Digital Input Circuit

Abstract

To suppress heat generation in a digital input circuit for taking in opening / closing information of a voltage-applied contact.SOLUTION: In a digital input circuit 20, a first resistor 24 is connected between a first external connection terminal 23P and a second external connection terminal 23N to which an external power source 11 and an external contact 12 are connected. A second resistor 25 is connected between the first external connection terminal 23P and a positive side input terminal 27P of an insulation element 26. A step-down power source circuit 28 includes an input terminal 29 for receiving voltage of the external power source 11 and an output terminal 30 for outputting step-down voltage obtained by stepping down voltage of the external power source 11. Current passing from the positive side input terminal 27P of the insulation element 26 through a negative side input terminal 27N is inputted to an output terminal 30 of the step-down power source circuit 28.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to digital input circuits. [Background technology]

[0002] A digital input circuit is a circuit for detecting the opening and closing of an external contact. For example, the digital input circuit obtains contact opening and closing information by detecting the current flowing through the external contact using an insulating element such as a photocoupler. The digital input circuit is also called a contact input circuit.

[0003] In digital input circuits, it is necessary to prevent contact failure caused by an oxide film that has formed on the contacts. To do this, the oxide film covering the contacts is removed by passing a current above a certain value when the contacts are turned on. Alternatively, a voltage above a certain value is applied between the contacts when the contacts are in the off state. The oxide film is destroyed by simply applying a high voltage for a short period of time.

[0004] However, in a steady state, there is no need to increase the current flowing through the contacts and insulating elements. Even in a steady state, if the same current and voltage as when removing the oxide film are applied to the digital input circuit, the device will generate heat. This can shorten the device's lifespan and place restrictions on the layout of the digital input circuit to dissipate the heat.

[0005] One example of a solution to the above problem is the technology described in Japanese Patent Laid-Open No. 11-234925 (Patent Document 1). Specifically, a digital input circuit according to one embodiment of this document comprises means for applying a low voltage to the contacts during a period in which contact information is normally acquired, and voltage switching means for intermittently applying a high voltage to the contacts for a short period of time to destroy the insulating film of the contacts. Another embodiment of the digital input circuit according to this document comprises means for applying a low voltage to the contacts during a period in which contact information is normally acquired, and high voltage application means for applying a high voltage to the contacts when the contacts change from off to on to destroy the insulating film of the contacts. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 11-234925 Summary of the Invention [Problem to be solved by the invention]

[0007] The external contacts to which the digital input circuit of the above-mentioned Japanese Patent Laid-Open No. 11-234925 (Patent Document 1) can be applied are so-called no-voltage contacts that are not provided with an external power source. In this case, the power supply voltage supplied to the external contacts is supplied from a power supply circuit such as a battery provided on the digital input circuit side.

[0008] On the other hand, the contacts of the electric power equipment installed in power plants and substations are so-called voltage contacts that are connected to an external power source. Moreover, the wiring distance from the external contacts to the digital input circuit is long, ranging from several hundred meters to several kilometers, so modifying the existing wiring to accommodate voltage-free contacts leads to increased costs. For this reason, it is difficult to apply the technology described in the above-mentioned Japanese Patent Laid-Open Publication No. 11-234925 (Patent Document 1) to the digital input circuits of electric power equipment.

[0009] The present disclosure has been made in consideration of the above-mentioned problems, and its purpose is to provide a digital input circuit for inputting opening and closing information of voltage-applied contacts provided in electric power equipment, etc., which is capable of suppressing heat generation even when a high power supply voltage is used to destroy the insulating coating of the contacts. [Means for solving the problem]

[0010] In one embodiment, the digital input circuit includes a first external connection terminal and a second external connection terminal, a first resistor, an isolation element, a monitoring circuit, a second resistor, and a step-down power supply circuit. The first external connection terminal and the second external connection terminal are connected to the positive and negative terminals of an external power supply, respectively. An external contact is connected between the positive terminal and the first external connection terminal or between the negative terminal and the second external connection terminal. The first resistor is connected between the first external connection terminal and the second external connection terminal. The isolation element has a positive input terminal and a negative input terminal and outputs a signal corresponding to a current flowing from the positive input terminal to the negative input terminal. The monitoring circuit monitors the open / closed state of the external contact based on the signal output from the isolation element. The second resistor is connected between the first external connection terminal and the positive input terminal of the isolation element. The step-down power supply circuit has an input terminal to which the voltage of an external power supply is input, and an output terminal to which a stepped-down voltage obtained by stepping down the voltage of the external power supply is output. A current passing from the positive input terminal to the negative input terminal of the isolation element is input to the output terminal of the step-down power supply circuit. [Effects of the Invention]

[0011] According to the above embodiment, by providing a first resistor, a second resistor, and a step-down power supply circuit, it is possible to provide a digital input circuit that can suppress heat generation even when a high power supply voltage is used to break down the insulating coating of the contacts. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a block diagram showing a configuration of a digital input circuit according to a first embodiment. [Figure 2] 2 is a diagram for explaining an example of the configuration of the step-down power supply circuit of FIG. 1; FIG. [Figure 3] 2 is a diagram for explaining the operation of the digital input circuit of FIG. 1. FIG. [Figure 4] FIG. 10 is a block diagram showing a configuration of a digital input circuit according to a second embodiment. [Figure 5] FIG. 11 is a block diagram showing a configuration of a digital input circuit according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Each embodiment will be described in detail below with reference to the drawings. The same or corresponding parts will be denoted by the same reference characters and description thereof will not be repeated.

[0014] Embodiment 1 [Digital input circuit configuration] Fig. 1 is a block diagram showing the configuration of a digital input circuit 20 according to the first embodiment. Referring to Fig. 1, the digital input circuit 20 includes external connection terminals 23 (23P, 23N), a high-voltage resistor 24, a current-limiting resistor 25, an insulating element 26, and a step-down power supply circuit 28. These components are provided in a DI (Digital Input) area 21 of the digital input circuit 20. The digital input circuit 20 further includes a monitoring circuit 35 provided in an LSI (Large-Scale Integration) area 22.

[0015] As shown in FIG. 1, an external power supply 11 provided in an external circuit 10 is connected between a positive external connection terminal 23P and a negative external connection terminal 23N. As shown in FIG. 1, an external contact 12 is connected between the positive terminal 11P of the external power supply 11 and the positive external connection terminal 23P. Unlike the case of FIG. 1, the external contact 12 may be connected between the negative terminal 11N of the external power supply 11 and the negative external connection terminal 23N. The negative terminal 11N of the external power supply 11 is connected to ground GND. In the present disclosure, the positive external connection terminal 23P is also referred to as a first external connection terminal, and the negative external connection terminal 23N is also referred to as a second external connection terminal.

[0016] High-voltage resistor 24 is connected between positive-side external connection terminal 23P and negative-side external connection terminal 23N. High-voltage resistor 24 has a relatively high resistance value to suppress the current flowing through external contact 12 when external contact 12 is on. In the present disclosure, high-voltage resistor 24 is also referred to as a first resistor.

[0017] The current-limiting resistor 25 is connected between the positive external connection terminal 23P and the positive input terminal 27P of the insulating element 26. As will be described later, the voltage applied to the current-limiting resistor 25 is relatively low. The resistance value of the current-limiting resistor 25 is determined so that the minimum necessary current flows through the external contact 12 at this low voltage. In the present disclosure, the current-limiting resistor 25 is also referred to as a second resistor.

[0018] The isolation element 26 transmits signals from the DI region 21 to the LSI region 22 while ensuring insulation between the DI region 21 and the LSI region 22. Examples of the isolation element 26 include, but are not limited to, a photocoupler or an isolator integrated circuit (IC). The isolation element 26 includes an input element connected between the positive input terminal 27P and the negative input terminal 27N, and an output element coupled to the input element by optical coupling, magnetic coupling, capacitive coupling, or the like. The output element outputs the signal transmitted from the input element to the monitoring circuit 35 via the output terminal 27OUT. In the case of a photocoupler, the input element is a light-emitting element such as an LED (light-emitting diode), and the output element is a light-receiving element such as a photodiode or a phototransistor.

[0019] The monitoring circuit 35 detects the open / closed state of the external contact 12 based on a signal output from the output terminal 27OUT of the insulating element 26. The monitoring circuit 35 may be configured, for example, based on a microcomputer including a CPU (Central Processing Unit) and memory, or may be configured by a dedicated circuit, or may be configured using an FPGA (Field Programmable Gate Array). The monitoring circuit 35 may also be configured by a combination of two or more of these.

[0020] The step-down power supply circuit 28 is connected between the positive-side external connection terminal 23P and the negative-side input terminal 27N of the isolation element 26. The step-down power supply circuit 28 generates a step-down voltage by stepping down the voltage of the external power supply 11 input to its input terminal 29 with respect to the potential of the ground GND, and outputs the generated step-down voltage from its output terminal 30. By making the magnitude of the step-down voltage as close as possible to the output voltage of the external power supply 11, it is possible to suppress power consumption in the current-limiting resistor 25. The output terminal 30 of the step-down power supply circuit 28 also functions as a current sink into which current flows. That is, the current that passes from the positive-side input terminal 27P to the negative-side input terminal 27N of the isolation element 26 is input to the output terminal 30 of the step-down power supply circuit 28.

[0021] FIG. 2 is a diagram illustrating an example of the configuration of the step-down power supply circuit 28 of FIG. 1. The step-down power supply circuit 28 of FIG. 1 can be configured using isolated DC power supplies 37, 42, 47, and 52 as shown in FIGS. 2(A) to 2(D). The input and output of the isolated DC power supplies may be either positive or negative voltages. The isolated DC power supplies 37, 42, 47, and 52 are configured using isolated DC / DC converters such as flyback or forward types, for example.

[0022] 2(A), an insulated DC power supply (positive input, positive output) 37 includes an input-side positive terminal 38, an input-side ground terminal (GND1) 39, an output-side positive terminal 40, and an output-side ground terminal (GND2) 41. As an example, a case will be described in which, in normal use, when a positive voltage of +110 V is input to the input-side positive terminal 38, a positive voltage of +5 V is output from the output-side positive terminal 40. In this case, the output-side positive terminal 40 has an output voltage difference of +5 V with respect to the output-side ground terminal (GND2) 41.

[0023] 2(A), the input side positive terminal 38 and the output side positive terminal 40 are connected by a wire, and the input side ground terminal (GND1) 39 is connected to the ground terminal GND of the external circuit 10 of FIG. 1. A positive voltage of 110 V is applied to the input side positive terminal 38 and the output side positive terminal 40. In this case, there is a voltage difference of +5 V between the output side positive terminal 40 and the output side ground terminal (GND2) 41, so when viewed from the ground terminal GND of the external circuit 10, the voltage of the output side ground terminal (GND2) 41 becomes +105 V.

[0024] Therefore, the isolated DC power supply 37 in Fig. 2(A) outputs a voltage obtained by stepping down the input voltage of +110V to +105V from the output-side ground terminal (GND2) 41, and since the output-side ground terminal (GND2) 41 is a current sink, it can be used as the step-down power supply circuit 28 in Fig. 1. In this case, a combined voltage of only 5V is applied to the current-limiting resistor 25 and the isolation element 26 in Fig. 1, so heat generation can be suppressed more than when 110V is applied.

[0025] 2(B), an insulated DC power supply (positive input, negative output) 42 includes an input-side positive terminal 43, an input-side ground terminal (GND1) 44, an output-side ground terminal (GND2) 45, and an output-side negative terminal 46. As an example, a case will be described in which, in normal use, when a positive voltage of +110 V is input to the input-side positive terminal 43, a negative voltage of −5 V is output from the output-side negative terminal 46. In this case, the output-side negative terminal 46 has an output voltage difference of −5 V with respect to the output-side ground terminal (GND2) 45.

[0026] 2(B), input side positive terminal 43 and output side ground terminal (GND2) 45 are connected by a wire, and input side ground terminal (GND1) 44 is connected to the ground terminal GND of external circuit 10 of FIG. 1. A positive voltage of 110 V is applied to input side positive terminal 43 and output side ground terminal (GND2) 45. In this case, there is a voltage difference of −5 V between output side negative terminal 46 and output side ground terminal (GND2) 45, so when viewed from the ground terminal GND of external circuit 10, the voltage of output side negative terminal 46 becomes +105 V.

[0027] Therefore, isolated DC power supply 42 in Fig. 2(B) outputs a voltage obtained by stepping down an input voltage of +110V to +105V from output-side negative terminal 46, and since output-side negative terminal 46 is a current sink, it can be used as step-down power supply circuit 28 in Fig. 1. In this case, a combined voltage of only 5V is applied to current-limiting resistor 25 and isolation element 26 in Fig. 1, so heat generation can be suppressed more than when 110V is applied.

[0028] 2(C), an insulated DC power supply (negative input, positive output) 47 includes an input ground terminal (GND1) 48, an input negative terminal 49, an output positive terminal 50, and an output ground terminal (GND2) 51. As an example, a case will be described in which, in normal use, when a negative voltage of −110 V is input to the input negative terminal 49, a positive voltage of +5 V is output from the output positive terminal 50. In this case, the output positive terminal 50 has an output voltage difference of +5 V with respect to the output ground terminal (GND2) 51.

[0029] 2(C), the input ground terminal (GND1) 48 and the output positive terminal 50 are connected by wiring. The input negative terminal 49 is connected to the ground terminal GND of the external circuit 10 in FIG. 1, and a positive voltage of +110 V is applied to the input ground terminal (GND1) 48. In this case, there is a voltage difference of +5 V between the output positive terminal 50 and the output ground terminal (GND2) 51, so when viewed from the ground terminal GND of the external circuit 10, the voltage of the output ground terminal (GND2) 51 is +105 V.

[0030] Therefore, isolated DC power supply 47 in Fig. 2(C) outputs a voltage obtained by stepping down an input voltage of +110V to +105V from output-side ground terminal (GND2) 51, and furthermore, because output-side ground terminal (GND2) 51 is a current sink, it can be used as step-down power supply circuit 28 in Fig. 1. In this case, a combined voltage of only 5V is applied to current-limiting resistor 25 and isolation element 26 in Fig. 1, so heat generation can be suppressed more than when 110V is applied.

[0031] 2(D), an insulated DC power supply (negative input, negative output) 52 includes an input ground terminal (GND1) 53, a negative input terminal 54, an output ground terminal (GND2) 55, and a negative output terminal 56. As an example, a case will be described in which, in normal use, when a negative voltage of −110 V is input to the negative input terminal 54, a negative voltage of −5 V is output from the negative output terminal 56. In this case, the negative output terminal 56 has an output voltage difference of −5 V with respect to the output ground terminal (GND2) 55.

[0032] 2(D), input ground terminal (GND1) 53 and output ground terminal (GND2) 55 are connected by wiring. Input negative terminal 54 is connected to ground terminal GND of external circuit 10 in FIG. 1, and a positive voltage of +110 V is applied to input ground terminal (GND1) 53. In this case, there is a voltage difference of -5 V between output negative terminal 56 and output ground terminal (GND2) 55, so when viewed from ground terminal GND of external circuit 10, the voltage of output negative terminal 56 becomes +105 V.

[0033] Therefore, isolated DC power supply 52 in Fig. 2(D) outputs a voltage obtained by stepping down an input voltage of +110V to +105V from output-side negative terminal 56, and since output-side negative terminal 56 is a current sink, it can be used as step-down power supply circuit 28 in Fig. 1. In this case, a combined voltage of only 5V is applied to current-limiting resistor 25 and isolation element 26 in Fig. 1, so heat generation can be suppressed more than when 110V is applied.

[0034] As described above, the step-down power supply circuit 28 of FIG. 1 can be configured using the isolated DC power supplies 37, 42, 47, and 52 as shown in FIGS. 2(A) to 2(D). Specifically, in the above example, the absolute value of the input voltage is 110 V, the absolute value of the output voltage is 5 V, and combinations of positive and negative input voltages and positive and negative output voltages (a total of four combinations) are shown. Needless to say, the specific configuration of the step-down power supply circuit 28 of FIG. 1 is not limited to the above example. In general, the step-down power supply circuit 28 of FIG. 1 can be configured by using a power conversion device in which the absolute value of the output voltage is smaller than the absolute value of the input voltage and in which the output terminal functions as a current sink.

[0035] [Digital input circuit operation] Fig. 3 is a diagram for explaining the operation of the digital input circuit 20 of Fig. 1. Hereinafter, the operation of the digital input circuit 20 will be described with reference to Fig. 3.

[0036] When the external contact 12 is ON, three current paths a, b, and c are generated in the digital input circuit 20, as shown in FIG. 3. First, current path a runs from the external power supply 11 through the external contact 12, the current-limiting resistor 25, and the input-side elements of the isolation element 26 (i.e., from the positive input terminal 27P to the negative input terminal 27N), before reaching the output terminal 30 of the step-down power supply circuit 28. By setting the step-down voltage generated at the output terminal 30 by the step-down power supply circuit 28 to a value slightly smaller than the output voltage of the external power supply 11, the voltage applied to the current-limiting resistor 25 can be reduced. Therefore, by setting the resistance value of the current-limiting resistor 25 to the minimum current value necessary to prevent poor contact at the external contact 12, power consumption in the current-limiting resistor 25 can be reduced.

[0037] Current path b is a path that runs from external power supply 11 through external contact 12 and high-voltage resistor 24 and returns to external power supply 11. Therefore, by setting the resistance value of high-voltage resistor 24 to a sufficiently high value, power consumption in high-voltage resistor 24 can be reduced.

[0038] Current path c is a path that runs from external power supply 11 through external contact 12 to input terminal 29 of step-down power supply circuit 28. As described above, power consumption can be reduced in the case of current path a, and therefore power consumption can be reduced by improving the power conversion efficiency of step-down power supply circuit 28.

[0039] Next, when the external contact 12 is OFF, no current flows through the above current paths a, b, and c, because they all pass through the external contact 12. Therefore, almost no heat is generated due to power consumption by the digital input circuit 20. Furthermore, both ends of the external contact 12 are connected to each other via the external power supply 11 and the high-voltage resistor 24. Therefore, a relatively high voltage from the external power supply 11 is applied to the external contact 12. As a result, the oxide film formed on the external contact 12 is destroyed.

[0040] [Effects of the First Embodiment] As described above, according to the digital input circuit 20 of the first embodiment, power consumption can be reduced when the external contact 12 is on, and current can be prevented from flowing when the external contact 12 is off. This makes it possible to reduce heat generation inside the digital input circuit 20.

[0041] Furthermore, when the external contact 12 is OFF, a relatively high voltage from the external power supply 11 is applied to the external contact 12, which destroys the oxide film on the external contact 12. When the external contact 12 is ON, a current sufficient to prevent poor contact can be passed through the external contact 12. Therefore, poor contact due to the oxide film on the external contact 12 can be prevented.

[0042] Furthermore, the digital input circuit 20 is compatible with voltage contacts provided in power equipment, etc. Therefore, there is no need to change the wiring of the external circuit 10 in order to obtain contact information using the digital input circuit 20.

[0043] Embodiment 2 Fig. 4 is a block diagram showing the configuration of a digital input circuit 20A according to embodiment 2. The digital input circuit 20A in Fig. 4 differs from the digital input circuit 20 in Fig. 1 in that it further includes a voltage detection circuit 31 and a semiconductor switch 32 for preventing chattering.

[0044] The semiconductor switch 32 is provided on the current path between the input terminal 29 of the step-down power supply circuit 28 and the positive-side external connection terminal 23P at a position that does not overlap with the current path from the positive-side external connection terminal 23P to the positive-side input terminal 27P of the step-down power supply circuit 28. The semiconductor switch 32 is formed, for example, by a FET (Field Effect Transistor).

[0045] The voltage detection circuit 31 detects the voltage at the positive-side external connection terminal 23P, and determines whether the voltage at the positive-side external connection terminal 23P has become constant at the voltage value of the external power supply 11 after the external contact 12 is turned on. When the voltage detection circuit 31 detects a constant external power supply voltage, that is, after confirming that no chattering is occurring, it turns on the semiconductor switch 32. This causes the voltage of the external power supply 11 to be supplied to the step-down power supply circuit 28, causing the step-down power supply circuit 28 to start operating.

[0046] Other configurations in FIG. 4 are similar to those in FIG. 1, so the same or corresponding parts are given the same reference numerals and description thereof will not be repeated.

[0047] As described above, according to the digital input circuit 20A of the second embodiment, by adding the voltage detection circuit 31 and the semiconductor switch 32 for preventing chattering, the digital input circuit 20A can be operated more stably.

[0048] Embodiment 3 FIG. 5 is a block diagram showing the configuration of a digital input circuit 20B according to a third embodiment. The digital input circuit 20B of FIG. 5 further includes a positive-side external connection terminal 23P2 separate from the positive-side external connection terminal 23P. The external circuit 10 is provided with a wiring that connects the positive terminal 11P of the external power supply 11 to the separate positive-side external connection terminal 23P2 without passing through the external contact 12. The input terminal 29 of the step-down power supply circuit 28 is connected to the separate positive-side external connection terminal 23P2 instead of the positive-side external connection terminal 23P. In the present disclosure, the separate positive-side external connection terminal 23P2 is also referred to as a third external connection terminal. Since the rest of FIG. 5 is the same as that of FIG. 1, the same or corresponding parts are designated by the same reference numerals and description thereof will not be repeated.

[0049] In the case of the digital input circuit 20 of the first embodiment shown in Fig. 1, the step-down power supply circuit 28 starts up after the external contact 12 is switched on. This causes a delay in the timing at which the monitoring circuit 35 determines that the external contact 12 has been switched on. As a result, problems may arise in the case of a digital input circuit mounted on a system that requires high-speed response.

[0050] According to the digital input circuit 20B of the third embodiment shown in FIG. 5, an additional wire is required to connect the positive terminal 11P of the external power supply 11 to the digital input circuit 20B without going through the external contact 12, but this has the advantage of enabling a high-speed response of the monitoring circuit 35.

[0051] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of this application is defined by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0052] 10 external circuit, 11 external power supply, 11P positive terminal, 11N negative terminal, 12 external contact, 20, 20A, 20B digital input circuit, 21 DI area, 22 LSI area, 23P positive external connection terminal (first external connection terminal), 23N negative external connection terminal (second external connection terminal), 23P2 another positive external connection terminal (third external connection terminal), 24 high voltage resistor (first resistor), 25 current limiting resistor (second resistor), 26 isolation element, 27N negative input terminal, 27OUT output terminal, 27P positive input terminal, 28 step-down power supply circuit, 29 input terminal, 30 output terminal, 31 voltage detection circuit, 32 semiconductor switch, 35 monitoring circuit, 37, 42, 47, 52 isolated DC power supply, 38, 43 Input side positive terminal, 39, 44, 48, 53 Input side ground terminal, 40, 50 Output side positive terminal, 41, 45, 51, 55 Output side ground terminal, 46, 56 Output side negative terminal, 49, 54 Input side negative terminal, GND Ground terminal for external circuit.

Claims

1. a first external connection terminal and a second external connection terminal connected to a positive terminal and a negative terminal of an external power supply, respectively, and an external contact connected between the positive terminal and the first external connection terminal or between the negative terminal and the second external connection terminal; and a first resistor connected between the first external connection terminal and the second external connection terminal; an isolation element having a positive input terminal and a negative input terminal, and outputting a signal corresponding to a current flowing from the positive input terminal to the negative input terminal; a monitoring circuit that monitors the open / closed state of the external contact based on the signal output from the insulating element; a second resistor connected between the first external connection terminal and the positive input terminal of the isolation element; a step-down power supply circuit having an input terminal to which the voltage of the external power supply is input and an output terminal to output a stepped-down voltage obtained by stepping down the voltage of the external power supply, wherein a current passing from the positive input terminal to the negative input terminal of the isolation element is input to the output terminal of the step-down power supply circuit.

2. the input terminal of the step-down power supply circuit is connected to the first external connection terminal; 2. The digital input circuit according to claim 1, wherein the voltage of the external power supply applied to the first external connection terminal when the external contact is on is input to the input terminal of the step-down power supply circuit.

3. a semiconductor switch connected between the input terminal of the step-down power supply circuit and the first external connection terminal; 3. The digital input circuit according to claim 2, further comprising: a voltage detection circuit that detects a voltage at the first external connection terminal and turns on the semiconductor switch after confirming that no chattering occurs when the external contact is on.

4. a third external connection terminal connected to the positive terminal of the external power supply; 2. The digital input circuit according to claim 1, wherein the input terminal of the step-down power supply circuit is connected to the third external connection terminal.

5. 5. The digital input circuit according to claim 1, wherein the step-down power supply circuit includes an isolated DC power supply.

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