, has an obvious drawback, that is, existing diagnostic instruments cannot check vehicles with adjustable differential buses. Once a vehicle with an adjustable differential bus fails, an ordinary diagnostic instrument cannot normally read the status of the ECU from the OBD diagnostic port, which brings inconvenience to vehicle maintenance. Therefore, in vehicle maintenance, there is an urgent need for an automotive diagnostic instrument that can be used for an adjustable differential bus circuit, so that vehicle maintenance personnel can quickly diagnose and search for vehicles with adjustable differential buses through the above-mentioned diagnostic instrument to obtain the cause of vehicle failure and provide assistance for vehicle maintenance. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a differential bus system for automotive diagnosis, which solves the problem that existing automotive diagnostic instruments cannot perform automotive diagnosis on vehicles with adjustable differential buses, expands the usage scenarios of the diagnostic instrument, and improves the versatility of the diagnostic instrument, thereby overcoming the deficiencies of the prior art.

[0005] To solve the above technical problem, the present invention provides a differential bus system for automotive diagnosis, which includes a main control MCU circuit, an adjustable differential bus circuit, and an OBD diagnostic interface connected in sequence,

[0006] wherein the main control MCU circuit includes two IO interfaces, one IO interface is an output IO interface for outputting square wave pulses, and the other IO interface is an input IO interface for receiving square wave pulses;

[0007] The adjustable differential bus circuit includes a differential bus high - end transmission circuit, a differential bus low - end transmission circuit, a differential bus matching terminal, and a differential bus receiving circuit. Among them, the output IO interface of the master MCU circuit is connected in parallel to the input interfaces of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit. The output interfaces of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit are respectively connected to the OBD diagnostic interface, and the differential bus matching terminal is connected between the output interfaces of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit. The differential bus receiving circuit receives the differential signal output by the adjustable differential bus circuit and transmits it to the input IO interface of the master MCU circuit.

[0008] When the master MCU circuit has no data output, the voltages of the output interfaces of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit are equal, and the voltage difference is 0. At this time, the voltage difference on the adjustable differential bus circuit represents the binary number 0 in the data stream; when the master MCU circuit has data output, there is a voltage difference V between the output interface of the differential bus high - end transmission circuit and the output interface of the differential bus low - end transmission circuit. At this time, the voltage difference V on the adjustable differential bus circuit represents the binary number 1 in the data stream. By simulating the binary data in the data stream through the voltage difference of the adjustable differential bus circuit, the data stream transmission from the output IO interface of the master MCU circuit to the OBD diagnostic interface is realized.

[0009] As an improvement of the present invention, the differential bus matching terminal includes a terminal resistor R5. One end of the terminal resistor R5 is connected to the output interface of the differential bus high - end transmission circuit, and the other end is connected to the output interface of the differential bus low - end transmission circuit. The terminal resistor R5 is used to improve the differential signal transmission waveform and reduce high - speed signal reflection.

[0010] As a further improvement of the present invention, the differential bus high-end transmission circuit includes a resistor R1, a resistor R2, a resistor R3, a resistor R4, a triode Q1, a triode Q2, a fuse resistor F1, and at least one diode D1. One end of the resistor R2 is connected to the output IO interface of the main control MCU circuit, and the other end of the resistor R2 is connected in parallel with one end of the resistor R3 and the base of the triode Q2. The other end of the resistor R3 is connected in parallel with the emitter of the triode Q2 and one end of the resistor R4 and then connected to the ground terminal GND; the collector of the triode Q2 is connected to one end of the resistor R1, and the other end of the resistor R1 is connected to the base of the triode Q1; the emitter of the triode Q1 is connected to one end of the fuse resistor F1, and the other end of the fuse resistor F1 is connected to the power input terminal VCCIN; the collector of the triode Q1 is connected in parallel with the other end of the resistor R4 and the positive electrode of the diode D1; the negative electrode of the diode D1 is connected in parallel with one end of the terminal resistor R5, and the negative electrode of the diode D1 is connected to the output interface of the differential bus high-end transmission circuit.

[0011] As an improvement of the present invention, when the output IO interface of the main control MCU circuit outputs a low level, after voltage division by the resistor R2 and the resistor R3, the voltage applied to the base of the triode Q2 is a low level. At this time, the triode Q2 is cut off, and the triode Q2 is in an open circuit state. At the same time, the triode Q1 will also be in an open circuit state. At this time, the positive electrode of the diode D1 is at zero level. At this time, the voltage of the output interface in the differential bus high-end transmission circuit is in a static state, and the voltage of the output interface in the differential bus high-end transmission circuit is V_OUT; when the output IO interface of the main control MCU circuit outputs a high level, after voltage division by the resistor R2 and the resistor R3, the voltage applied to the base of the triode Q2 is a high level, and the base of the triode Q2 is forward-biased and conducts. At this time, the collector level of the triode Q2 is equal to the ground terminal GND. At this time, the base of the triode Q1 is at a low level, and the triode Q1 is in a conducting state. At this time, the collector voltage of the triode Q1 is equal to the power input terminal VCCIN. Since the power input terminal VCCIN is greater than V_OUT, the diode D1 conducts forward. Since the forward voltage drop of the diode D1 is a constant value P, the voltage of the negative electrode of the diode D1 is VCCIN–P. At this time, the voltage of the output interface in the differential bus high-end transmission circuit will be increased to VCCIN–P.

[0012] As a further improvement of the present invention, the differential bus low - end transmission circuit includes a resistor R6, a resistor R7, a triode Q3 and at least one diode D2. One end of the resistor R6 is connected to the output IO interface of the main control MCU circuit, and the other end of the resistor R6 is in parallel with one end of the resistor R7 and the base of the triode Q3; the other end of the resistor R7 is connected to the emitter of the triode Q3, and at the same time the other end of the resistor R7 is in parallel to the ground terminal GND; the collector of the triode Q3 is connected to one end of the resistor R9, and at the same time the collector of the triode Q3 is in parallel to the negative electrode of the diode D2; the positive electrode of the diode D2 is in parallel with the other end of the terminal resistor R5, and the positive electrode of the diode D2 is connected to the output interface of the differential bus low - end transmission circuit.

[0013] As an improvement of the present invention, when the output IO interface of the main control MCU circuit outputs a low level, after voltage division by the resistor R6 and the resistor R7, the voltage applied to the base of the triode Q3 is a low level. At this time, the triode Q3 is cut off and the triode Q3 is in an open - circuit state. Therefore, the negative electrode of the diode D2 is in an open - circuit state. Since the levels of the output interfaces of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit remain equal in the static state of the adjustable differential bus circuit, the voltage of the output interface in the differential bus high - end transmission circuit is V_OUT; when the output IO interface of the main control MCU circuit outputs a high level, after voltage division by the resistor R6 and the resistor R7, the voltage applied to the base of the triode Q3 is a high level. At this time, the triode Q3 is forward - biased and conducts. Therefore, the collector voltage of the triode Q3 is equal to the ground terminal GND, that is, the negative - electrode voltage of the diode D2 is equal to the ground terminal GND at this time. Since the forward voltage drop of a single diode D2 is a constant value P, the voltage of the positive electrode of the diode D2 is pulled down to P through the diode D2. Therefore, the voltage of the output interface in the differential bus low - end transmission circuit will be pulled down to P.

[0014] As an improvement of the present invention, when the main control MCU circuit has no data transmission, the voltages of the output interfaces of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit are both V_OUT. At this time, the voltage difference on the adjustable differential bus circuit is 0, and this voltage difference represents the binary number 0 in the data stream; when the main control MCU circuit has data transmission, the voltage of the output interface of the differential bus low - end transmission circuit is VCCIN–P, and the voltage of the output interface of the differential bus low - end transmission circuit is P. Therefore, the voltage difference V between the high - end differential bus voltage and the low - end differential bus voltage is VCCIN–2P. At this time, the voltage difference V on the adjustable differential bus circuit represents the binary number 1 in the data stream. The data transmission process from the output IO interface of the main control MCU circuit to the OBD diagnostic interface is realized through the change of the voltage difference of the adjustable differential bus circuit.

[0015] As a further improvement of the present invention, the differential bus receiving circuit includes a resistor R8, a resistor R9, a resistor R10, a resistor R11, a resistor R12 and a comparator U1A. One end of the resistor R9 is connected to the negative electrode of the diode D2, and the other end of the resistor R9 is connected to the second pin of the comparator U1A. One end of the resistor R11 is connected to the power input terminal VCCIN, and the other end of the resistor R11 is connected in parallel with one end of the resistor R12 and the third pin of the comparator U1A. The other end of the resistor R12 is connected in parallel with the fourth pin of the comparator U1A and then connected to the ground terminal GND. The eighth pin of the comparator U1A is connected to the power input terminal VCCIN. One end of the resistor R8 is connected to the power supply VCC3V3, and the other end of the resistor R8 is connected in parallel with the first pin of the comparator U1A and one end of the resistor R10. The other end of the resistor R10 is connected to the input IO interface of the main control MCU circuit.

[0016] As a further improvement of the present invention, when the binary data transmitted by the adjustable differential bus circuit is 1, the triode Q3 in the differential bus low-end sending circuit is in the conducting state. At this time, the voltage at the negative electrode of the diode D2 in the differential bus low-end sending circuit is P. The voltage at the negative electrode of the diode D2 is loaded onto the second pin of the comparator U1A through the resistor R9. Since the voltage at the third pin of the comparator U1A is greater than the voltage at the second pin of the comparator U1A, the first pin of the comparator U1A outputs a high level and is connected to the input IO interface of the main control MCU circuit through the resistor R10, making the input IO interface of the main control MCU circuit at a high level. Then the input IO interface of the main control MCU circuit receives the binary data 1. When the binary data transmitted by the adjustable differential bus circuit is 0, the triode Q3 in the differential bus low-end sending circuit is in the cut-off state. At this time, the voltage at the negative electrode of the diode D2 in the differential bus low-end sending circuit is equal to the voltage V_OUT of the output interface of the differential bus low-end sending circuit. The voltage at the negative electrode of the diode D2 is loaded onto the second pin of the comparator U1A through the resistor R9. At this time, the voltage at the third pin of the comparator U1A is less than the voltage at the second pin of the comparator U1A, making the first pin of the comparator U1A output a low level and connected to the input IO interface of the main control MCU circuit through the resistor R10. The input IO interface of the main control MCU circuit is at a low level, and then the input IO interface of the main control MCU circuit receives the binary data 0.

[0017] As a further improvement of the present invention, the resistance values of the resistor R11 and the resistor R12 are taken according to the formula R12÷(R11 + R12)xVCCIN = V_OUT / 2, so that after voltage division by the resistor R12 and the resistor R11, the voltage at the third pin of the comparator U1A is equal to half of the output interface voltage of the differential bus high-end sending circuit or the differential bus low-end sending circuit in the static state.

[0018] After adopting such a design, the present invention has at least the following advantages:

[0019] (1) By providing an adjustable differential bus system that can be used in a diagnostic instrument, the present invention enables the diagnostic instrument to search for or diagnose faults in vehicles using an adjustable differential bus through the OBD diagnostic interface. Thereby, maintenance personnel can quickly and accurately judge the vehicle status, and can quickly and efficiently detect and repair faults and status of vehicles using adjustable differential bus circuits, which brings convenience to vehicle maintenance and improves maintenance efficiency.

[0020] (2) The adjustable differential bus circuit in the present invention realizes the data stream output from the main control MCU circuit by simulating the voltage difference change between the differential bus high - end transmission circuit and the differential bus low - end transmission circuit, and transmits it to the OBD diagnostic interface through the adjustable differential bus circuit, realizing the data stream transmission between the main control MCU circuit and the OBD diagnostic interface, and further realizing the function of fault searching or diagnosing for vehicles. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The above is only an overview of the technical solution of the present invention. In order to understand the technical means of the present invention more clearly, the following further detailed description of the present invention will be given in combination with the drawings and specific embodiments.

[0022] Figure 1 FIG. is a schematic structural diagram of a differential bus system for vehicle diagnosis in an embodiment of the present invention.

[0023] Figure 2 FIG. is a schematic circuit diagram of the differential bus system in an embodiment of the present invention.

[0024] Figure 3 FIG. is a schematic structural diagram of the adjustable differential bus circuit in an embodiment of the present invention.

[0025] Figure 4 FIG. is a schematic structural diagram of the differential bus high - end transmission circuit in an embodiment of the present invention.

[0026] Figure 5 FIG. is a schematic structural diagram of the differential bus low - end transmission circuit in an embodiment of the present invention.

[0027] Figure 6 FIG. is a schematic structural diagram of the differential bus receiving circuit in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] Examples of the embodiments of the present invention are shown in the drawings, where the same or similar reference numerals represent the same or similar components or components with the same or similar functions throughout. The embodiments described below with reference to the drawings are exemplary and are only used to explain the present invention and should not be construed as limiting the present invention.

[0029] In the description of the present invention, it should be noted that unless otherwise clearly specified and defined, the terms "installation", "connection", and "coupling" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0030] Combined with Figure 1 As shown, in this embodiment, a differential bus system for vehicle diagnosis is specifically disclosed, which includes a main control MCU circuit, an adjustable differential bus circuit, and an OBD diagnostic interface connected in sequence.

[0031] Specifically, the main control MCU circuit includes two IO interfaces. One IO interface is an output IO interface for outputting square wave pulses, and the other IO interface is an input IO interface for receiving square wave pulses. The two IO interfaces in the main control MCU circuit are used for data transmission and data reception.

[0032] Combined with Figure 2 and Figure 3 As shown, the adjustable differential bus circuit includes a differential bus high - end transmission circuit, a differential bus low - end transmission circuit, a differential bus matching terminal, and a differential bus reception circuit. The adjustable differential bus circuit is mainly used to convert the pulses sent in the main control MCU circuit into a high - end pulse signal and a low - end pulse signal, so as to form a differential signal form. The adjustable differential bus circuit is connected to the OBD diagnostic interface and sends the data stream for diagnosis to the OBD diagnostic interface in a differential signal manner. At the same time, the adjustable differential bus circuit also inputs the differential signal data stream in the adjustable differential bus circuit into the main control MCU circuit through the differential bus reception circuit for vehicle diagnosis.

[0033] Among them, the output IO interface of the main control MCU circuit is connected in parallel to the input interfaces of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit. The output interfaces of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit are respectively connected to the OBD diagnostic interface, and the differential bus matching terminal is connected between the output interfaces of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit. The differential bus reception circuit receives the differential signal output by the adjustable differential bus circuit and transmits it to the input IO interface of the main control MCU circuit.

[0034] When the main control MCU circuit has no data output, the voltages of the output interface of the differential bus high-end transmission circuit and the output interface of the differential bus low-end transmission circuit are equal, and the voltage difference is 0. At this time, the voltage difference on the adjustable differential bus circuit represents the binary number 0 in the data stream; when the main control MCU circuit has data output, there is a voltage difference V between the output interface of the differential bus high-end transmission circuit and the output interface of the differential bus low-end transmission circuit. At this time, the voltage difference V on the adjustable differential bus circuit represents the binary number 1 in the data stream. The voltage difference of the adjustable differential bus circuit is used to simulate the binary data in the data stream, so as to realize the data stream transmission from the output IO interface of the main control MCU circuit to the OBD diagnostic interface.

[0035] The differential bus matching terminal includes a terminal resistor R5. One end of the terminal resistor R5 is connected to the output interface of the differential bus high-end transmission circuit, and the other end is connected to the output interface of the differential bus low-end transmission circuit. The terminal resistor R5 is used to improve the differential signal transmission waveform and reduce high-speed signal reflection. In this embodiment, the preferred value of the terminal resistor R5 is 100 ohms.

[0036] Combined with Figure 4 As shown, the differential bus high-end transmission circuit includes a resistor R1, a resistor R2, a resistor R3, a resistor R4, a triode Q1, a triode Q2, a fuse resistor F1 and at least one diode D1. In this embodiment, preferably, the resistors R1 and R2 have a value of 4.7K ohms, the resistor R3 has a value of 10K ohms, the resistor R4 has a value of 1K ohm, the triode Q1 is selected as MMBT3906, the triode Q2 is selected as MMBT3904W, the diode D1 is selected as SS14, and the fuse resistor F1 has parameters of 0.1A / 60.

[0037] Specifically, one end of the resistor R2 is connected to the output IO interface of the main control MCU circuit, and the other end of the resistor R2 is connected in parallel with one end of the resistor R3 and the base of the triode Q2. The other end of the resistor R3 is connected in parallel with the emitter of the triode Q2 and one end of the resistor R4 and then connected to the ground terminal GND; the collector of the triode Q2 is connected to one end of the resistor R1, and the other end of the resistor R1 is connected to the base of the triode Q1; the emitter of the triode Q1 is connected to one end of the fuse resistor F1, and the other end of the fuse resistor F1 is connected to the power input terminal VCCIN; the collector of the triode Q1 is connected in parallel with the other end of the resistor R4 and the positive pole of the diode D1; the negative pole of the diode D1 is connected in parallel with one end of the terminal resistor R5, and the negative pole of the diode D1 is connected to the output interface OUT-P of the differential bus high-end transmission circuit.

[0038] The working process of the differential bus high-end transmission circuit is as follows:

[0039] When the output IO interface of the main control MCU circuit outputs a low level, after being divided by resistors R2 and R3, the voltage applied to the base of transistor Q2 is a low level. At this time, transistor Q2 is cut off, and transistor Q2 is in an open-circuit state. At the same time, transistor Q1 will also be in an open-circuit state. At this time, the positive electrode of diode D1 is at zero level, and diode D1 is in reverse cut-off. Since the voltage of the output interface in the differential bus high-end transmission circuit remains a constant value in the static state, in this embodiment, the voltage of the output interface in the differential bus high-end transmission circuit in the static state is set as V_OUT, that is, the voltage of the output interface in the differential bus high-end transmission circuit is V_OUT. It should be noted that the actual voltage of V_OUT is related to the vehicle model, and the measured value of the actual vehicle shall prevail.

[0040] When the output IO interface of the main control MCU circuit outputs a high level, after being divided by resistors R2 and R3, the voltage applied to the base of transistor Q2 is a high level. The base of transistor Q2 is forward-biased and conducts. At this time, the collector level of transistor Q2 is basically equal to the ground terminal GND. At this time, the base of transistor Q1 is at a low level, and transistor Q1 is in a conducting state. At this time, the collector voltage of transistor Q1 is basically equal to the power supply input terminal VCCIN. Since the power supply input terminal VCCIN is greater than V_OUT, diode D1 conducts forward. Since the forward voltage drop of diode D1 is a constant value P, the voltage of the negative electrode of diode D1 is VCCIN–P. At this time, the voltage of the output interface in the differential bus high-end transmission circuit will be increased to VCCIN–P. For example, assuming that the forward voltage drop of diode D1 is a constant value of 0.3V, after the power supply input terminal VCCIN passes through diode D1, the voltage of the negative electrode of diode D1 is VCCIN–0.3V. Since VCCIN–0.3V is much greater than V_OUT, the voltage of the output interface in the differential bus high-end transmission circuit will be increased to VCCIN–0.3V.

[0041] Combined with Figure 5 As shown, in this embodiment, the differential bus low-end transmission circuit includes resistor R6, resistor R7, transistor Q3 and at least one diode D2. In this embodiment, preferably, the value of resistor R6 is 4.7K ohms, the value of resistor R7 is 10K ohms, transistor Q3 is selected as MMBT3904W, and diode D2 is selected as SS14.

[0042] Specifically, one end of the resistor R6 is connected to the output IO interface of the main control MCU circuit, and the other end of the resistor R6 is connected in parallel with one end of the resistor R7 and the base of the triode Q3; the other end of the resistor R7 is connected to the emitter of the triode Q3, and at the same time, the other end of the resistor R7 is connected in parallel to the ground terminal GND; the collector of the triode Q3 is connected to one end of the resistor R9, and at the same time, the collector of the triode Q3 is connected in parallel to the negative electrode of the diode D2; the positive electrode of the diode D2 is connected in parallel with the other end of the terminal resistor R5, and the positive electrode of the diode D2 is connected to the output interface OUT-N of the differential bus low-end sending circuit.

[0043] The working process of the differential bus low-end sending circuit is as follows:

[0044] When the output IO interface of the main control MCU circuit outputs a low level, after voltage division by the resistor R6 and the resistor R7, the voltage applied to the base of the triode Q3 is a low level. At this time, the triode Q3 is cut off, and the triode Q3 is in an open circuit state. Therefore, the negative electrode of the diode D2 is in an open circuit state. Since the levels of the output interfaces of the differential bus high-end sending circuit and the differential bus low-end sending circuit in the adjustable differential bus circuit remain equal in the static state, the voltage of the output interface in the differential bus high-end sending circuit is V_OUT. At this time, the voltage difference between the differential bus high-end sending circuit and the differential bus low-end sending circuit in the adjustable differential bus circuit is 0. It should be noted that the actual voltage of V_OUT is related to the vehicle model and is subject to the measured value of the actual vehicle.

[0045] When the output IO interface of the main control MCU circuit outputs a high level, after voltage division by the resistor R6 and the resistor R7, the voltage applied to the base of the triode Q3 is a high level. At this time, the triode Q3 is forward-biased and conducts. Therefore, the collector voltage of the triode Q3 is equal to the ground terminal GND, that is, the negative electrode voltage of the diode D2 is equal to the ground terminal GND at this time. Since the forward voltage drop of a single diode D2 is a constant value P, the voltage of the positive electrode of the diode D2 is pulled down to P through the diode D2, and finally the voltage of the output interface in the differential bus low-end sending circuit will be pulled down to P.

[0046] Combining the working processes of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit, in this embodiment, when there is no data transmission in the master MCU circuit, that is, the output IO interface of the master MCU circuit outputs a low level at this time. At this time, the voltages of the output interfaces of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit are both V_OUT. At this time, the voltage difference on the adjustable differential bus circuit is 0, and this voltage difference represents the binary number 0 in the data stream; when there is data transmission in the master MCU circuit, that is, the output IO interface of the master MCU circuit outputs a high level at this time. At this time, the voltage of the output interface of the differential bus low - end transmission circuit is VCCIN–P, and the voltage of the output interface of the differential bus low - end transmission circuit is P. Therefore, the voltage difference V between the high - end differential bus voltage and the low - end differential bus voltage is V = VCCIN–2P. At this time, the voltage difference V on the adjustable differential bus circuit represents the binary number 1 in the data stream. The data transmission process from the output IO interface of the master MCU circuit to the OBD diagnostic interface is realized through the change of the voltage difference of the adjustable differential bus circuit.

[0047] Furthermore, as shown in Figure 6 In this embodiment, the differential bus receiving circuit includes resistor R8, resistor R9, resistor R10, resistor R11, resistor R12 and comparator U1A. Preferably in this embodiment, resistor R8 is selected as 4.7K ohms, resistor R9 is selected as 10K ohms, resistor R10 is selected as 1K ohm, and the resistance values of resistor R11 and resistor R12 are calculated according to the following formula:

[0048] R12÷(R11 + R12)xVCCIN = V_OUT / 2;

[0049] According to the above formula, resistor R11 and resistor R12 are selected to make the voltage of the 3rd pin of comparator U1A equal to half of the output interface voltage of the differential bus high - end transmission circuit or the differential bus low - end transmission circuit in the static state. In this embodiment, resistor R11 is preferably selected as an 18K resistor, and resistor R12 is preferably selected as a 2K resistor.

[0050] Specifically, one end of the resistor R9 is connected to the negative electrode of the diode D2, and the other end of the resistor R9 is connected to the second pin of the comparator U1A; one end of the resistor R11 is connected to the power input terminal VCCIN, and the other end of the resistor R11 is connected in parallel with one end of the resistor R12 and the third pin of the comparator U1A; the other end of the resistor R12 is connected in parallel with the fourth pin of the comparator U1A and then connected to the ground terminal GND, and the eighth pin of the comparator U1A is connected to the power input terminal VCCIN; one end of the resistor R8 is connected to the power supply VCC3V3, and the other end of the resistor R8 is connected in parallel with the first pin of the comparator U1A and one end of the resistor R10, and the other end of the resistor R10 is connected to the input IO interface of the main control MCU circuit. The differential bus receiving circuit is mainly used to receive the differential signal data stream transmitted by the adjustable differential bus circuit and transmit the differential signal data stream to the main control MCU circuit, so that the diagnostic instrument can perform automotive fault analysis based on the differential signal data stream.

[0051] The working process of the differential bus receiving circuit is as follows:

[0052] During design, the voltage value V_OUT / 2 is obtained by voltage division of the resistor R12 and the resistor R11. Therefore, the voltage of the third pin of the integrated comparator U1A is also V_OUT / 2. When the binary data transmitted by the adjustable differential bus circuit is 1, the triode Q3 in the differential bus low-end transmission circuit is in the conducting state. At this time, the negative electrode voltage of the diode D2 in the differential bus low-end transmission circuit is P, and the negative electrode voltage of the diode D2 is loaded onto the second pin of the comparator U1A through the resistor R9. Since the voltage of the third pin of the comparator U1A is greater than the voltage of the second pin of the comparator U1A, the first pin of the comparator U1A outputs a high level and is connected to the input IO interface of the main control MCU circuit through the resistor R10, making the input IO interface of the main control MCU circuit a high level. Then, the input IO interface of the main control MCU circuit receives the binary data 1.

[0053] When the binary data transmitted by the adjustable differential bus circuit is 0, the triode Q3 in the differential bus low-end transmission circuit is in the cut-off state. At this time, the negative electrode voltage of the diode D2 in the differential bus low-end transmission circuit is equal to the voltage V_OUT of the output interface of the differential bus low-end transmission circuit. The negative electrode voltage of the diode D2 is loaded onto the second pin of the comparator U1A through the resistor R9. At this time, the voltage of the third pin of the comparator U1A is less than the voltage of the second pin of the comparator U1A, so that the first pin of the comparator U1A outputs a low level and is connected to the input IO interface of the main control MCU circuit through the resistor R10. The input IO interface of the main control MCU circuit is a low level, and then the input IO interface of the main control MCU circuit receives the binary data 0.

[0054] The above are only the preferred embodiments of the present invention, and do not impose any formal limitations on the present invention. Any simple modifications, equivalent changes or decorations made by those skilled in the art using the technical content disclosed above all fall within the protection scope of the present invention.

Claims

1. A differential bus system for vehicle diagnosis, characterized in that, it comprises a master MCU circuit, an adjustable differential bus circuit and an OBD diagnostic interface connected in sequence, wherein the master MCU circuit includes two IO interfaces, one IO interface is an output IO interface for outputting square wave pulses, and the other IO interface is an input IO interface for receiving square wave pulses; the adjustable differential bus circuit includes a differential bus high - end sending circuit, a differential bus low - end sending circuit, a differential bus matching terminal and a differential bus receiving circuit. The output IO interface of the master MCU circuit is connected in parallel to the input interfaces of the differential bus high - end sending circuit and the differential bus low - end sending circuit. The output interfaces of the differential bus high - end sending circuit and the differential bus low - end sending circuit are respectively connected to the OBD diagnostic interface, and the differential bus matching terminal is connected between the output interfaces of the differential bus high - end sending circuit and the differential bus low - end sending circuit. The differential bus receiving circuit receives the differential signals generated by the differential bus high - end sending circuit and the differential bus low - end sending circuit and transmits them to the input IO interface of the master MCU circuit; when the master MCU circuit has no data output, the voltages of the output interfaces of the differential bus high - end sending circuit and the differential bus low - end sending circuit are equal, and the voltage difference is 0. At this time, the voltage difference on the adjustable differential bus circuit represents the binary number 0 in the data stream; when the master MCU circuit has data output, there is a voltage difference V between the output interface of the differential bus high - end sending circuit and the output interface of the differential bus low - end sending circuit. At this time, the voltage difference V on the adjustable differential bus circuit represents the binary number 1 in the data stream. The binary data in the data stream is simulated through the voltage difference of the adjustable differential bus circuit, thereby realizing the data stream transmission from the output IO interface of the master MCU circuit to the OBD diagnostic interface.

2. The differential bus system according to claim 1, characterized in that, the differential bus matching terminal includes a terminal resistor R5. One end of the terminal resistor R5 is connected to the output interface of the differential bus high - end sending circuit, and the other end is connected to the output interface of the differential bus low - end sending circuit. The terminal resistor R5 is used to improve the differential signal transmission waveform and reduce high - speed signal reflection.

3. The differential bus system according to claim 2, characterized in that, The differential bus high - end transmission circuit includes resistor R1, resistor R2, resistor R3, resistor R4, triode Q1, triode Q2, fuse resistor F1 and at least one diode D1. One end of the resistor R2 is connected to the output IO interface of the main control MCU circuit. The other end of the resistor R2 is connected in parallel with one end of the resistor R3 and the base of the triode Q2. The other end of the resistor R3 is connected in parallel with the emitter of the triode Q2 and one end of the resistor R4 and then connected to the ground terminal GND. The collector of the triode Q2 is connected to one end of the resistor R1. The other end of the resistor R1 is connected to the base of the triode Q1. The emitter of the triode Q1 is connected to one end of the fuse resistor F1. The other end of the fuse resistor F1 is connected to the power input terminal VCCIN. The collector of the triode Q1 is connected in parallel with the other end of the resistor R4 and the positive electrode of the diode D1. The negative electrode of the diode D1 is connected in parallel with one end of the terminal resistor R5, and the negative electrode of the diode D1 is connected to the output interface of the differential bus high - end transmission circuit.

4. The differential bus system according to claim 3, characterized in that, when the output IO interface of the main control MCU circuit outputs a low level, after voltage division by the resistor R2 and the resistor R3, the voltage applied to the base of the triode Q2 is a low level. At this time, the triode Q2 is cut off, and the triode Q2 is in an open - circuit state. At the same time, the triode Q1 will also be in an open - circuit state. At this time, the positive electrode of the diode D1 is at zero level. At this time, the voltage of the output interface in the differential bus high - end transmission circuit is in a static state, and the voltage of the output interface in the differential bus high - end transmission circuit is V_OUT. When the output IO interface of the main control MCU circuit outputs a high level, after voltage division by the resistor R2 and the resistor R3, the voltage applied to the base of the triode Q2 is a high level. The base of the triode Q2 is forward - biased and conducts. At this time, the collector level of the triode Q2 is equal to the ground terminal GND. At this time, the base of the triode Q1 is at a low level, and the triode Q1 is in a conducting state. At this time, the collector voltage of the triode Q1 is equal to the power input terminal VCCIN. Since the power input terminal VCCIN is greater than V_OUT, the diode D1 conducts forward. Since the forward voltage drop of the diode D1 is a constant value P, the voltage of the negative electrode of the diode D1 is VCCIN–P. At this time, the voltage of the output interface in the differential bus high - end transmission circuit will be increased to VCCIN–P.

5. The differential bus system according to claim 4, characterized in that, The differential bus low - end transmission circuit includes a resistor R6, a resistor R7, a triode Q3, and at least one diode D2. One end of the resistor R6 is connected to the output IO interface of the main control MCU circuit, and the other end of the resistor R6 is in parallel with one end of the resistor R7 and the base of the triode Q3; the other end of the resistor R7 is connected to the emitter of the triode Q3, and at the same time, the other end of the resistor R7 is in parallel to the ground terminal GND; the collector of the triode Q3 is connected to one end of the resistor R9, and at the same time, the collector of the triode Q3 is in parallel to the negative electrode of the diode D2; the positive electrode of the diode D2 is in parallel with the other end of the terminal resistor R5, and the positive electrode of the diode D2 is connected to the output interface of the differential bus low - end transmission circuit.

6. The differential bus system according to claim 5, wherein, when the output IO interface of the main control MCU circuit outputs a low level, after voltage division by the resistor R6 and the resistor R7, the voltage applied to the base of the triode Q3 is a low level. At this time, the triode Q3 is cut off, and the triode Q3 is in an open - circuit state. Therefore, the negative electrode of the diode D2 is in an open - circuit state. Since the levels of the output interfaces of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit remain equal in the static state of the adjustable differential bus circuit, the voltage of the output interface in the differential bus high - end transmission circuit is V_OUT; when the output IO interface of the main control MCU circuit outputs a high level, after voltage division by the resistor R6 and the resistor R7, the voltage applied to the base of the triode Q3 is a high level. At this time, the triode Q3 is forward - biased and conducts. Therefore, the collector voltage of the triode Q3 is equal to the ground terminal GND, that is, the negative electrode voltage of the diode D2 is equal to the ground terminal GND at this time. Since the forward voltage drop of a single diode D2 is a constant value P, the voltage at the positive electrode of the diode D2 is pulled down to P through the diode D2. Therefore, the voltage of the output interface in the differential bus low - end transmission circuit will be pulled down to P.

7. The differential bus system according to claim 6, wherein, when the main control MCU circuit has no data transmission, the voltages of the output interfaces of the differential bus high - end transmission circuit and the differential bus low - end transmission circuit are both V_OUT. At this time, the voltage difference on the adjustable differential bus circuit is 0, and this voltage difference represents the binary number 0 in the data stream; when the main control MCU circuit has data transmission, the voltage of the output interface of the differential bus low - end transmission circuit is VCCIN–P, and the voltage of the output interface of the differential bus low - end transmission circuit is P. Therefore, the voltage difference V between the high - end differential bus voltage and the low - end differential bus voltage is VCCIN–2P. At this time, the voltage difference V on the adjustable differential bus circuit represents the binary number 1 in the data stream. The data transmission process from the output IO interface of the main control MCU circuit to the OBD diagnostic interface is realized through the change of the voltage difference of the adjustable differential bus circuit.

8. The differential bus system according to claim 7, wherein, The differential bus receiving circuit includes resistor R8, resistor R9, resistor R10, resistor R11, resistor R12 and comparator U1A. One end of resistor R9 is connected to the negative electrode of diode D2, and the other end of resistor R9 is connected to the second pin of comparator U1A. One end of resistor R11 is connected to the power input terminal VCCIN, and the other end of resistor R11 is connected in parallel with one end of resistor R12 and the third pin of comparator U1A. The other end of resistor R12 is connected in parallel with the fourth pin of comparator U1A and then connected to the ground terminal GND. The eighth pin of comparator U1A is connected to the power input terminal VCCIN. One end of resistor R8 is connected to power supply VCC3V3, and the other end of resistor R8 is connected in parallel with the first pin of comparator U1A and one end of resistor R10. The other end of resistor R10 is connected to the input IO interface of the main control MCU circuit.

9. The differential bus system according to claim 8, wherein, when the binary data transmitted by the adjustable differential bus circuit is 1, the triode Q3 in the differential bus low-end transmission circuit is in the conducting state. At this time, the voltage at the negative electrode of diode D2 in the differential bus low-end transmission circuit is P. The voltage at the negative electrode of diode D2 is loaded onto the second pin of comparator U1A through resistor R9. Since the voltage at the third pin of comparator U1A is greater than the voltage at the second pin of comparator U1A, the first pin of comparator U1A outputs a high level and is connected to the input IO interface of the main control MCU circuit through resistor R10, making the input IO interface of the main control MCU circuit at a high level. Then the input IO interface of the main control MCU circuit receives the binary data 1. When the binary data transmitted by the adjustable differential bus circuit is 0, the triode Q3 in the differential bus low-end transmission circuit is in the cut-off state. At this time, the voltage at the negative electrode of diode D2 in the differential bus low-end transmission circuit is equal to the voltage V_OUT of the output interface of the differential bus low-end transmission circuit. The voltage at the negative electrode of diode D2 is loaded onto the second pin of comparator U1A through resistor R9. At this time, the voltage at the third pin of comparator U1A is less than the voltage at the second pin of comparator U1A, making the first pin of comparator U1A output a low level and connected to the input IO interface of the main control MCU circuit through resistor R10. The input IO interface of the main control MCU circuit is at a low level, and then the input IO interface of the main control MCU circuit receives the binary data 0.

10. The differential bus system according to claim 8, wherein, the resistance values of resistor R11 and resistor R12 are taken according to the formula R12÷(R11 + R12) x VCCIN = V_OUT / 2, so that after voltage division by resistor R12 and resistor R11, the voltage at the third pin of comparator U1A is equal to half of the output interface voltage of the differential bus high-end transmission circuit or the differential bus low-end transmission circuit in the static state.

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