A serial port multiplexing and switching circuit

Abstract

In response to the current situation where the State Grid's standardized power distribution terminals do not have additional interfaces to lead out the internal serial ports of the device, the present invention discloses a serial port multiplexing switching circuit, which belongs to the technical field of electrical digital data processing. The serial port multiplexing switching circuit includes: a serial port switching auxiliary module, a physical switch, a device internal preset module, and a logical switch. The physical switch connects the logical selection module to the RS232 serial port receiving port of the maintenance serial port and the RS232 serial port grounding port of the maintenance serial port, thereby achieving the purpose of switching the serial ports of each debugging serial port in the power distribution terminal to the same physical channel and serial port communication with the maintenance serial port. The present invention can achieve switching between different serial ports on the same serial port physical channel through a hardware circuit without affecting the normal function of the defined interface and without relying on the software running state, thereby achieving the function of communicating with the device CPU through different serial ports. The circuit is simple, reliable, and low in cost.

Description

Technical Field

[0001] The invention discloses a serial port multiplexing switching circuit for a power distribution terminal, belonging to the technical field of electrical digital data processing. Background Art

[0002] In 2021, State Grid Corporation of China will improve equipment selection standards, improve the quality control system, enhance the durability of distribution network equipment, highlight the core of good equipment, and comprehensively promote the standardization of distribution equipment based on the principles of safety, reliability, durability, unified standards, and universal interchangeability in accordance with the distribution network operation service management requirements. The State Grid Standardized Distribution Terminal Equipment Specification has clear requirements for the external dimensions, connection terminal interface form, and detailed terminal definitions of the FTU, and all external terminals are defined. There is only one maintenance serial port leading out from the panel, and there are no spare terminals for manufacturers to customize. In other words, the debugging serial port of the State Grid Standardized Distribution Terminal Equipment has no redundant interfaces. On the other hand, the distribution terminal device usually adopts an integrated design with an IP rating of IP67. The device cannot be easily disassembled for maintenance, which makes it difficult for the State Grid Standardized Distribution Terminal Device to be commissioned and maintained on site.

[0003] Given the aforementioned issue of the State Grid standardized FTU lacking an additional interface for exposing the debug serial port, serial port multiplexing technology can be used to leverage existing interface definition channels and enable different serial ports to be brought out of the same physical interface. Serial port multiplexing technology can be implemented in both software and hardware. Software-based serial port multiplexing uses processor software to control analog switch switching channels, allowing different serial ports to be brought out of the same physical interface. However, this approach is prone to loss of analog switch control during processor malfunctions, resulting in low reliability and failing to meet the real-time debugging requirements of distribution terminal equipment. Hardware-based serial port multiplexing uses a rectifier bridge and T-type flip-flops to process signals received from the maintenance serial port wiring to generate analog switch switching control signals. This approach requires complex hardware circuitry and constant plugging and unplugging of interfaces to switch serial ports. It cannot meet the requirement for multiple debug serial ports within the distribution terminal to automatically connect to the maintenance serial port. Customized interface functionality is required to access signals transmitted by the maintenance serial port. In other words, existing hardware implementations of serial port multiplexing technology cannot be implemented in accordance with the interface definitions defined in the State Grid standardized distribution terminal specifications.

[0004] Therefore, the present invention aims to propose a serial port multiplexing circuit compatible with the State Grid standardized power distribution terminal equipment specifications to overcome the above-mentioned defects. Summary of the Invention

[0005] The purpose of the present invention is to address the deficiencies of the above-mentioned background technology and provide a serial port multiplexing switching circuit. Without affecting the implementation of the defined interface function and without relying on the software running status, the circuit automatically switches different debugging serial ports in the distribution terminal equipment to access the same serial port physical channel through a simple hardware circuit, thereby achieving the purpose of leading out the debugging serial port inside the terminal when there is no additional interface in the State Grid standardized distribution terminal, realizing reliable communication between different debugging serial ports of the State Grid standardized distribution terminal and the device CPU, and solving the technical problem that the State Grid standardized distribution terminal equipment has no additional interface to lead out the debugging serial port.

[0006] The present invention adopts the following technical solutions to achieve the above-mentioned purpose:

[0007] A serial port multiplexing switching circuit comprises a serial port switching auxiliary module and a device internal preset module. It can realize the switching function between the same serial port physical channel and different debugging serial ports in the power distribution terminal device without relying on software.

[0008] The serial port switching auxiliary module is located outside the State Grid standardized power distribution terminal. In addition to the standardized terminals for connecting to the power distribution terminal device and serial port debugging tools, it also consists of internal wiring and a physical switch. According to State Grid's standardized specifications, the RS232 serial port transmit port, RS232 serial port receive port, and RS232 serial port ground port of the maintenance serial port are connected to the serial port switching auxiliary module. A physical switch controls the connection between the RS232 serial port receive port of the maintenance serial port and the active pulse terminal pin or the reactive pulse terminal pin, enabling serial port switching between pre-configured modules within the device.

[0009] The preset modules inside the device include: three groups of debugging serial ports, a logic selection module, and a logic switch. One end of the logic switch is connected to the RS232 serial port transmitting port and the RS232 serial port receiving port defined in the standard specifications of the power distribution terminal, and the other end of the logic switch is connected to a group of debugging serial ports after receiving the control signal sent by the logic selection module. The logic selection module realizes the selection of different serial port channels for the same serial port physical loop through a gate circuit. Specifically: when the physical switch controls the RS232 serial port receiving port of the maintenance serial port to be connected to the active pulse terminal pin, the logic selection module accesses the RS232 serial port receiving port of the maintenance serial port through the active pulse terminal pin, and the logic selection module accesses the RS232 serial port ground port of the maintenance serial port through the pulse common port. After the serial port switching auxiliary module is connected to the PC with an external serial port debugging tool, a voltage characteristic greater than 3V is generated between the RS232 serial port ground port of the maintenance serial port and the RS232 serial port receiving port of the maintenance serial port, so that the logic selection module generates the selection of the first group of debugging serial ports. The control signal of the port accessing the RS232 physical channel; when the physical switch controls the RS232 serial port receiving port of the maintenance serial port to be connected to the reactive pulse terminal pin, the logic selection module accesses the RS232 serial port receiving port of the maintenance serial port through the reactive pulse terminal pin, and the logic selection module accesses the RS232 serial port ground port of the maintenance serial port through the pulse common port. The voltage characteristics of the serial port switching auxiliary module after the external serial port debugging tool is connected to the PC are used to make the logic selection module generate a control signal for selecting the second group of debugging serial ports to access the RS232 physical channel; when the physical switch is suspended, the logic selection module generates a control signal for selecting the third group of debugging serial ports to access the RS232 physical channel.

[0010] Furthermore, as a further improvement to the serial port multiplexing switching circuit, the logic selection module includes: a first logic selection unit and a second logic selection unit. When the physical switch is thrown to the active pulse terminal pin, the first logic selection unit is connected to the active pulse port and the pulse common port; when the physical switch is thrown to the reactive pulse terminal pin, the second logic selection unit is connected to the reactive pulse port and the pulse common port.

[0011] Furthermore, as a further improvement solution of the serial port multiplexing switching circuit, the first logic selection unit includes: a first optocoupler, a first resistor, a second resistor, a first capacitor, a first freewheeling diode, and a first anti-reverse diode. When the physical switch controls the RS232 serial port receiving port of the maintenance serial port to be connected to the active pulse terminal pin, the first logic selection unit accesses the RS232 serial port receiving port of the maintenance serial port through the active pulse terminal pin, and the first logic selection unit accesses the RS232 serial port ground port of the maintenance serial port through the pulse common port. The loop formed by the RS232 serial port ground port of the maintenance serial port, the pulse common port, the first freewheeling diode, the P active pulse port, the physical switch, and the RS232 serial port receiving port of the maintenance serial port causes the first optocoupler to work, and the first logic selection unit generates a 1-bit low-level signal RS232_SEL1. At this time, the second logic selection unit does not work and generates a 1-bit high-level signal RS232_SEL2. The logic selection module outputs the control signal RS232_SEL composed of RS232_SEL1 and RS232_SEL2 for selecting the first group of debugging serial ports to access the RS232 physical channel to the logic switch.

[0012] Furthermore, as a further improvement solution of the serial port multiplexing switching circuit, the second logic selection unit includes: a third optocoupler, a fourth resistor, a fifth resistor, a second capacitor, a second freewheeling diode, and a second anti-reverse diode. When the physical switch controls the RS232 serial port receiving port of the maintenance serial port to be connected to the reactive pulse terminal pin, the second logic selection unit accesses the RS232 serial port receiving port of the maintenance serial port through the reactive pulse terminal pin, and the second logic selection unit accesses the RS232 serial port ground port of the maintenance serial port through the pulse common port. The loop formed by the RS232 serial port ground port of the maintenance serial port, the pulse common port, the second freewheeling diode, the Q reactive pulse port, the physical switch, and the RS232 serial port receiving port of the maintenance serial port makes the third optocoupler work, and the second logic selection unit generates a 1-bit low level RS232_SEL2. At this time, the first logic selection unit does not work and outputs a 1-bit high level RS232_SEL1. The logic selection module outputs the control signal RS232_SEL composed of RS232_SEL1 and RS232_SEL2 for selecting the second group of debugging serial ports to access the RS232 physical channel to the logic switch.

[0013] Furthermore, as a further improvement scheme of the serial port multiplexing switching circuit, when the preset module inside the device includes N groups of debugging serial ports, and N is an integer greater than 3, the physical switch selects a single-pole N-1 throw physical switch, and the single-pole N-1 throw physical switch is used to control the RS232 serial port receiving port of the maintenance serial port to be connected to the P active pulse terminal pin or the Q reactive pulse terminal pin or other idle pulse terminal pins, so as to realize the serial port switching of the preset module inside the device.

[0014] The present invention adopts the above technical solution and has the following beneficial effects:

[0015] (1) The serial port multiplexing switching circuit disclosed in the present invention utilizes the pulse port of the power distribution terminal equipment to lead out the standardized debugging serial port of the power distribution terminal equipment, switches the standardized debugging serial ports in the power distribution terminal equipment to the same physical channel through the gate logic selection unit, and realizes reliable communication between the standardized debugging serial port of the power distribution terminal and the device CPU through a simple and low-cost hardware circuit without using additional physical channels and without changing the State Grid standardized interface definition specifications.

[0016] (2) The serial port multiplexing switching circuit disclosed in the present invention can realize automatic switching of the standardized maintenance serial port of the power distribution terminal equipment without relying on the software running status. When the device is operating abnormally, the switching function can still be realized, providing support for the debugging and maintenance of the device and obtaining the device printing information, and effectively expanding the device's external serial port resources.

[0017] (3) The serial port multiplexing switching circuit disclosed in the present invention is simple to implement. The external serial port switching auxiliary module can realize the function through passive components. The circuit is reliable. The switching of the standardized maintenance serial port of the distribution terminal equipment is realized through a physical switch, and the cost is extremely low. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a module composition diagram of the serial port multiplexing switching circuit of the present invention.

[0019] Figure 2 This is an equivalent circuit diagram when the first logic selection unit in the serial port multiplexing switching circuit of the present invention is connected to the RS232 serial port receiving port and the active pulse port of the maintenance serial port.

[0020] Figure 3 This is an equivalent circuit diagram when the second logic selection unit in the serial port multiplexing switching circuit of the present invention is connected to the RS232 serial port receiving port and the reactive pulse port of the maintenance serial port.

[0021] Explanation of the numbers in the figure: RS232_TX is the RS232 serial port sending port, RS232_RX is the RS232 serial port receiving port, RS232_GND is the RS232 serial port ground port, SWITCH1 is the physical switch, SWITCH2 is the logical switch, P is the active pulse port, Q is the reactive pulse port, COM is the pulse common port, D1 is the first freewheeling diode, D2 is the second freewheeling diode, D3 is the first anti-reverse diode, D4 is the second anti-reverse diode, R1~R6 are the first to sixth resistors, C1 and C2 are the first and second capacitors, U1~U4 are the first to fourth optocouplers. DETAILED DESCRIPTION

[0022] The technical solution of the invention is described in detail below with reference to the accompanying drawings.

[0023] The serial port multiplexing switching circuit disclosed in the present invention is as follows Figure 1 As shown, it includes a serial port switching auxiliary module and a pre-set module inside the device. The serial port switching auxiliary module is located outside the State Grid standardized power distribution terminal and includes: standardized terminals connected to the power distribution terminal device and the serial port debugging tool, internal wiring, and a physical switch SWITCH1. The standardized terminals are used to lead out the RS232 serial port transmitting port RS232_TX, RS232 serial port receiving port RS232_RX, and RS232 serial port ground port RS232_GND defined in the standard specification, thereby realizing the maintenance of the serial port RS232 serial port transmitting port RS232_TX, RS232 serial port receiving port RS232_RX, and RS232 serial port ground port RS232_GND. 232_RX, RS232 serial port ground port RS232_GND are electrically connected to the RS232 serial port transmitting port RS232_TX, RS232 serial port receiving port RS232_RX, and RS232 serial port ground port RS232_GND defined in the standardization specification of the power distribution terminal. The RS232 serial port receiving port of the maintenance serial port is connected to the active pulse terminal pin or the reactive pulse terminal pin or other pulse terminal pin through the physical switch SWITCH1 to realize the serial port switching of the preset module inside the device.

[0024] like Figure 1 As shown, the pre-installed modules within the device include: a first group of debug serial ports (serial port 0-TXD and serial port 0-RXD), a second group of debug serial ports (serial port 1-TXD and serial port 1-RXD), a third group of debug serial ports (serial port 2-TXD and serial port 2-RXD), a logic switch SWITCH2, a logic selection module, and the active pulse port P, reactive pulse port Q, and pulse common port COM defined in the standard specification. The physical switch SWITCH1 is used to connect the first or second logic selection unit in the logic selection module to the RS232 serial port receiving port RS232_RX of the maintenance serial port and the RS232 serial port ground port RS232_GND of the maintenance serial port, and the pulse common port COM is electrically connected to the RS232 serial port ground port RS232_GND of the maintenance serial port.

[0025] like Figure 1As shown, the logic selection module includes a first logic selection unit and a second logic selection unit. When the physical switch SWITCH1 controls the RS232 serial port receiving port of the maintenance serial port to be connected to the terminal pin of the active pulse port P, the first logic selection unit is connected to the RS232 serial port receiving port of the maintenance serial port through the terminal pin of the active pulse port, and the first logic selection unit is connected to the RS232 serial port ground port RS232_GND of the maintenance serial port through the pulse common port COM. After the serial port switching auxiliary module is connected to the PC with an external serial port debugging tool, the RS232 serial port of the maintenance serial port is connected to the RS232 serial port ground port RS232_GND of the maintenance serial port. A voltage characteristic greater than 3V is generated between the 2 serial port ground port RS232_GND and the RS232 serial port receiving port RS232_RX of the maintenance serial port, so that the first logic selection unit generates a 1-bit low level RS232_SEL1, and the second logic selection unit generates a 1-bit high level RS232_SEL2. RS232_SEL1 and RS232_SEL2 form the control signal RS232_SEL for selecting the first group of debug serial ports to access the RS232 physical channel; when the physical switch SWITCH1 controls the RS232 serial port of the maintenance serial port, the RS232_SEL1 is connected to the RS232 physical channel. When the receiving port RS232_RX is connected to the terminal pin of the reactive pulse port Q, the second logic selection unit module is connected to the RS232 serial port receiving port RS232_RX of the maintenance serial port through the terminal pin of the reactive pulse port Q, and the second logic selection unit is connected to the RS232 serial port ground port RS232_GND of the maintenance serial port through the pulse common port COM. The voltage characteristics of the serial port switching auxiliary module after the external serial port debugging tool is connected to the PC are used to make the second logic selection unit generate a low level RS232_SEL2 and the first logic selection unit generate a high level. RS232_SEL1 is level, RS232_SEL1 and RS232_SEL2 form the control signal RS232_SEL for selecting the second group of debugging serial ports to access the RS232 physical channel; when the physical switch SWITCH1 is suspended, RS232_SEL1 output by the first logic selection unit and RS232_SEL2 output by the second logic selection unit are both high, RS232_SEL1 and RS232_SEL2 form the control signal RS232_SEL for selecting the third group of debugging serial ports to access the RS232 physical channel.When the logic switch SWITCH2 receives the control signal RS232_SEL for the first group of debugging serial ports to access the RS232 physical channel, the first group of debugging serial ports is led out and connected to the RS232 serial port transmitting port RS232_TX and the RS232 serial port receiving port RS232_RX defined in the standardized specifications in the preset module inside the device; when the logic switch SWITCH2 receives the control signal RS232_SEL for the second group of debugging serial ports to access the RS232 physical channel, the second group of debugging serial ports is led out and connected to the RS232 serial port transmitting port RS232_TX and the RS232 serial port receiving port RS232_RX defined in the standardized specifications in the preset module inside the device; when the logic switch SWITCH2 receives the control signal RS232_SEL for the third group of debugging serial ports to access the RS232 physical channel, the third group of debugging serial ports is led out and connected to the RS232 serial port transmitting port RS232_TX and the RS232 serial port receiving port RS232_RX defined in the standardized specifications in the preset module inside the device.

[0026] When the physical switch SWITCH1 connects the first logic selection unit in the logic selection module between the RS232 serial port receiving port RS232_RX of the maintenance serial port and the RS232 serial port ground port RS232_GND of the maintenance serial port, the equivalent circuit after the first logic selection unit is connected to the RS232 serial port receiving port RS232_RX of the maintenance serial port and the RS232 serial port ground port RS232_GND of the maintenance serial port is as follows: Figure 2 As shown, the first logic selection unit includes a first optocoupler U1, a first anti-reverse diode D3, a first freewheeling diode D1, a first resistor R1, a second resistor R2, and a first capacitor C1, wherein the second resistor R2 is a pull-up resistor, the first capacitor C1 is a filter capacitor, the first resistor R1 is a current limiting resistor, the anode of the first freewheeling diode D1 is connected to the pulse common port COM, the cathode of the first freewheeling diode D1 is connected to one end of the first resistor R1, the other end of the first resistor R1 is connected to the anode of the first optocoupler U1, the cathode of the first optocoupler U1 is connected to the active pulse port P, the active pulse port P is connected to the anode of the first anti-reverse diode D3, the cathode of the first anti-reverse diode D3 is connected to the cathode of the first freewheeling diode D1, the emitter of the first optocoupler U1 is grounded, the collector of the first optocoupler U1 is connected to one end of the second resistor R2 and one electrode of the first capacitor C1 as the output end of RS232_SEL1, the other end of the second resistor R2 is connected to the 3.3V DC level, and the other electrode of the first capacitor C1 is grounded. There is a 3V voltage difference between the RS232 serial port RS232_GND of the maintenance serial port and the RS232 serial port receiving port RS232_RX of the maintenance serial port: V RS232_GND-RX ≥3V condition, we can know that there is a voltage difference between the anode and cathode of the first optocoupler U1. When RS232_SEL1 is low, the control signal RS232_SEL2 is high. The control signal RS232_SEL1 and RS232_SEL2 are combined to control the logic switch SWITCH2 to select the first debug serial port to be connected to the RS232 physical channel. D1 is the voltage drop of the first freewheeling diode D1, and CTR is the current transfer ratio of the first optocoupler. Figure 2 The second optocoupler U2 and the third resistor R3 shown belong to the active power pulse circuit. Since the present invention uses the active pulse port P as the receiving end of the RS232 serial port signal, the first anti-reverse diode D3 is connected to the collector of the second optocoupler U2 to prevent the active pulse from being transmitted to the first optocoupler U1.

[0027] When the physical switch SWITCH1 connects the second logic selection unit in the logic selection module between the RS232 serial port receiving port RS232_RX of the maintenance serial port and the RS232 serial port ground port RS232_GND of the maintenance serial port, the equivalent circuit after the second logic selection unit is connected to the RS232 serial port receiving port RS232_RX of the maintenance serial port and the RS232 serial port ground port RS232_GND of the maintenance serial port is as follows: Figure 3 As shown. The second logic selection unit includes a third optocoupler U3, a second anti-reverse diode D4, a second freewheeling diode D2, a fourth resistor R4, a fifth resistor R5, and a second capacitor C2. The fifth resistor R5 is a pull-up resistor, the second capacitor C2 is a filter capacitor, the fourth resistor R4 is a current-limiting resistor, the anode of the second freewheeling diode D2 is connected to the pulse common port COM, the cathode of the second freewheeling diode D2 is connected to one end of the fourth resistor R4, the other end of the fourth resistor R4 is connected to the anode of the third optocoupler U3, the cathode of the third optocoupler U3 is connected to the reactive pulse port Q, the reactive pulse port Q is connected to the anode of the second anti-reverse diode D4, the cathode of the second anti-reverse diode D4 is connected to the cathode of the second freewheeling diode D2, the emitter of the third optocoupler U3 is grounded, the collector of the third optocoupler U3 is connected to one end of the fifth resistor R5 and one electrode of the second capacitor C2 as the output end of RS232_SEL2, the other end of the fifth resistor R5 is connected to a 3.3V DC level, and the other electrode of the second capacitor C2 is grounded. There is a 3V voltage difference between the RS232 serial port RS232_GND of the maintenance serial port and the RS232 serial port receiving port RS232_RX of the maintenance serial port: V RS232_GND-RX ≥3V condition, we can know that there is a voltage difference between the anode and cathode of the third optocoupler U3. When the control signal RS232_SEL2 output by the second logic selection unit is set to low, the control signal RS232_SEL1 output by the first logic selection unit is set to high, and the control signal composed of RS232_SEL1 and RS232_SEL2 controls the logic switch SWITCH2 to select the second group of debug serial ports to be connected to the RS232 physical channel, V D2 is the voltage drop of the second freewheeling diode D2, and CTR is the current transfer ratio of the third optocoupler. Figure 2 The fourth optocoupler U4 and the sixth resistor R6 shown belong to a reactive power pulse circuit. Since the present invention uses the reactive pulse port Q as the receiving end of the RS232 serial port signal, a second anti-reverse diode D4 is connected to the collector of the fourth optocoupler U2 to prevent the reactive pulse from being transmitted to the third optocoupler U3.

[0028] For filter capacitors C1 and C2, according to Calculate the appropriate capacitance value, where f is the serial communication baud rate and R is the resistance value of the pull-up resistor R2 or R5.

[0029] The above implementation modes are merely exemplary descriptions of the present invention and do not limit its scope of protection. Those skilled in the art may also make partial changes thereto. For example, a single-pole multi-throw physical switch may be selected and combined with an idle pulse port of the power distribution terminal equipment to achieve the purpose of the invention of bringing out more internal debugging serial ports of the power distribution terminal and communicating with the RS232 serial port of the maintenance serial port. Any form of equivalent replacement that conforms to the purpose of the invention falls within the scope of protection of the present invention.

Claims

1. A serial port multiplexing switching circuit, characterized in that: include: A serial port switching auxiliary module includes terminals for leading out the RS232 serial port transmitting port, the RS232 serial port receiving port, and the RS232 serial port ground port of the maintenance serial port as defined in the standard specification, and a single-pole N-throw physical switch, one end of the single-pole N-throw physical switch being connected to the RS232 serial port receiving port of the maintenance serial port, and the other end of the single-pole N-throw physical switch being connected to a terminal pin of one of the N pulse ports of the power distribution terminal, where N is an integer greater than or equal to 2; A preset module within the device includes a logic selection module and a logic switch. The logic selection module is connected to the RS232 serial port receiving port and the RS232 serial port ground port of the maintenance serial port when the single-pole N-throw physical switch is connected to one pulse port of the power distribution terminal. When a test voltage is generated between the RS232 serial port ground port and the RS232 serial port receiving port of the maintenance serial port, the logic selection module outputs a control signal for selecting one group of debugging serial ports in the N+1 groups of debugging serial ports of the power distribution terminal to the control end of the logic switch; and, A logic switch, one end of which is connected to the RS232 serial port transmitting port and the RS232 serial port receiving port defined in the standard specifications of the distribution terminal, and the other end of which is connected to the corresponding debugging serial port when the control end receives a control signal for selecting one of the N+1 groups of debugging serial ports of the distribution terminal. The RS232 serial port transmitting port and the RS232 serial port receiving port defined in the standard specifications of the distribution terminal are electrically connected to the terminals of the RS232 serial port transmitting port and the RS232 serial port receiving port of the maintenance serial port in the serial port switching auxiliary module, and the pulse common port defined in the standard specifications of the distribution terminal is electrically connected to the terminal of the RS232 serial port ground port of the maintenance serial port in the serial port switching auxiliary module.

2. A serial port multiplexing switching circuit according to claim 1, characterized in that: When N=2, the logic selection module includes a first logic selection unit and a second logic selection unit, the first logic selection unit is connected between the active pulse port and the pulse common port of the power distribution terminal, and the second logic selection unit is connected between the reactive pulse port and the pulse common port of the power distribution terminal; When the other end of the single-pole N-throw physical switch is connected to the terminal pin of the active pulse port of the power distribution terminal, the active pulse port receives the test signal from the RS232 serial port receiving port of the maintenance serial port; When the other end of the single-pole N-throw physical switch is connected to the terminal pin of the reactive pulse port of the power distribution terminal, the reactive pulse port receives the test signal from the RS232 serial port receiving port of the maintenance serial port.

3. The serial port multiplexing switching circuit according to claim 2, characterized in that: The first logic selection unit includes: a first optocoupler, a first anti-reverse diode, a first freewheeling diode, a first resistor, a second resistor, and a first capacitor, wherein the anode of the first freewheeling diode is connected to the pulse common port, the cathode of the first freewheeling diode is connected to one end of the first resistor, the other end of the first resistor is connected to the anode of the first optocoupler, the cathode of the first optocoupler is connected to the active pulse port, the active pulse port is connected to the anode of the first anti-reverse diode, the cathode of the first anti-reverse diode is connected to the cathode of the first freewheeling diode, the emitter of the first optocoupler is grounded, the collector of the first optocoupler is connected to one end of the second resistor and one pole of the first capacitor as the output end of the first logic selection unit, the other end of the second resistor is connected to the DC level, and the other pole of the first capacitor is grounded.

4. The serial port multiplexing switching circuit according to claim 2, characterized in that: The second logic selection unit includes: a third optocoupler, a second anti-reverse diode, a second freewheeling diode, a fourth resistor, a fifth resistor, and a second capacitor, wherein the anode of the second freewheeling diode is connected to the pulse common port, the cathode of the second freewheeling diode is connected to one end of the fourth resistor, the other end of the fourth resistor is connected to the anode of the third optocoupler, the cathode of the third optocoupler is connected to the reactive pulse port, the reactive pulse port is connected to the anode of the second anti-reverse diode, the cathode of the second anti-reverse diode is connected to the cathode of the second freewheeling diode, the emitter of the third optocoupler is grounded, the collector of the third optocoupler is connected to one end of the fifth resistor and one pole of the second capacitor as the output end of the second logic selection unit, the other end of the fifth resistor is connected to the DC level, and the other pole of the second capacitor is grounded.

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