High-impedance switching anti-interference method for fire-fighting two-bus

Abstract

The invention discloses an anti-interference method for fire-fighting two-bus high impedance state switching, which belongs to the field of fire fighting, improves a traditional fire-fighting product bus communication mechanism, can greatly improve the reliability of system communication, can realize higher communication rate, has lower requirements on line characteristic parameters, is low in hardware circuit cost, and is suitable for large-scale popularization and application. Therefore, the system design cost and the construction cost are lower. The device mainly comprises a master station MCU, the master station MCU is connected with a fire-fighting two-bus master station chip through an optocoupler isolation circuit, the device further comprises an MOS tube VM1 and an MOS tube VM2, a G pole of the MOS tube VM1 is connected with a BUSH pin of the fire-fighting two-bus master station chip through a resistor R9, an S pole of the MOS tube VM1 is connected with a power supply through a resistor R5, a D pole of the MOS tube VM1 is connected with a D pole of the MOS tube VM2, and the D pole of the MOS tube VM2 is further connected with a diode VD4 and a resistor R20. The method is mainly used for fire-fighting two-bus anti-interference.

Description

Technical Field

[0001] The present invention relates to the field of fire protection, and more particularly to a method for high-impedance state switching anti-interference of a two-wire fire protection bus. Background Art

[0002] At present, most fire automatic alarm systems use two-wire buses to achieve communication and power supply between fire alarm controllers and on-site monitoring units. The two-wire fire protection bus is an asymmetric communication bus. The downlink code sending of the controller needs to consider both communication and power supply, so a voltage modulation method is adopted; in order to minimize voltage loss, the uplink return code of the monitoring unit uses a current loop method powered by the controller. In order to ensure reliable communication and increase the transmission distance, existing fire protection systems often choose twisted pairs as transmission cables. In actual engineering application scenarios, for the consideration of balancing the construction convenience of different control systems and saving costs, the communication lines between the fire control room and each building often do not use twisted pairs, but multi-core parallel lines with sheaths. However, in the existing technical architecture, since each loop card is powered by the same power supply system and the buses between each loop card are grounded together, this situation causes crosstalk signals to be generated. In the operation system of the two-wire fire protection bus, each loop needs to carry 200 detection devices. At the same time, the on-site construction environment presents distinct characteristics, specifically manifested as: long bus distances, sharing cable trays with high-voltage lines, and having multiple branch lines, etc.

[0003] With the in-depth development of the digital age, the challenges and requirements faced by fire alarm systems are increasing day by day. In this context, fire alarm controllers undertake more important tasks and need to obtain in real time the data collected by smoke and temperature detection devices to ensure that fire alarm information can be reported back in a timely and accurate manner. To ensure the smooth realization of this key function, extremely high real-time requirements are put forward for the process of uploading sampled data to the controller through the bus. This high real-time requirement, in turn, sets a higher standard for the bus communication rate. However, while increasing the data transmission rate, it inevitably causes the problem of crosstalk between lines. After the problem of crosstalk between lines occurs, it will have a negative impact on the uplink current loop return code, resulting in deviation or even error of the return code, and thus seriously reducing the communication success rate in long-distance, high-voltage line sharing, and multi-branch complex scenarios, making the entire two-wire fire protection bus system face technical bottlenecks and restricting the further optimization and improvement of its performance.

[0004] The prior art often adopts the following methods: 1. Each loop card of the fire alarm controller is equipped with an isolated power supply, and the loop cards are isolated from power and ground through devices such as transformers and optocouplers. Each controller can have a maximum of 20 loop cards installed, but this design has some drawbacks. For example, the cost of the isolated power supply circuit is high, it occupies a relatively large amount of cabinet space, generates a lot of heat, has a relatively high static power consumption, has a large inrush current when powered on, and has relatively high requirements for the power supply system, battery, and cabinet; 2. Lower the bus baud rate to 2400 bps, which will result in an overly low communication rate, causing a lag in reporting fire alarm events and a slow response in fire protection linkage; 3. Implement hardware filtering using devices such as magnetic rings and common-mode inductors. Such devices have problems such as large volume, high cost, and the filtering effect not meeting expectations. Moreover, the filtering circuit needs to consider the power supply for bus devices, and the design is relatively complex; The above three aspects of problems lead to high costs and limit the improvement of the bus communication rate. It is necessary to study a method that can not only improve the success rate of bus communication but also combine low cost and high bus communication rate. Summary of the Invention

[0005] The purpose of the present invention is to provide a high-impedance state switching anti-interference method for a fire protection two-wire bus, which improves the traditional bus communication mechanism of fire protection products, can greatly enhance the reliability of system communication, achieve a higher communication rate, has lower requirements for line characteristic parameters, and has a low hardware circuit cost. Therefore, it has a lower system design cost and construction cost.

[0006] The present invention is realized through the following technical solutions: A high-impedance state switching anti-interference method for a fire protection two-wire bus includes a master station MCU. The master station MCU is connected to the fire protection two-wire bus master chip through an optocoupler isolation circuit. It also includes MOS transistor VM1 and MOS transistor VM2. The G pole of MOS transistor VM1 is connected to the BUSH pin of the fire protection two-wire bus master chip through resistor R9. The S pole of MOS transistor VM1 is connected to the power supply through resistor R5. The D pole of MOS transistor VM1 is connected to the D pole of MOS transistor VM2. The D pole of MOS transistor VM2 is also respectively connected to diode VD4 and resistor R20. Diode VD4 is connected to the REVIN pin of the fire protection two-wire bus master chip. Resistor R20 is connected to the CDET pin of the fire protection two-wire bus master chip. The G pole of MOS transistor VM2 is connected to the BUSL pin of the fire protection two-wire bus master chip through resistor R32. The S pole of MOS transistor VM2 is grounded through resistor R28. A switching circuit is connected in parallel between the D pole of MOS transistor VM2 and resistor R28; a high-impedance state switching circuit is provided between the power supply panel and the fire protection two-wire bus master chip; It includes the following adjustment methods: S1. When R / T = 0, turn on the transmission modulation circuit and turn off the reception demodulation circuit to make the fire protection two-wire bus master chip in the transmission state; S2. When R / T = 1, turn on the receiving demodulation circuit and turn off the transmitting modulation circuit, so that the fire two-bus master chip is in the receiving state.

[0007] Further, the switching circuit includes an MOS transistor VM5. The G pole of the MOS transistor VM5 is connected to the R / T pin of the fire two-bus master chip. The D pole of the MOS transistor VM5 is connected to a resistor R33. The S pole of the MOS transistor VM5 and the resistor R33 are connected in parallel with a diode VD8.

[0008] Further, step S1 further includes that the modulation voltage in the master station transmitting state is 0V - VCC. The modulation principle is as follows: The TTL signal of the master station MCU passes through the optocoupler isolation circuit TXD_MCU and is input into the fire two-bus master chip through the TXD pin, forming a modulation signal on the bus. If TXD = 1, the P-type MOS transistor VM1 is turned on and the N-type MOS transistor VM2 is turned off, and the bus outputs 1. If TXD = 0, the P-type MOS transistor VM1 is turned off and the N-type MOS transistor VM2 is turned on, and the bus outputs 0.

[0009] Further, step S2 further includes that the fire two-bus master chip performs constant-voltage current demodulation in the receiving state. The demodulation principle is as follows: When the master station is in the receiving state, the fire two-bus master chip receives the bus signal through the REVIN pin. After demodulation, the TTL signal is output through the RXD pin and given to the master station MCU through the optocoupler isolation circuit RXD_MCU. If a current is detected at the REVIN pin, then set RXD = 0. If no current is detected at the REVIN pin and the current exceeds the threshold current of 10 mA and the hardware current limit is 30 mA, then set RXD = 1.

[0010] Further, the high-impedance state switching circuit includes a totem pole circuit and a MOS transistor circuit. The MOS transistor circuit includes two MOS transistors D1A and D1B connected in series with each other and connected to the positive pole of the power supply disk, and two MOS transistors D6A and D6B connected in series with each other and connected to the negative pole of the power supply disk. The totem pole circuit includes a triode VT1, a triode VT2, and a triode VT3. The B pole of the triode VT1 and the B pole of the triode VT2 are connected between the resistor R8 and the C pole of the triode VT3. The C pole of the triode VT1 and the resistor R8 are both connected to the S poles of the MOS transistor D1A and the MOS transistor D1B. The E poles of the triode VT1 and the triode VT2 are connected and are respectively connected to the G poles of the MOS transistor D1A and the MOS transistor D1B through the resistor R10. The E pole of the triode VT3 is grounded through the resistor R17. The B pole of the triode VT3 is respectively connected to the G poles of the MOS transistor D6A and the MOS transistor D6B. The B pole of the triode VT3 is also connected with an inverter D2, and the inverter D2 is connected to the R / T pin of the fire two-bus master station chip. The MOS transistors D1A and D1B are P-type MOS transistors, and the MOS transistors D6A and D6B are N-type MOS transistors.

[0011] Further, a bypass resistor R1 is connected in parallel with the MOS transistors D1A and D1B, and a bypass resistor R76 is connected in parallel with the MOS transistors D6A and D6B.

[0012] Further, the following adjustment method is also included: S3. When R / T = 0, the bus is in the sending state: After the R / T signal passes through the inverter D2 and the totem pole circuit, both the P-type MOS transistor and the N-type MOS transistor are turned on, so as to supply power to the subsequent circuit. When turned on, its internal resistance is in the milliohm level, and the voltage drop of the MOS transistor is extremely low, so it will not heat up; S4. When R / T = 1, the bus is in the receiving state: The R / T signal passes through the inverter D2 and the driving totem pole circuit, so that both the P-type MOS transistor and the N-type MOS transistor are in the cut-off state. After being cut off, the resistance value between the power supply disk and the fire two-bus master station chip is equivalent to the bypass resistance of the MOS transistor. When the MOS transistor is cut off, the body diode of the MOS transistor has the performance of reverse current cut-off, playing the role of current isolation; In the receiving state, the on-site monitoring component is powered by its own electrolytic capacitor and does not require power supply from the master station circuit. At the same time, the pull current of the return code is limited to 30 mA, and the longest time for a single return code is limited within 10 ms to ensure that the return code voltage has no fluctuation and the voltage change of the electrolytic capacitor is less than 1 V. When the bus switches from the receiving state to the transmitting state, the MOS transistor is turned on to charge the electrolytic capacitor of the master station and the electrolytic capacitor of the on-site monitoring component.

[0013] Further, the resistance value of the bypass resistor is: R1 = R76 = 10 kΩ. During the power-on period of the loop card, the MOS transistor is default in the cut-off state, and the current charges the master station circuit through the 10 kΩ resistor; when the voltage rises to the threshold value, the inverter D2 starts, and its output level prompts the MOS transistor to conduct automatically, thereby powering the master station circuit; if the electrolytic capacitor of the master station loses power due to abnormal conditions, resulting in the continuous cut-off of the MOS transistor, the loop card can still be powered on again through the 10 kΩ resistor, enhancing the reliability of the system.

[0014] Further, it also includes that when the length of the fire-fighting two-wire line exceeds 1000 m and a short circuit occurs at its end, the system will detect whether there is a situation where the return code level is pulled down due to continuous pull current in the return code slot. If the RXD is continuously demodulated to a low level and the duration exceeds 30 s, it is determined that the bus is short-circuited.

[0015] Further, it also includes that there are two situations of bus short circuit: First, a short circuit occurs in the transmission state. At this time, the overcurrent detection circuit will play a role, and the device will continuously maintain the transmission state without entering the reception state, so it will not cause abnormal pulling down of the electrolytic voltage; Second, in the reception state, the pull current of the return code is limited to 30 mA by hardware, and the maximum width of a single return code is limited within 10 ms, and it will not cause abnormal pulling down of the electrolytic voltage either.

[0016] Compared with the prior art, the beneficial effects of the present invention are: Aiming at the characteristics that the downstream code sending of the bus is not affected by crosstalk current and the upstream return code is easily affected by crosstalk current, in the fire-fighting bus control system, by adding a crosstalk elimination circuit in the loop card, the achieved high-impedance state switching effect is: when the bus is in the transmission state, the master station chip and the power supply panel are not isolated through the bypass resistor; when the bus is in the reception state, the master station chip and the power supply panel are isolated through the bypass resistor, completing the GND isolation between loop cards; Connect the loop card to the power supply panel through the crosstalk elimination circuit. The crosstalk elimination circuit has the characteristic of time-sharing isolation. By improving the grounding conditions between each loop card, the signal crosstalk between each loop card is effectively reduced, ensuring the communication quality; The present invention improves the traditional bus communication mechanism of fire-fighting products, can greatly enhance the reliability of system communication and achieve a higher communication rate, has lower requirements for line characteristic parameters, and has a low hardware circuit cost. Therefore, it has a lower system design cost and construction cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is the circuit schematic diagram of the master station MCU and the fire-fighting two-wire master station chip of the present invention; Figure 2 is the circuit schematic diagram of the high-impedance state switching circuit of the present invention; Figure 3This is the impact of crosstalk on the upstream return code current loop when GND isolation is not added in the present invention; Figure 4 This is the interference waveform of crosstalk on the upstream return code current loop after the high-impedance state switching circuit is added in the present invention; Figure 5 This is the test environment diagram of the present invention. Specific implementation manners

[0018] The present invention will be further described below with reference to the accompanying drawings.

[0019] Example 1: As Figure 1 – Figure 2 shown, a high-impedance state switching anti-interference method for a fire two-wire bus includes a master station MCU. The master station MCU is connected to a fire two-wire bus master chip through an opto-isolation circuit, and also includes an MOS transistor VM1 and an MOS transistor VM2. The G pole of the MOS transistor VM1 is connected to the BUSH pin of the fire two-wire bus master chip through a resistor R9. The S pole of the MOS transistor VM1 is connected to the power supply through a resistor R5. The D pole of the MOS transistor VM1 is connected to the D pole of the MOS transistor VM2. The D pole of the MOS transistor VM2 is also respectively connected to a diode VD4 and a resistor R20. The diode VD4 is connected to the REVIN pin of the fire two-wire bus master chip. The resistor R20 is connected to the CDET pin of the fire two-wire bus master chip. The G pole of the MOS transistor VM2 is connected to the BUSL pin of the fire two-wire bus master chip through a resistor R32. The S pole of the MOS transistor VM2 is grounded through a resistor R28. A switching circuit is connected in parallel between the D pole of the MOS transistor VM2 and the resistor R28; a high-impedance state switching circuit is provided between the power supply panel and the fire two-wire bus master chip; It includes the following adjustment methods: S1. When R / T = 0, turn on the transmission modulation circuit and turn off the reception demodulation circuit to make the fire two-wire bus master chip in the transmission state; S2. When R / T = 1, turn on the reception demodulation circuit and turn off the transmission modulation circuit to make the fire two-wire bus master chip in the reception state; Figure 3 shows the impact of crosstalk on the upstream return code current loop when GND isolation is not added. As Figure 3 can be seen, the return code current is severely interfered, specifically manifested as a large crosstalk current pulse width and many oscillations; The described switching circuit includes an MOS transistor VM5. The G pole of the MOS transistor VM5 is connected to the R / T pin of the fire two-wire bus master chip. The D pole of the MOS transistor VM5 is connected to a resistor R33. A diode VD8 is connected in parallel between the S stage of the MOS transistor VM5 and the resistor R33; Step S1 further includes that when the master station is in the sending state, the modulation voltage is 0V - VCC. Modulation principle: The TTL signal of the master station MCU passes through the optocoupler isolation circuit TXD_MCU and is input into the fire two-bus master station chip through the TXD pin, forming a modulation signal on the bus. If TXD = 1, the P-type MOS transistor VM1 is turned on and the N-type MOS transistor VM2 is turned off, and the bus outputs 1. If TXD = 0, the P-type MOS transistor VM1 is turned off and the N-type MOS transistor VM2 is turned on, and the bus outputs 0. Step S2 further includes that when the fire two-bus master station chip is in the receiving state, it is a constant-voltage current demodulation. Demodulation principle: When the master station is in the receiving state, the fire two-bus master station chip receives the bus signal through the REVIN pin, and after demodulation, the TTL signal is output through the RXD pin and given to the master station MCU through the optocoupler isolation circuit RXD_MCU. If a current is detected at the REVIN pin, then set RXD = 0. If no current is detected at the REVIN pin and the current exceeds the threshold current of 10 mA and the hardware current limit is 30 mA, then set RXD = 1. The high-impedance state switching circuit includes a totem pole circuit and a MOS transistor circuit. The MOS transistor circuit includes two mutually series-connected MOS transistors D1A and D1B connected to the positive pole of the power supply panel, and two mutually series-connected MOS transistors D6A and D6B connected to the negative pole of the power supply panel. The totem pole circuit includes a triode VT1, a triode VT2, and a triode VT3. The B pole of the triode VT1 and the B pole of the triode VT2 are connected between the resistor R8 and the C pole of the triode VT3. The C pole of the triode VT1 and the resistor R8 are both connected to the S poles of the MOS transistor D1A and the MOS transistor D1B. The E poles of the triode VT1 and the triode VT2 are connected and are respectively connected to the G poles of the MOS transistor D1A and the MOS transistor D1B through the resistor R10. The E pole of the triode VT3 is grounded through the resistor R17. The B pole of the triode VT3 is respectively connected to the G poles of the MOS transistor D6A and the MOS transistor D6B. The B pole of the triode VT3 is also connected to an inverter D2, and the inverter D2 is connected to the R / T pin of the fire two-bus master station chip. The MOS transistors D1A and D1B are P-type MOS transistors, and the MOS transistors D6A and D6B are N-type MOS transistors. Use the totem pole circuit to drive the two P-type MOS transistors D1A and D1B to control the P-type MOS transistors to enter the on or off state. The duration of the process of the bus switching from the receiving state to the transmitting state is limited by the bus protocol and needs to be completed within 10 us. During this period, the P-type MOS transistors must be changed from the off state to the on state. Because if this fails, the large current in the sending state will pull down the electrolytic voltage of the master station. In view of this, the totem pole circuit is used to drive the P-type MOS transistors, and this circuit can shorten the switching time of the on or off state of the MOS transistors to within 2 us. The high-impedance state switching borrows existing hardware resources and does not require separate control by the master station MCU program, imposing no additional requirements on the performance of the master station MCU. The high-impedance state switching scheme borrows general circuit components such as MOS transistors, bipolar transistors, and resistors, with low cost and small space occupation. Since the MOS transistor has a large input impedance, the static power consumption of the circuit is small, and the power-on impact current is small. Compared with the isolated power supply scheme, it has lower requirements for the power supply system, storage battery, and cabinet. The MOS transistors D1A and D1B are shunted with a bypass resistor R1, and the MOS transistors D6A and D6B are shunted with a bypass resistor R76. It also includes the following adjustment method: S3. When R / T = 0, the bus is in the transmit state: After the R / T signal passes through the inverter D2 and the totem pole circuit, both the P-type MOS transistor and the N-type MOS transistor are turned on, thereby supplying power to the subsequent circuit. When conducting, its internal resistance is in the milliohm level, and the voltage drop of the MOS transistor is extremely low, so it will not heat up. S4. When R / T = 1, the bus is in the receive state: The R / T signal passes through the inverter D2 and the driving totem pole circuit, causing both the P-type MOS transistor and the N-type MOS transistor to be in the cut-off state. After cut-off, the resistance value between the power supply panel and the fire fighting two-wire master station chip is equivalent to the bypass resistor of the MOS transistor. When the MOS transistor is cut off, the body diode of the MOS transistor has the performance of reverse current cut-off, achieving the effect of current isolation. In the receive state, the on-site monitoring component is powered by its own electrolytic capacitor and does not require power supply from the master station circuit. At the same time, the pull current for code return is limited to 30 mA, and the longest time for a single code return is limited within 10 ms to ensure that the code return voltage has no fluctuation and the voltage change of the electrolytic capacitor is less than 1 V. When the bus switches from the receive state to the transmit state, the MOS transistor is turned on to charge the electrolytic capacitors of the master station and the on-site monitoring component. The resistance value of the bypass resistor is: R1 = R76 = 10 kΩ. During the power-on period of the loop card, the MOS transistor is default to the cut-off state, and the current charges the master station circuit through the 10 kΩ resistor. When the voltage rises to the threshold value, the inverter D2 is activated, and its output level prompts the MOS transistor to automatically turn on, thereby supplying power to the master station circuit. If the electrolytic capacitor of the master station loses power due to abnormal conditions, causing the MOS transistor to remain cut off, the loop card can still be powered on again through the 10 kΩ resistor, enhancing the reliability of the system. It also includes that when the length of the fire fighting two-wire line exceeds 1000 m and a short circuit occurs at its end, due to the limitation of the line resistance, the short-circuit current is less than the threshold value of the overcurrent detection circuit, resulting in the overcurrent detection circuit being unable to complete the detection. At this time, the system will detect whether there is a situation where the code return level is pulled low due to continuous pull current in the code return slot. If the RXD is continuously demodulated to a low level and the duration exceeds 30 s, it is determined that the bus is short-circuited. There are also two cases of bus short circuit: First, a short circuit occurs in the transmission state. At this time, the overcurrent detection circuit will play a role, and the device will continuously maintain the transmission state without entering the reception state, so the electrolytic voltage will not be abnormally pulled down. Second, in the reception state, the return code pull current is limited to 30 mA by hardware, and the longest single return code width is limited within 10 ms, which will not cause the electrolytic voltage to be abnormally pulled down either.

[0020] Regarding the characteristics that the downstream code sending is not affected by crosstalk current and the upstream return code is easily affected by crosstalk current, the achieved high-impedance state switching effect is as follows: When the bus is in the transmission state, the master station chip and the power supply panel are conducted through a MOS transistor, and the ground between the master station chip and the power supply panel is not isolated; when the bus is in the reception state, the master station chip and the power supply panel are isolated through a bypass resistor, thereby completing the ground isolation between each loop card. By taking isolation measures, the ground unbalanced current between each loop card is successfully eliminated, and thus the common-mode interference noise is effectively suppressed. After calculation based on the wire length and signal transmission, it is confirmed that the differential-mode noise width is less than 10 us. The master station can easily filter out the differential-mode noise interference through program filtering, thereby completely solving the crosstalk problem. In addition, this measure has a significant inhibitory effect on interference such as on-site strong electricity co-management noise, burst interference, and conducted interference.

[0021] Figure 4 It shows the interference waveform of crosstalk on the upstream return code current loop after adding the high-impedance state switching circuit.

[0022] Figure 4 And Figure 2 For comparison, it can be seen from the figure that: 1. The differential-mode current caused by crosstalk still exists, while the common-mode current is almost gone. The differential-mode current pulse duration is about 10 us, but it will not cause misdemodulation of RXD (received data).

[0023] 2. The oscillation phenomenon is effectively and greatly attenuated. Compared with the situation without adding isolation, the square wave waveform of the current can be visually observed with the naked eye.

[0024] 3. Using the anti-interference method of high-impedance state switching for the fire two-bus can replace the isolated power supply scheme and has an obvious effect in suppressing crosstalk between bus lines.

[0025] Test racks 1 - 20: Each test rack is installed with 252 on-site monitoring units, all at the end of the long line: Before optimization: 1. Loops with fast registration speed: Most loops have a registration speed of registering one on-site monitoring unit per second, and all loops complete registration in 2 minutes and 33 seconds; 2. Loops with slow registration speed: Loops 3, 5, and 14 complete registration in 9 minutes and 36 seconds; 3. The slowest registering loops: Loop 9, Loop 10, and Loop 11, which complete registration in 20 minutes.

[0026] As Figure 5 shown, after optimization using the method of the present invention: 1. Six seriously interfered loops, namely Loop 3, Loop 5, Loop 14, Loop 9, Loop 10, and Loop 11, complete registration in 3 minutes and 24 seconds.

[0027] 2. When the on-site monitoring unit conducts a registration roll call operation at the fire alarm controller end, the speed is significantly increased and the effect is remarkable.

[0028] 3. During the conducted immunity test of the fire fighting two-wire bus, no communication abnormality occurs.

[0029] In summary: During the power-on process of the loop card, the bypass resistor of the MOS transistor is used to charge the master station circuit. When the voltage reaches the threshold value, the inverter starts to work, and the corresponding level of its output causes the MOS transistor to automatically turn on, thereby supplying power to the master station circuit. If the electrolytic capacitor of the master station loses power due to an abnormal situation, resulting in the MOS transistor remaining cut off continuously, the loop card can still be powered on again through the bypass resistor.

[0030] When the fire fighting two-wire bus is in the sending state, the master station performs a high-power output without applying isolation; when the fire fighting two-wire bus is in the receiving state, the resistance between the power supply panel and the fire fighting two-wire bus master station chip is the bypass resistor of the MOS. When the MOS transistor is cut off, the common-mode interference between the loop cards is isolated by the bypass resistor, isolating the common-mode interference current between the loop cards.

[0031] When the fire fighting two-wire bus is in the receiving state, the on-site monitoring component is powered by its own electrolytic capacitor without the need for the master station circuit to supply power. When the on-site monitoring component pulls back the code current, the master station chip has hardware current limiting, and the master station chip limits the width of the longest single code return slot to ensure that the code return voltage fluctuation is small.

[0032] When the bus changes from the receiving state to the sending state, the MOS transistor is turned on within an extremely short time to charge the electrolytic capacitor and the on-site monitoring component.

Claims

1. A high-impedance state switching anti-interference method for a fire protection two-wire bus, including a master station MCU, the master station MCU is connected to a fire protection two-wire bus master station chip through an optocoupler isolation circuit, and is characterized in that: It also includes MOS transistor VM1 and MOS transistor VM2. The G pole of MOS transistor VM1 is connected to the BUSH pin of the fire two-bus master station chip through resistor R9. The S pole of MOS transistor VM1 is connected to the power supply through resistor R5. The D pole of MOS transistor VM1 is connected to the D pole of MOS transistor VM2. The D pole of MOS transistor VM2 is also respectively connected to diode VD4 and resistor R20. Diode VD4 is connected to the REVIN pin of the fire two-bus master station chip. Resistor R20 is connected to the CDET pin of the fire two-bus master station chip. The G pole of MOS transistor VM2 is connected to the BUSL pin of the fire two-bus master station chip through resistor R32. The S pole of MOS transistor VM2 is grounded through resistor R28. A switching circuit is connected in parallel between the D pole of MOS transistor VM2 and resistor R28. A high-impedance state switching circuit is provided between the power supply panel and the fire two-bus master station chip; It includes the following adjustment methods: S1. When R / T = 0, turn on the transmission modulation circuit and turn off the reception demodulation circuit to make the fire two-bus master station chip in the transmission state; S2. When R / T = 1, turn on the reception demodulation circuit and turn off the transmission modulation circuit to make the fire two-bus master station chip in the reception state.

2. A high-impedance state switching anti-interference method for a fire fighting two-wire bus according to claim 1, characterized in that: The said switching circuit includes MOS transistor VM5. The G pole of MOS transistor VM5 is connected to the R / T pin of the fire two-bus master station chip. The D pole of MOS transistor VM5 is connected to resistor R33. A diode VD8 is connected in parallel between the S pole of MOS transistor VM5 and resistor R33.

3. A high-impedance state switching anti-interference method for a fire fighting two-wire bus according to claim 1, characterized in that: Step S1 also includes that the modulation voltage in the master station transmission state is 0V - VCC. Modulation principle: The TTL signal of the master station MCU passes through the optocoupler isolation circuit TXD_MCU and is input into the fire two-bus master station chip through the TXD pin to form a modulation signal on the bus; If TXD = 1, P-type MOS transistor VM1 conducts and N-type MOS transistor VM2 turns off, and the bus outputs 1; if TXD = 0, P-type MOS transistor VM1 turns off and N-type MOS transistor VM2 conducts, and the bus outputs 0.

4. A method for anti-interference with high impedance state switching of a two-wire fire protection bus according to claim 1, characterized in that: Step S2 also includes that it is constant-voltage current demodulation in the reception state of the fire two-bus master station chip. Demodulation principle: When the master station is in the reception state, the fire two-bus master station chip receives the bus signal through the REVIN pin, and after demodulation, the TTL signal is output through the RXD pin and given to the master station MCU through the optocoupler isolation circuit RXD_MCU; if a current is detected at the REVIN pin, then set RXD = 0; if no current is detected at the REVIN pin and it exceeds the threshold current of 10 mA and the hardware current limit is 30 mA, then set RXD = 1.

5. A method for high-impedance state switching anti-interference of a fire fighting two-wire bus according to claim 1, characterized in that: The described high-impedance state switching circuit includes a totem pole circuit and a MOS transistor circuit. The MOS transistor circuit includes two MOS transistors D1A and D1B connected in series with each other and connected to the positive pole of the power supply disk, and two MOS transistors D6A and D6B connected in series with each other and connected to the negative pole of the power supply disk. The totem pole circuit includes a triode VT1, a triode VT2, and a triode VT3. The B pole of the triode VT1 and the B pole of the triode VT2 are connected between the resistor R8 and the C pole of the triode VT3. The C pole of the triode VT1 and the resistor R8 are both connected to the S poles of the MOS transistor D1A and the MOS transistor D1B. The E poles of the triode VT1 and the triode VT2 are connected and are respectively connected to the G poles of the MOS transistor D1A and the MOS transistor D1B through the resistor R10. The E pole of the triode VT3 is grounded through the resistor R17. The B pole of the triode VT3 is respectively connected to the G poles of the MOS transistor D6A and the MOS transistor D6B. The B pole of the triode VT3 is also connected with an inverter D2, and the inverter D2 is connected to the R / T pin of the fire two-bus master station chip. The MOS transistor D1A and the MOS transistor D1B are P-type MOS transistors, and the MOS transistor D6A and the MOS transistor D6B are N-type MOS transistors.

6. A method for anti-interference of high impedance state switching of a two-wire fire protection bus according to claim 5, characterized in that: A bypass resistor R1 is connected in parallel with the MOS transistors D1A and D1B, and a bypass resistor R76 is connected in parallel with the MOS transistors D6A and D6B.

7. A high-impedance state switching anti-interference method for a fire fighting two-wire bus according to claim 6, characterized in that: The following adjustment method is also included: S3. When R / T = 0, the bus is in the sending state: After the R / T signal passes through the inverter D2 and the totem pole circuit, both the P-type MOS transistor and the N-type MOS transistor are turned on, so as to supply power to the subsequent circuit. When turned on, its internal resistance is in the milliohm level, and the voltage drop of the MOS transistor is extremely low, so it will not heat up. S4. When R / T = 1, the bus is in the receiving state: The R / T signal passes through the inverter D2 and drives the totem pole circuit, so that both the P-type MOS transistor and the N-type MOS transistor are in the cut-off state. After being cut off, the resistance value between the power supply disk and the fire two-bus master station chip is equivalent to the bypass resistance of the MOS transistor. When the MOS transistor is cut off, the body diode of the MOS transistor has the performance of reverse current cut-off, playing the role of current isolation. In the receiving state, the on-site monitoring component is powered by its own electrolytic capacitor and does not require power supply from the master station circuit. At the same time, the return code pull current is limited to 30 mA, and the longest time for a single return code is limited within 10 ms to ensure that the return code voltage has no fluctuation and the voltage change of the electrolytic capacitor is less than 1 V. When the bus switches from the receiving state to the transmitting state, the MOS transistor is turned on to charge the electrolytic capacitor of the master station and the electrolytic capacitor of the on-site monitoring component.

8. A method for high-impedance state switching and anti-interference of a fire-fighting two-wire bus according to claim 7, characterized in that: The resistance value of the bypass resistor is: R1 = R76 = 10 kΩ. During the power-on period of the loop card, the MOS transistor is default to the cut-off state, and the current charges the master station circuit through the 10 kΩ resistor. When the voltage rises to the threshold value, the inverter D2 starts, and its output level prompts the MOS transistor to automatically turn on, so as to supply power to the master station circuit. If the electrolytic capacitor of the master station loses power due to abnormal conditions, resulting in the continuous cut-off of the MOS transistor, the loop card can still be powered on again through the 10 kΩ resistor to increase the reliability of the system.

9. A high-impedance state switching anti-interference method for a fire fighting two-wire bus according to claim 7, characterized in that: It also includes that when the length of the fire two-wire line exceeds 1000m and a short circuit occurs at its end, the system will detect whether there is a situation where the return code level is pulled down due to continuous pull current in the return code slot. If the RXD is continuously demodulated to a low level and the duration exceeds 30s, it is determined that there is a bus short circuit.

10. A method for high-impedance state switching anti-interference of a fire fighting two-wire bus according to claim 7, characterized in that: It also includes that there are two situations of bus short circuit: First, a short circuit occurs in the transmission state. At this time, the overcurrent detection circuit will play a role, and the device will continuously maintain the transmission state without entering the reception state, so it will not cause abnormal pulling down of the electrolytic voltage; Second, in the reception state, the return code pull current is limited to 30mA by hardware, and the longest single return code width is limited within 10ms, which will not cause abnormal pulling down of the electrolytic voltage either.

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