A fire alarm system loop bus isolator

Abstract

The invention discloses a loop bus isolator for a fire alarm system, which relates to the technical field of fire alarms. The isolator comprises an input loop conditioning circuit, a main control unit, an IO expansion circuit, a branch short-circuit detection circuit, a branch isolation control circuit and a status indication circuit. The loop bus processed by the input loop conditioning circuit and the filtered high-level signal are connected to a microprocessor of the main control unit. The microprocessor is connected to the IO expansion circuit via an SPI interface. The output signal of the IO expansion circuit is connected as a control signal to the branch short-circuit detection circuit, the branch short-circuit isolation control circuit and the status indication circuit. The sampling signal of the branch short-circuit detection circuit is connected to an A / D port of the main control unit. The branch isolation control circuit is used to isolate a short-circuited branch. The invention divides a loop bus into multiple branch outputs that can be independently isolated and protected, has higher reliability and lower cost, and parameters such as a short-circuit threshold and a fault alarm sensitivity can be set and queried in real time by software, making debugging and maintenance convenient.

Description

Technical Field

[0001] The present invention relates to the technical field of fire alarm technology, and in particular to a loop bus isolator for a fire alarm system. Background Art

[0002] With the increasing frequency and severity of fires in recent years, people are placing increasing emphasis on early fire detection and alarms. Fire alarm products are increasingly being used in production and daily life. This increased emphasis on fire alarms has led to higher requirements for fire detection reliability. Fire alarm system loop bus isolators can isolate and protect devices connected to the isolator's back-end branches from abnormalities such as short circuits, preventing them from affecting the normal fire detection of other devices on the loop and improving system reliability.

[0003] Currently, fire alarm system loop bus isolators have the following shortcomings: (1) Most of them use a single-branch isolation method with one input and one output. A fire alarm loop bus can generally connect 252 field devices. If more isolation partitions are needed, more bus isolators will be needed, which is costly. (2) Most of them use solid-state relays. Due to the characteristics of the relay itself, the response time is slow, the mechanical life is short, and it is easy to trigger false operation in a vibration environment. (3) Most of them rely on pure hardware circuits to achieve isolation. The short-circuit judgment threshold parameters are fixed, which makes the application inflexible and inconvenient for subsequent maintenance. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a fire alarm system loop bus isolator adopting a one-input and multiple-output multi-branch isolation mode.

[0005] In order to solve the above technical problems, the present invention provides a fire alarm system loop bus isolator, including an input loop conditioning circuit, a main control unit, an IO expansion circuit, a branch short-circuit detection circuit, a branch isolation control circuit and a status indication circuit. The loop bus processed by the input loop conditioning circuit and the filtered high-level signal are connected to the microprocessor of the main control unit. The microprocessor of the main control unit is connected to the IO expansion circuit via an SPI interface. The output signal of the IO expansion circuit is connected as a control signal to the branch short-circuit detection circuit, the branch short-circuit isolation control circuit and the status indication circuit. The sampling signal of the branch short-circuit detection circuit is connected to the A / D port of the main control unit. The branch isolation control circuit is used to isolate the short-circuited branch. The status indication circuit is used to indicate the operating status of the microprocessor of the main control unit and the short-circuit fault of the branch.

[0006] According to a preferred embodiment of the present invention, the branch short circuit detection circuit includes a signal sampling control circuit. In the signal sampling control circuit, the output signal AD1ON of the cascade shift register of the IO expansion circuit is simultaneously connected to one end of the resistor R22 and the resistor R25, the other end of the resistor R22 is connected to the G pole of the N-MOS tube U4, the other end of the resistor R25 is connected to GND, the S pole of the N-MOS tube U4 is connected to GND, the D pole of the N-MOS tube U4 is simultaneously connected to the capacitor C10, the resistor R19 and one end of the resistor R20, the other end of the resistor R20 is connected to the positive pole of the voltage regulator tube D12, the negative pole of the voltage regulator tube D12 is connected to 1B- of the branch loop output, and the branch load RL is connected between 1B- and B+.

[0007] According to a preferred embodiment of the present invention, the branch short circuit detection circuit includes a signal sampling circuit. In the signal sampling circuit, one end of the resistor R19 is connected to one end of the capacitor C10 and is also connected to the D pole of the N-MOS tube U4 of the sampling control circuit. When the sampling control switch is turned on, the N-MOS tube U4 is turned on and the S pole is connected to GND. The other end of the resistor R19 is connected to the other end of the capacitor C10 and is also connected to one end of the resistor R16 and PIN3 of the combination diode D11. PIN1 of the combination diode D11 is grounded, PIN2 of the combination diode D11 is connected to a 5V voltage, and the other end of the resistor R16 is connected to 1B- of the branch loop output.

[0008] According to a preferred embodiment of the present invention, in the branch isolation control circuit, the output signal 1ON of the cascade shift register of the IO expansion circuit is simultaneously connected to one end of the resistor R17 and the resistor R21, the other end of the resistor R17 is connected to the G pole of the N-MOS transistor Q1B, the other end of the resistor R21 is connected to GND, the S pole of the N-MOS transistor Q1B is connected to GND, the D pole of the N-MOS transistor Q1B is connected to 1B-, and the branch load RL is connected between 1B- and B+.

[0009] According to a preferred embodiment of the present invention, the input loop conditioning circuit includes an EMC protection circuit of a port. In the EMC protection circuit, B+ of the input loop is connected to one end of a bidirectional TVS tube in the EMC protection circuit, and the other end of the bidirectional TVS tube is connected to B- of the input loop.

[0010] According to a preferred embodiment of the present invention, the input loop conditioning circuit further includes a MOS tube reverse polarity protection circuit. In the MOS tube reverse polarity protection circuit, B+ of the input loop is connected to one end of a sampling resistor R15, the other end of the resistor R15 is connected to one end of a resistor R18, the other end of the resistor R18 is connected to GND, GND is connected to the S poles of Q5A and Q5B, the D poles of Q5A and Q5B are connected and connected to B-, the G poles of Q5A and Q5B are connected and connected to one end of a resistor R110, the other end of the resistor R110 is connected to a control signal Crtl_ON of a microprocessor of a main control unit, the control signal Crtl_ON is also connected to one end of a resistor R109, and the other end of the resistor R109 is connected to GND.

[0011] According to a preferred embodiment of the present invention, the main control unit further includes a power supply LDO, and the power supply LDO is used to provide power for the loop bus isolator.

[0012] According to a preferred embodiment of the present invention, the microprocessor of the main control unit is an LK213 chip.

[0013] According to a preferred embodiment of the present invention, the IO expansion circuit includes a plurality of cascaded shift latches.

[0014] According to a preferred embodiment of the present invention, the status indication circuit includes an electrically connected microprocessor operation status indication circuit and a branch short circuit fault indication circuit.

[0015] The technical effects of the present invention are:

[0016] 1. The present invention relates to a loop bus isolator for a fire alarm system. The loop bus processed by the input loop conditioning circuit and the filtered high-level signal are connected to the microprocessor of the main control unit. The microprocessor is connected to the IO expansion circuit via the SPI interface. The output signal of the IO expansion circuit is connected as a control signal to the branch short-circuit detection circuit, the branch short-circuit isolation control circuit and the status indication circuit. The sampling signal of the branch short-circuit detection circuit is connected to the A / D port of the main control unit. The branch short-circuit isolation control circuit is connected to multiple branches. The present invention divides a loop bus into multiple branch outputs that can be independently isolated and protected through an isolator, thereby achieving the beneficial effect of a multi-branch isolation method with one input and multiple outputs.

[0017] 2. The present invention provides a fire alarm system loop bus isolator, which contains a microprocessor, which is connected to the loop bus and can communicate with each other. A controller or configuration tool can read and write parameters of the fire alarm system loop bus isolator through the loop bus. The short circuit judgment threshold, the time for reporting a short circuit fault, etc. can be flexibly set and read in real time, which is very convenient for subsequent debugging and maintenance.

[0018] 3. The fire alarm system loop bus isolator of the present invention realizes isolation control by using a MOS tube switch. Compared with a relay, it has a faster response, a longer service life and a higher reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a principle block diagram of a fire alarm system loop bus isolator according to the present invention;

[0020] Figure 2 The present invention is a schematic diagram of a short-circuit detection and isolation control circuit for the first branch of a loop bus isolator of a fire alarm system.

[0021] Reference numerals: RL - load; R15 - resistor; R16 - resistor; R18 - resistor; R19 - resistor; R20 - resistor; C9 - capacitor; C10 - capacitor; D11 - combination diode; D12 - voltage regulator; D13 - combination diode; Q1 - MOS tube; Q5 - MOS tube; U4 - MOS tube. DETAILED DESCRIPTION

[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it. However, the embodiments are not intended to limit the invention.

[0023] like Figure 1-Figure 2As shown, the present invention presents a fire alarm system loop bus isolator with one input and eight outputs. It receives a loop bus signal and divides it into eight output branches, which are then connected to a number of field components. The fire alarm system loop bus isolator includes an input loop conditioning circuit, a main control unit, an I / O expansion circuit, a branch short-circuit detection circuit, a branch isolation control circuit, and a status indication circuit. The input loop conditioning circuit includes a port surge protection circuit, an anti-reverse connection circuit, and a loop rectification and filtering circuit; the main control unit includes a power supply LDO, a loop protocol analysis circuit and a microprocessor. The microprocessor selected in the present invention is the company's customized development chip LK213 chip, which has integrated the power supply LDO and the loop protocol analysis circuit. The output voltage of the power supply LDO is 5VDC; the IO expansion circuit includes three cascaded 74HC595 shift latches, which are cascaded and expanded by the microprocessor of the main control unit via SPI; the branch short-circuit detection circuit includes a signal sampling control circuit for eight branches, a signal sampling circuit and a signal sampling circuit for the input loop bus trunk; the branch isolation control circuit includes a MOS tube switch control circuit for eight branches; the status indication circuit includes an isolator's own operating status indication circuit and an eight-branch short-circuit fault indication circuit. The loop bus processed by the conditioning circuit of the input loop bus and the filtered high-level signal are connected to the microprocessor chip of the main control unit; the sampling signal of the branch short-circuit detection circuit is connected to the A / D port of the microprocessor chip of the main control unit; the IO expansion circuit is expanded using a 74HC595 cascade method, and its data signal, clock signal and latch signal are connected to the SPI interface of the microprocessor chip of the main control unit, and its output signal is connected as a control signal to the branch short-circuit detection circuit, the branch short-circuit isolation control circuit and the status indication circuit.

[0024] Furthermore, the fire alarm system loop bus isolator is designed with fixing holes, input terminals, and output terminals on the PCB. It can be manufactured in the form of a board, or isolation card, that can be installed within the controller. After the controller's loop bus output is connected to the isolation card, it is output via eight branches to connect to field loop bus devices. If eight branches are insufficient, one or more isolation cards can be added. The fire alarm system loop bus isolator can also be manufactured as a finished module, or isolation module, that is installed at the construction site. After the field loop bus is connected to the isolation module, it is output via eight branches, each connected to nearby field loop bus devices.

[0025] Furthermore, the main control unit's microprocessor integrates a loop protocol parsing circuit. The loop waveform is a pulse waveform containing both communication and non-communication protocols. The communication segment is used for communication between the loop board and loop bus devices, while the non-communication segment is used for loop line detection. The fire alarm system loop bus isolator utilizes the non-communication segment to perform short-circuit detection and algorithmic determination on the eight branches. Branches that meet the short-circuit criteria are identified as short-circuited, and the corresponding MOSFET is controlled to shut down and isolate. Once the short circuit is detected and resolved, the corresponding MOSFET is controlled to turn on, resuming normal operation.

[0026] Furthermore, the IO expansion circuit includes three cascaded 74HC595 chips, which expand 24 output control signals, corresponding to the signal sampling control start signals of the eight branches, the MOS tube switch control signals and the short-circuit fault indicator light drive signals. The three cascaded 74HC595 chips are connected to the main control unit microprocessor via SPI.

[0027] Furthermore, the branch short-circuit detection circuit includes a signal sampling circuit for the loop bus trunk, a signal sampling control start-up circuit for each of the eight branches, and a signal sampling circuit. The signal sampling circuit includes a 1:10 voltage divider circuit consisting of two resistors, a capacitor filtering circuit, and a sampling limiter circuit. The signal sampling control circuit includes a switch circuit consisting of a control signal output by a 74HC595, a resistor, and a MOS transistor, and a high-level detection channel circuit consisting of a 6.2V voltage regulator diode, a 1.2K resistor, and a load RL. The 6.2V voltage regulator diode is used to filter out the 6V level and low-level 0V waveforms in the loop, allowing only high-level waveforms to pass.

[0028] Furthermore, the branch isolation control circuit includes a control signal output by 74HC595, a resistor and a MOS tube; the short-circuited branch is isolated by turning off the MOS tube.

[0029] Furthermore, the status indicator light circuit includes a main control unit microprocessor, a control signal output by an IO expansion circuit 74HC595, a current limiting resistor, and an LED.

[0030] The following example illustrates a fire alarm system loop bus isolator of the present invention, but it is not limited to this in actual engineering use.

[0031] like Figure 1-Figure 2The figure shows a schematic block diagram of the fire alarm system loop bus isolator and a practical schematic diagram of the first branch short-circuit detection and isolation control circuit. The fire alarm system loop bus enters the isolator, communicating with and powering the main control unit's microprocessor. The microprocessor uses the company's customized LK213 chip, which integrates a loop bus protocol parser and LDO. Once operational, the microprocessor provides 5V power to the board and peripheral components. Crtl_ON is also controlled high, ensuring proper conduction of the anti-reverse MOSFET Q5, connecting GND to B-.

[0032] Please refer to the following Figure 2 , the short-circuit detection and isolation control principle of the first branch is elaborated in detail.

[0033] The input circuit conditioning circuit consists of the port's EMC protection circuit, a MOS transistor reverse polarity protection circuit, and a rectifier and filter circuit consisting of diodes, resistors, and capacitors. The input circuit's B+ terminal is connected to one end of the bidirectional TVS diode in the EMC protection circuit, while the other end is connected to the input circuit's B- terminal, forming the input circuit's primary EMC protection. The input circuit's B+ terminal is connected to one end of sampling resistor R15, which is connected to one end of R18. The other end of R18 is connected to GND, which is connected to the S terminals of Q5A and Q5B. The D terminals of Q5A and Q5B are connected and connected to B-. The G terminals of Q5A and Q5B are connected and connected to one end of resistor R110. The other end of resistor R110 is connected to the Crtl_ON control signal from the main control unit's microprocessor. The Crtl_ON control signal is also connected to one end of R109, which is connected to GND. This constitutes the reverse polarity protection circuit. The input circuit B+ / GND after anti-reverse connection treatment is connected to the circuit protocol analysis circuit of the main control unit.

[0034] The branch short-circuit detection circuit includes a signal sampling control circuit and a signal sampling circuit. The signal sampling control circuit is used to control the on and off of signal sampling. It is composed of a control signal output by the IO expansion circuit, a switch circuit composed of a resistor and a MOS transistor, and a high-level detection channel circuit composed of a 6.2V voltage regulator, a resistor, and a load. The signal sampling control circuit principles of multiple branches are the same. Taking the first branch as an example, the output signal AD1ON of the cascade shift register of the IO expansion circuit is connected to one end of resistor R22 and resistor R25. The other end of resistor R22 is connected to the G terminal of N-MOS transistor U4, the other end of resistor R25 is connected to GND, the S terminal of N-MOS transistor U4 is connected to GND, and the D terminal of N-MOS transistor U4 is connected to one end of capacitor C10, resistor R19, and resistor R20. The other end of resistor R20 is connected to the positive terminal of voltage regulator transistor D12, and the negative terminal of D12 is connected to 1B- of the branch circuit output. The first branch load RL is connected between 1B- and B+. The signal sampling circuit, used for signal sampling, includes a 1:10 voltage divider circuit formed by two resistors, a capacitor filtering circuit, and a sampling voltage limiter circuit. The signal sampling control circuit principles for the loop bus trunk and the multi-loop branches are the same. Taking branch 1 as an example, one end of resistor R19 is connected to one end of capacitor C10 and also to the D terminal of N-MOS transistor U4 in the sampling control circuit. When the sampling control switch is turned on, N-MOS transistor U4 conducts, and U4's S terminal is connected to GND. The other end of resistor R19 is connected to the other end of capacitor C10, and is also connected to one end of R16 and pin 3 of D11. Pin 1 of D11 is grounded, and pin 2 of D11 is connected to 5V. The other end of R16 is connected to 1B- of the branch loop output. Resistors R19 and R16 form a 1:10 voltage divider. Other voltage divider ratios are also possible.

[0035] The branch isolation control circuit includes a MOS transistor switch control circuit. The isolation control circuit principles for multiple loop branches are the same. Taking branch 1 as an example, the output signal 1ON of the cascaded shift register of the IO expansion circuit is connected to one end of resistors R17 and R21. The other end of resistor R17 is connected to the G terminal of N-MOS transistor Q1B, and the other end of resistor R21 is connected to GND. The S terminal of N-MOS transistor Q1B is connected to GND, and the D terminal of N-MOS transistor Q1B is connected to 1B-. The first branch load RL is connected between 1B- and B+.

[0036] The microprocessor chip communicates with the circuit during the communication phase and performs line detection during the non-communication phase. The main control unit sends a command, via the IO expansion circuit, to control the 1ON output pin of the 74HC595 chip to a low level, effectively shutting off MOS transistor Q1B. At this point, the output of the first branch of the circuit—the high-end remains B+, while the low-end 1B-—is no longer directly connected to GND. When the microprocessor analyzes the circuit level as high, it controls the AD1ON output pin of the 74HC595 chip to a high level, activating the high-level short-circuit detection channel. The path is as follows: B+ passes through load RL to 1B-, then to Zener diode D12 (voltage: 6.2V), then to resistor R20 (resistance: 1.2K), and finally to MOS transistor U4. After being controlled by AD1ON, it is connected to GND. Finally, it is connected through MOS transistor Q5 and returns to B-. Resistors R16 (1MΩ) and R19 (100KΩ) in the sampling voltage divider circuit are connected in parallel with the Zener diode D12 and resistor R20. Capacitor C10 is connected in parallel with resistor R19. AD1 is the voltage across R19, corresponding to (V1B-)*1 / 11. Diode D11 acts as a voltage limiter to protect AD1 from abnormal voltages that could damage the microprocessor. AD1 is connected to the A / D port of the main control unit's microprocessor. Software sampling and readings can be used to infer AD1's voltage value, VAD1. Based on the voltage divider ratio of resistors R16 / R19, the 1B- voltage value, V1B-, can be calculated as 11*VAD1. Similarly, the ADLB+ sampling circuit follows the following path: B+ passes through R15 (1MΩ) and resistor R18 (100KΩ) in the sampling voltage divider circuit, to GND, and finally, through MOSFET Q5, back to B-. Capacitor C9 is connected in parallel with resistor R18. ADLB+ is the voltage across resistor R18, corresponding to (VB+)*1 / 11. Diode D13 provides voltage limiting protection for ADLB+, preventing abnormal voltage from entering the microprocessor and damaging it. ADLB+ is connected to the A / D port of the main control unit's microprocessor. Software sampling and readings can be used to infer the voltage across ADLB+ (VADLB+). Based on the voltage divider ratio of resistors R15 / R18, the voltage across B+ (VB+) can be calculated as 11*VADLB+. This allows the output voltage of the first branch to be calculated: load RL voltage VRL = (VB+) - (V1B-). Given the voltage across voltage regulator diode D12 (VD12) = 6.2V, the voltage across resistor R20 VR20 = (V1B-) - VD12. The ratio of load RL voltage VRL to resistor R20 voltage VR20 serves as the basis for short-circuit determination.

[0037] B+ / B- are input loop lines, with a pulse waveform, a maximum high-level voltage of 28V, a mid-level of 6V, and a low-level of 0V. Loop detection is performed at a high level in the non-communication segment, with a maximum high-level output of 28V. The minimum high level allowed by the end device of the loop bus is 13V. Depending on the loop load current, the claimed long line also varies. Here, the short-circuit threshold of the loop branch is defined as RS. The short-circuit judgment criterion is as follows:

[0038]

[0039] The simplified formula is as follows:

[0040]

[0041] VB+ can be calculated based on the ADLB+ sampling value, (VB+)=11*VADLB+.

[0042] V1B- can be calculated based on the AD1 sampling value, (V1B-)=11*VAD1.

[0043] VD12=6.2V.

[0044] By sampling AD1 and ADLB+ through software, the expression on the left side of the above formula 2 is substituted. If the formula 2 is satisfied, it is determined that the current branch load resistance is less than the short-circuit threshold RS, and the branch is determined to be short-circuited. Otherwise, the branch is normal.

[0045] The short-circuit resistance threshold in the present invention is software-adjustable, i.e., the claimed short-circuit resistance RS can be set according to specific site conditions. The main control unit microprocessor in the present invention can communicate with the controller circuit, allowing flexible parameter setting and real-time query parameter reading. The specific explanation is described as follows:

[0046] When the total current of the full loop load on site is 0.3A, the allowable long-line resistance is (28V-13V) / 0.3A=50 ohms. Taking into account sampling errors and component accuracy deviations, and leaving margin, the short-circuit threshold can be set to 60 ohms. The corresponding short-circuit criterion is as follows:

[0047]

[0048] Simplify the following formula 4:

[0049]

[0050] By sampling AD1 and ADLB+ through software, the left side of the expression in the above formula 4 can be substituted. If the expression in formula 4 is satisfied, it is determined that the load resistance is less than the short-circuit resistance threshold of 60 ohms, and the branch is short-circuited. Otherwise, the branch is normal.

[0051] When the total current of the full loop load on site is 0.1A, the allowable long-line resistance is (28V-13V) / 0.1A=150 ohms. The greater the long-line resistance of the loop bus, the longer the loop bus wiring can be. Taking into account sampling errors and component accuracy deviations, and leaving margin, the short-circuit threshold can be set to 160 ohms. The corresponding short-circuit criterion is as follows: Formula 5:

[0052]

[0053] Simplify the following formula six:

[0054]

[0055] By sampling AD1 and ADLB+ through software, the left side of the expression in the above formula 6 can be substituted. If the expression in formula 6 is satisfied, it is determined that the load resistance is less than the short-circuit resistance threshold of 160 ohms, and the branch is short-circuited. Otherwise, the branch is normal.

[0056] The above embodiments are merely preferred embodiments for the purpose of fully illustrating the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. The scope of protection of the present invention shall be subject to the claims.

Claims

1. A fire alarm system loop bus isolator, characterized in that: The system comprises an input loop conditioning circuit, a main control unit, an IO expansion circuit, a branch short-circuit detection circuit, a branch isolation control circuit, and a status indication circuit. The loop bus processed by the input loop conditioning circuit and the filtered high-level signal are connected to the microprocessor of the main control unit. The microprocessor of the main control unit is connected to the IO expansion circuit via an SPI interface. The output signal of the IO expansion circuit is connected as a control signal to the branch short-circuit detection circuit, the branch short-circuit isolation control circuit, and the status indication circuit. The sampling signal of the branch short-circuit detection circuit is connected to the A / D port of the main control unit. The branch isolation control circuit is used to isolate a short-circuited branch. The status indication circuit is used to indicate the operating status of the microprocessor of the main control unit and a short-circuit fault of a branch. The input loop conditioning circuit also includes a MOS tube reverse connection protection circuit. In the MOS tube reverse connection protection circuit, B+ of the input loop is connected to one end of a sampling resistor R15, the other end of the sampling resistor R15 is connected to one end of a resistor R18, the other end of the resistor R18 is connected to GND, GND is connected to the S poles of Q5A and Q5B, the D poles of Q5A and Q5B are connected and connected to B-, the G poles of Q5A and Q5B are connected and connected to one end of a resistor R110, the other end of the resistor R110 is connected to the control signal Crtl_ON of the main control unit microprocessor, the control signal Crtl_ON is also connected to one end of a resistor R109, and the other end of the resistor R109 is connected to GND; The branch short circuit detection circuit includes a signal sampling control circuit. In the signal sampling control circuit, the output signal AD1ON of the cascade shift register of the IO expansion circuit is connected to one end of the resistor R22 and the resistor R25 at the same time. The other end of the resistor R22 is connected to the G pole of the N-MOS tube U4. The other end of the resistor R25 is connected to GND. The S pole of the N-MOS tube U4 is connected to GND. The D pole of the N-MOS tube U4 is connected to one end of the capacitor C10, the resistor R19 and the resistor R20 at the same time. The other end of the resistor R20 is connected to the positive pole of the voltage regulator tube D12. The negative pole of the voltage regulator tube D12 is connected to the 1B output of the branch loop. -, 1B- and B+ are connected to a branch load RL; one end of a resistor R19 is connected to one end of a capacitor C10 and is also connected to the D-pole of an N-MOS transistor U4 of the sampling control circuit. When the sampling control switch is turned on, the N-MOS transistor U4 is turned on and its S-pole is connected to GND. The other end of the resistor R19 is connected to the other end of the capacitor C10 and is also connected to one end of a resistor R16 and PIN3 of a combination diode D11. PIN1 of the combination diode D11 is grounded, and PIN2 of the combination diode D11 is connected to a 5V voltage. The other end of the resistor R16 is connected to 1B-, the output of the branch loop; The claimed long line varies depending on the loop load current. Here, the short-circuit threshold of the loop branch is defined as RS. The short-circuit criterion is as follows: ； Where: VB+ can be calculated based on the VADLB+ sampling value, (VB+)=11*VADLB+, VADLB+ is the voltage of resistor R18; V1B- can be calculated based on the VAD1 sampling value, (V1B-)=11*VAD1, VAD1 is the voltage of R19; If the current branch load resistance is less than the short-circuit threshold RS, the branch is determined to be short-circuited; otherwise, the branch is normal.

2. A fire alarm system loop bus isolator according to claim 1, characterized in that: In the branch isolation control circuit, the output signal 1ON of the cascade shift register of the IO expansion circuit is connected to one end of both resistors R17 and R21. The other end of resistor R17 is connected to the G terminal of the N-MOS transistor Q1B. The other end of resistor R21 is connected to GND. The S terminal of the N-MOS transistor Q1B is connected to GND. The D terminal of the N-MOS transistor Q1B is connected to 1B-. A branch load RL is connected between 1B- and B+.

3. A fire alarm system loop bus isolator according to claim 1, characterized in that: The input loop conditioning circuit includes an EMC protection circuit of a port. In the EMC protection circuit, B+ of the input loop is connected to one end of a bidirectional TVS tube in the EMC protection circuit, and the other end of the bidirectional TVS tube is connected to B- of the input loop.

4. A fire alarm system loop bus isolator according to claim 1, characterized in that: The main control unit further includes a power supply LDO, which is used to provide power to the loop bus isolator.

5. A fire alarm system loop bus isolator according to claim 1, characterized in that: The microprocessor of the main control unit is an LK213 chip.

6. A fire alarm system loop bus isolator according to claim 1, characterized in that: The IO expansion circuit includes a plurality of cascaded shift latches.

7. A fire alarm system loop bus isolator according to claim 1, characterized in that: The state indication circuit includes an electrically connected microprocessor operation state indication circuit and a branch short circuit fault indication circuit.

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