Digital input module

Abstract

In this digital input module, contact detection is performed on the basis of a dark current that flows through external wiring when a contact detection voltage is applied to an external input terminal to which an external resistor is connected in parallel with an external device. A first power supply unit for applying the contact detection voltage is configured such that the contact detection voltage is switched on the basis of a setting signal.

Description

Digital Input Module

[0001] This disclosure relates to a digital input module. This application claims priority to Japanese Patent Application No. 2024-049993, filed with the Japan Patent Office on March 26, 2024, the contents of which are incorporated herein by reference.

[0002] In a plant facility such as a power plant, various components such as devices and instruments are operated under the control and management of a control device. The control device for controlling such a plant facility includes a digital input (DI) module for receiving digital input data from each component to be controlled.

[0003] Some digital input modules of this type have a function for detecting the contacts of external input terminals to which external devices can be connected. Contact detection is performed by measuring the dark current when a contact detection voltage is applied to the external input terminal using power supplied from a power source. This contact detection voltage can be applied by a power source built into the digital input module, but in the past, the voltage value and polarity of the contact detection voltage were fixed.

[0004] On the other hand, depending on the specifications of the plant equipment, contact detection voltages with different voltage values ​​and polarities may be required. To meet such demands, for example, Patent Document 1 discloses a digital input module that provides a plurality of power supply circuits capable of applying different contact detection voltages, and that can switch the contact detection voltage by selecting one of the power supply circuits as needed.

[0005] Japanese Utility Model Application Publication No. 55-103734

[0006] In the above-mentioned Patent Document 1, multiple power supply circuits must be provided within the module in order to switch the contact detection voltage, which tends to make the circuit configuration complex, increasing the circuit scale within the digital input module and increasing costs.

[0007] At least one embodiment of the present disclosure has been made in consideration of the above circumstances, and has an object to provide a digital input module capable of switching contact detection voltages with a simple configuration.

[0008] In order to solve the above problem, a digital input module according to at least one embodiment of the present disclosure comprises: an external input terminal to which an external device is connected; an external resistor connected in parallel with the external device to the external input terminal; a first power supply unit for applying a contact detection voltage to the external input terminal; a contact detection unit for detecting a contact of the external device based on a dark current flowing in an external wiring to which the contact detection voltage is applied when the contact detection voltage is applied to the external input terminal; and a setting signal acquisition unit for acquiring a setting signal for setting the contact detection voltage, wherein the first power supply unit is configured to switch the contact detection voltage based on the setting signal.

[0009] According to at least one embodiment of the present disclosure, it is possible to provide a digital input module capable of switching contact detection voltages with a simple configuration.

[0010] 1 is a configuration diagram of a digital input module according to one embodiment; FIG. 2 is a block diagram showing the internal configuration of a module control unit of FIG. 1; FIG. 3 is a circuit diagram showing an example of the configuration of a maximum current limiting unit of FIG. 1; FIG. 4 is a diagram showing an example of a reference value to be compared with a determination voltage in the contact detection unit of FIG. 1; and FIG. 5 is a schematic diagram showing the internal circuit of the digital input module of FIG. 1 that is multiplexed and connected to an external device.

[0011] Hereinafter, several embodiments of the present invention will be described with reference to the accompanying drawings. However, the configurations described as the embodiments or shown in the drawings are merely illustrative examples and are not intended to limit the scope of the present invention.

[0012] 1 is a configuration diagram of a digital input module 1 according to one embodiment. The digital input module 1 is a digital input (DI) module used in a control device for controlling plant equipment such as a power plant, for example, to receive digital input data Din from external devices 2, such as various devices and instruments, that the plant equipment is equipped with. The digital input module 1 includes an external input terminal CHI to which the external device 2, which is the source of the digital input data Din, is connected.

[0013] The digital input module 1 has a primary function of receiving digital input data Din input from an external device 2, but also has a contact detection function for the external input terminal CHI. The contact detection function is a function for determining the contact state of the external input terminal CHI, which may be an open contact state (open state) in which the external input terminal CHI is not connected to the external device 2, a closed contact state (open state) in which the external device 2 is normally connected to the external input terminal CHI, a disconnected state (open state) in which the external device 2 is connected to the external input terminal CHI but an abnormality such as a disconnection has occurred in the external wiring (for example, the wiring between the external input terminal CHI and the external device 2), or a transition state which is an intermediate state between these states. The contact detection result by the contact detection function can be output to another external device from an output terminal (not shown) as appropriate.

[0014] 1 mainly shows components related to the contact detection function as the internal configuration of the digital input module 1. The digital input module 1 includes a first power supply unit 4, a module control unit 10, a maximum current limiting unit 20, and a leakage current regulating unit 30.

[0015] The first power supply unit 4 is a built-in power supply provided in the digital input module 1, and is configured to apply a contact detection voltage Vd to the external input terminal CHI. The contact detection voltage Vd is a voltage applied to detect a contact at the external input terminal CHI, and as will be described later, is switchable between a plurality of pre-prepared voltage values ​​based on a control signal Sc from the module control unit 10.

[0016] The first power supply unit 4 is a transformer power supply that can switch the contact detection voltage Vd by switching at least one of the transformation ratio and polarity. In this embodiment, two types of transformation ratio and polarity of the transformer power supply can be selected, and the contact detection voltage Vd can be switched between four different voltage values ​​corresponding to each combination. For example, if 12 V and 24 V can be switched by changing the transformation ratio of the transformer power supply and positive and negative can be switched by changing the polarity of the transformer power supply, the contact detection voltage Vd can be switched between four voltage values: +24 V, +48 V, −24 V, and −48 V.

[0017] When the first power supply unit 4 is configured as a transformer power supply, the switching of the contact detection voltage Vd by the first power supply unit 4 can be performed by switching a switching circuit formed of semiconductor elements such as FETs based on a control signal Sc generated by the module control unit 10 in response to a setting signal St input from outside the module control unit 10. By using a switching circuit formed of semiconductor elements in this way, it is possible to switch the transformation ratio and polarity without providing a center tap in the transformer power supply. As a result, the configuration of the transformer power supply that constitutes the first power supply unit 4 can be simplified, and a digital input module 1 with reduced circuit size and cost can be realized.

[0018] If the first power supply unit 4 is configured as a switching power supply, the contact detection voltage Vd may be switched by a combination of switching the value of the feedback resistor via a photocoupler and switching the polarity using a bridge FET. Such a configuration of the first power supply unit 4 is also simple, which can contribute to reducing the circuit size and cost of the digital input module 1.

[0019] The module control unit 10 is a control unit for controlling the digital input module 1, and is configured to include a processor such as a CPU (Central Processing Unit). These processors function as a control unit for controlling the operation of the digital input module 1 by executing programs stored in a storage device (not shown).

[0020] Fig. 2 is a block diagram showing the internal configuration of the module control unit 10 shown in Fig. 1. The module control unit 10 includes a setting signal acquisition unit 12, a first power supply control unit 14, a contact detection unit 16, and a second power supply unit 18.

[0021] The setting signal acquisition unit 12 is configured to acquire a setting signal St input from an external device. The setting signal St is input to set the switchable contact detection voltage Vd in the first power supply unit 4. The setting signal St is generated by another component of the plant equipment and acquired by the setting signal acquisition unit 12. The other component that generates the setting signal St is not limited, but may be, for example, a higher-level control device (not shown) that comprehensively controls the entire plant equipment including the digital input module 1. Generally, the contact detection voltage Vd is specified to have an appropriate voltage value depending on the specifications of the plant equipment. Therefore, the higher-level control device generates a setting signal St corresponding to an appropriate voltage value, thereby allowing the other components of the plant equipment to recognize the appropriate voltage value. By acquiring such a setting signal St using the setting signal acquisition unit 12, the module control unit 10 can recognize the contact detection voltage Vd appropriate for the plant equipment.

[0022] The first power supply control unit 14 is configured to control the first power supply unit 4. In the present embodiment, the first power supply control unit 14 generates a control signal Sc for controlling the first power supply unit 4 so that the contact detection voltage Vd output from the first power supply unit 4 has a voltage value corresponding to the setting signal St, based on the setting signal St acquired by the setting signal acquisition unit 12. This control signal Sc is sent to the first power supply unit 4, thereby performing switching control so that the contact detection voltage Vd output from the first power supply unit 4 has a voltage value corresponding to the setting signal St.

[0023] The contact detection unit 16 is configured to detect the contact of the external input terminal CHI. As shown in FIG. 1 , an external resistor R0 is connected to the external input terminal CHI in parallel with the external device 2. The contact detection unit 16 detects the contact based on a dark current Ia that flows through the external wiring when a contact detection voltage Vd is applied to the external input terminal CHI. The contact detection unit 16 actually detects the dark current Ia by converting the dark current Ia into a voltage using a shunt resistor R1 that is connected in series with the external wiring, and obtaining a determination voltage Vj that is obtained as the potential difference across the shunt resistor R1.

[0024] The contact detection unit 16 also has a second power supply unit 18 used to measure the determination voltage Vj. The second power supply unit 18 is a single power supply capable of outputting a constant voltage value with a fixed voltage value and polarity. The contact detection unit 16 receives the determination voltage Vj measured in this manner and compares it with a preset reference value, thereby performing contact detection.

[0025] The maximum current limiting unit 20 is configured to limit the maximum value of the dark current Ia that flows through the internal circuitry of the digital input module 1 when the contact detection voltage Vd is applied. As described above, the contact detection voltage Vd applied by the first power supply unit 4 can be switched based on the setting signal St. Therefore, the magnitude or direction of the dark current Ia that flows may change depending on the contact detection voltage Vd, but by providing the maximum current limiting unit 20 in series with the circuit through which the dark current Ia flows, the maximum value of the dark current Ia is limited to a constant value regardless of the contact detection voltage Vd.

[0026] Fig. 3 is a circuit diagram showing an example of the configuration of the maximum current limiting unit 20 in Fig. 1. In this example, the maximum current limiting unit 20 is configured as a bridge diode circuit including a constant current diode (CRD).

[0027] In this way, the maximum value of the dark current Ia that flows when the contact detection voltage Vd is applied is limited by the maximum current limiting unit 20. This reduces the effects on the contact detection voltage Vd generated by the first power supply unit 4 of fluctuations in the input voltage input to the external input terminal CHI and the load current of the external device 2 connected to the external input terminal CHI. As a result, the contact detection voltage Vd can be accurately applied to the external input terminal CHI, and the contact of the external device 2 connected to the external input terminal CHI can be accurately detected.

[0028] Furthermore, as described above, when the first power supply unit 4 is configured as a transformer power supply, if the first power supply unit 4 frequently selects a specific voltage value as the contact detection voltage Vd, the magnetic balance of the core constituting the transformer power supply becomes uneven, which may result in a larger required core size and increased iron loss heat generation in the core. In contrast, in this embodiment, the maximum current limiter 20 limits the maximum value of the dark current Ia, thereby limiting the load current on the secondary side of the transformer power supply and making it uniform, thereby stabilizing the magnetic balance of the core. As a result, the core size of the transformer power supply constituting the first power supply unit 4 can be reduced, and iron loss heat generation in the core can be effectively reduced.

[0029] As described above, the polarity of the contact detection voltage Vd applied by the first power supply unit 4 is variable based on the setting signal St. Therefore, the sign of the judging voltage Vj input to the contact detection unit 16 changes depending on the polarity of the contact detection voltage Vd (for example, if the contact detection voltage Vd is positive, the judging voltage Vj will also be positive, and if the contact detection voltage Vd is negative, the judging voltage Vj will also be negative). The judging voltage Vj is measured by applying a single voltage from the second power supply unit 18 to the shunt resistor R1, as described above. If the sign of the judging voltage Vj to be measured changes in this way, it may be necessary to provide a second power supply unit 18 with reversible polarity or two second power supplies 18 with opposite polarities in order to reverse the polarity of the single voltage from the second power supply unit 18, which may result in a complex configuration and be disadvantageous in terms of cost.

[0030] Therefore, as shown in FIG. 1 , the contact detection unit 16 includes a bias voltage application unit 22 for applying a bias voltage Vb to the determination voltage Vj. The bias voltage Vb is set so that the determination voltage Vj input to the contact detection unit 16 is always a positive voltage. In this embodiment, the bias voltage application unit 22 is configured as a voltage divider circuit including two resistors R2 and R3, and the bias voltage Vb can be adjusted by the resistance ratio between the two. Preferably, the bias voltage Vb is set to half the maximum value of the determination voltage Vj input to the contact detection unit 16.

[0031] In this way, by adding the bias voltage Vb to the determination voltage Vj by the bias voltage application unit 22, the determination voltage Vj input to the contact detection unit 16 is always a positive voltage, regardless of the polarity of the contact detection voltage Vd. As a result, a single power supply is sufficient for the second power supply unit 18 for measuring the determination voltage Vj, which simplifies the configuration and reduces costs.

[0032] 4 is a diagram showing examples of several reference values ​​that are compared with the determination voltage Vj in the contact detection unit 16 of FIG. 1. The determination voltage Vj input to the contact detection unit 16 (the determination voltage Vj is always a positive voltage due to the addition of the bias voltage Vb) is compared with these reference values ​​and used for contact detection. The reference values ​​that are compared with the determination voltage Vj are set in advance to correspond to each state of the contact.

[0033] In this embodiment, the contact states that are determined include a contact close region R1 indicating that the contact is in a closed state, a contact open region R3 indicating that the contact is in an open state, and a disconnected region R5 indicating that the external wiring connected to the external input terminal CHI is in a disconnected state, as well as a transition region R2 indicating an intermediate state between the contact close region R1 and the contact open region R3, and a transition region R4 indicating an intermediate state between the contact open region R3 and the disconnected region R5.

[0034] A reference value corresponding to each state of the contact is prepared for each polarity of the contact detection voltage Vd applied by the first power supply unit 4. Specifically, as reference values ​​corresponding to the case where the polarity of the contact detection voltage Vd is positive, a reference value Vref1 defining the boundary between the contact close region R1 and the transition region R2, a reference value Vref2 defining the boundary between the transition region R2 and the contact open region R3, a reference value Vref3 defining the boundary between the contact open region R3 and the transition region R4, and a reference value Vref4 defining the boundary between the transition region R4 and the open region R5 are prepared. Furthermore, as reference values ​​corresponding to the case where the polarity of the contact detection voltage Vd is negative, a reference value Vref5 defining the boundary between the contact close region R1 and the transition region R2, a reference value Vref6 defining the boundary between the transition region R2 and the contact open region R3, a reference value Vref7 defining the boundary between the contact open region R3 and the transition region R4, and a reference value Vref8 defining the boundary between the transition region R4 and the open region R5 are prepared.

[0035] A reference value Vref0 is provided as a reference value that defines the boundary between the disconnection region R5 when the polarity of the contact detection voltage Vd is positive and the disconnection region R5 when the polarity of the contact detection voltage Vd is negative. The reference values ​​Vref1 to Vref4 and the reference values ​​Vref5 to Vref8 are symmetrical to each other with respect to the reference value Vref0.

[0036] The digital input module 1 having the above configuration can be multiplexed and connected to an external device 2. Among the multiplexed digital input modules 1, leakage current Iw may occur between other digital input modules 1. If the contact detection voltage Vd is a fixed voltage, this leakage current Iw can be prevented by inserting a one-way diode. In this embodiment, since the polarity of the contact detection voltage Vd can be changed as described above, such a simple configuration cannot prevent the leakage current Iw, whose direction changes depending on the polarity of the contact detection voltage Vd. Therefore, the digital input module 1 of this embodiment includes a leakage current regulator 30, which can effectively solve the above problem, as described below.

[0037] 5 is a schematic diagram showing the internal circuitry of the digital input module 1 of FIG. 1 that is multiplexed and connected to an external device 2. In each of the multiplexed digital input modules 1, the contact detection voltage Vd is set to a common value.

[0038] In a digital input module 1 that is multiplexed and connected to an external device 2, there is a risk that a leakage current Iw may flow in or out from the external input terminal CHI to other digital input modules 1. In order to regulate such leakage current Iw, the digital input module 1 has a leakage current regulation unit 30.

[0039] The leakage current regulation unit 30 is configured to regulate the inflow and outflow of the leakage current Iw, and includes a first rectifier circuit 32, a second rectifier circuit 34, and a switching circuit 36. The first rectifier circuit 32 is configured as a circuit for regulating the leakage current Iw flowing in a first direction D1 (the direction flowing into the digital input module 1), and the second rectifier circuit 34 is configured as a circuit for regulating the leakage current Iw flowing in a second direction D2 (the direction flowing from the digital input module 1) opposite to the first direction D1. The first rectifier circuit 32 and the second rectifier circuit 34 are each configured as a bridge circuit including a plurality of diodes together with a constant current diode (CRD).

[0040] The switching circuit 36 ​​is configured to enable one of the first rectifier circuit 32 and the second rectifier circuit 34 and cut off the other in response to the contact detection voltage Vd applied by the first power supply unit 4. In this embodiment, the switching circuit 36 ​​is configured using an NPN transistor and a PNP transistor, which have opposite characteristics, and by individually cutting off the first rectifier circuit 32 and the second rectifier circuit 34, it is possible to preferably prevent the leakage current Iw even when the polarity of the contact detection voltage Vd changes.

[0041] The on / off switching control of each transistor constituting the switching circuit 36 ​​can be performed by the module control unit 10, but in order to simplify the structure and achieve it at low cost, the contact detection voltage Vd from the first power supply unit 4 is configured to be applied to the base terminal of each transistor, so that the switching circuit 36 ​​automatically switches depending on the polarity of the contact detection voltage Vd.

[0042] In this case, the base current of each transistor constituting the switching circuit 36 ​​flows into the shunt resistor R1 and thus flows through the external device 2. This base current may be added as an error to the digital signal input from the external device 2. Therefore, in this embodiment, a constant voltage source 40, such as a voltage clamp Zener diode, is provided to apply a constant voltage (e.g., 5.1 V) to the base terminal of the transistor. This makes the current flowing into the shunt resistor R1 constant, and the contact detection unit 16 may correct each reference value compared with the determination voltage Vj by subtracting this current value in advance in a calculation. This makes it possible to effectively suppress errors in the digital signal input from the external device 2.

[0043] The leakage current control unit 30 for controlling the leakage current Iw flowing between the multiplexed modules in this manner can effectively control the leakage current Iw between the multiplexed digital input modules 1 by switching between the first rectifier circuit 32 and the second rectifier circuit 34, which can control the leakage current Iw in the first direction D1 and the second direction D2, by the switching circuit 36 ​​in response to the contact detection voltage Vd applied by the first power supply unit 4.

[0044] As described above, according to the above embodiment, the first power supply unit 4 applies the contact detection voltage Vd to the external input terminal CHI, which is connected in parallel with the external device 2 and the external resistor R0, thereby performing contact detection based on the dark current Ia flowing through the external wiring. The contact detection voltage Vd can be switched by controlling the first power supply unit 4 based on the setting signal St obtained as a control signal from, for example, a higher-level control device. This makes it possible to realize a digital input module 1 that can switch the contact detection voltage Vd as needed, with a compact configuration that suppresses circuit size and costs.

[0045] In addition, within the scope of the present disclosure, the components in the above-described embodiments may be replaced with well-known components as appropriate, and the above-described embodiments may be combined as appropriate.

[0046] The contents described in each of the above embodiments can be understood, for example, as follows.

[0047] (1) A digital input module according to one embodiment includes: an external input terminal to which an external device is connected; an external resistor connected in parallel with the external device to the external input terminal; a first power supply unit for applying a contact detection voltage to the external input terminal; a contact detection unit for detecting a contact of the external device based on a dark current flowing in an external wiring to which the contact detection voltage is applied when the contact detection voltage is applied to the external input terminal; and a setting signal acquisition unit for acquiring a setting signal for setting the contact detection voltage, wherein the first power supply unit is configured to switch the contact detection voltage based on the setting signal.

[0048] According to the aspect (1) above, a contact detection voltage is applied from the first power supply unit to the external input terminal connected in parallel with an external device and an external resistor, thereby performing contact detection based on a dark current flowing through the external wiring. The contact detection voltage can be switched by controlling the first power supply unit based on a setting signal obtained as a control signal from, for example, a higher-level control device. This makes it possible to realize a digital input module that can switch the contact detection voltage as needed, with a compact configuration that suppresses circuit size and costs.

[0049] (2) In another aspect, in the aspect (1), the first power supply unit is a transformer power supply that can switch the contact detection voltage by switching at least one of a transformation ratio and a polarity.

[0050] According to the above aspect (2), the contact detection voltage can be suitably switched by switching at least one of the transformation ratio and polarity of the transformer power supply.

[0051] (3) In another aspect, the configuration of (2) above further comprises a maximum current limiting unit for limiting the maximum value of the dark current flowing through the external wiring.

[0052] According to the above aspect (3), the digital input module includes a maximum current limiting unit, which limits the maximum load current in the external wiring through which dark current flows, even when the contact detection voltage applied to the external input terminal is switched. This reduces the influence of fluctuations in the input voltage input to the external input terminal and the load current from an external device connected to the external input terminal on the contact detection voltage generated by the first power supply. As a result, the contact detection voltage can be accurately applied to the external input terminal, allowing for accurate detection of the contact of the external input terminal.

[0053] Furthermore, as described above, when the first power supply unit is configured as a transformer power supply, if the frequency of selecting a specific contact detection voltage in the first power supply unit increases, the magnetic balance of the core constituting the transformer power supply becomes uneven, which may result in a larger required core size or increased iron loss heat generation in the core. In contrast, in this aspect, the maximum load current is limited by the maximum current limiting unit, thereby limiting the load current on the secondary side of the transformer power supply and making it uniform, thereby stabilizing the magnetic balance of the core. As a result, the core size of the transformer power supply constituting the first power supply unit can be reduced, effectively reducing iron loss heat generation in the core.

[0054] (4) In another aspect, in any one of the aspects (1) to (3) above, the device further comprises: a shunt resistor through which the dark current flows; and a second power supply unit for applying a single voltage for measuring the dark current flowing through the shunt resistor, wherein the contact detection unit detects the contact by comparing a determination voltage obtained by converting the dark current into a voltage by the shunt resistor with a preset reference value.

[0055] According to the above aspect (4), the dark current flowing through the external wiring is converted into a voltage by the shunt resistor, and the voltage can be suitably measured as a determination voltage by applying a single voltage from the second power supply unit. The measured determination voltage can be compared with a preset reference value to suitably detect the contact of the external input terminal.

[0056] (5) In another aspect, in the aspect (4), a bias voltage application unit is provided for applying a bias voltage to the determination voltage, and the bias voltage is set so that the determination voltage is a positive voltage regardless of the contact detection voltage.

[0057] According to the above aspect (5), by applying a bias voltage to the determination voltage, the determination voltage can be always a positive voltage regardless of the voltage value or polarity of the contact detection voltage, which allows the second power supply unit for measuring the determination voltage to be a single power supply, thereby simplifying the configuration.

[0058] (6) In another aspect, in the aspect (5), the bias voltage is half the maximum value of the determination voltage.

[0059] According to the above aspect (6), by setting the bias voltage to 1 / 2 of the maximum value of the judgment voltage, the judgment voltage can always be a positive voltage regardless of the voltage value or polarity of the contact detection voltage.

[0060] (7) In another aspect, in any one of the aspects (1) to (6) above, a leakage current control unit is provided for controlling leakage current flowing into or out of the external input terminal between the digital input module and other digital input modules when the digital input module is multiplexed and connected to the external device.

[0061] According to the above aspect (7), the digital input module is provided with a leakage current control unit, so that when the digital input module is multiplexed with other digital input modules and connected to an external device, leakage current can be suitably prevented from being input or output between the digital input module and the other digital input modules.

[0062] (8) In another aspect, in the aspect (7) above, the leakage current control unit includes: a first rectifier circuit for controlling the leakage current flowing in a first direction; a second rectifier circuit for controlling the leakage current flowing in a second direction opposite to the first direction; and a switching circuit configured to enable one of the first rectifier circuit and the second rectifier circuit and cut off the other in accordance with the contact detection voltage.

[0063] According to the above aspect (8), the leakage current control unit for controlling the leakage current flowing between multiplexed modules can be preferably realized by a configuration in which a first rectifier circuit and a second rectifier circuit capable of controlling the leakage current in a first direction and a second direction are switched between each other by a switching circuit in accordance with the contact detection voltage applied from the first power supply unit.

[0064] REFERENCE SIGNS LIST 1 Digital input module 2 External device 4 First power supply unit 10 Module control unit 12 Setting signal acquisition unit 14 First power supply control unit 16 Contact detection unit 18 Second power supply unit 20 Maximum current limiter 30 Leakage current regulation unit 32 First rectifier circuit 34 Second rectifier circuit 36 ​​Switching circuit 40 Constant voltage source

Claims

1. A digital input module comprising: an external input terminal to which an external device is connected; an external resistor connected in parallel with the external device to the external input terminal; a first power supply unit for applying a contact detection voltage to the external input terminal; a contact detection unit for detecting the contact of the external device based on a dark current flowing in an external wiring to which the contact detection voltage is applied when the contact detection voltage is applied to the external input terminal; and a setting signal acquisition unit for acquiring a setting signal for setting the contact detection voltage, wherein the first power supply unit is configured to switch the contact detection voltage based on the setting signal.

2. The digital input module according to claim 1, wherein the first power supply unit is a transformer power supply that can switch the contact detection voltage by switching at least one of the transformation ratio and polarity.

3. The digital input module according to claim 1 or 2, further comprising a maximum current limiting section for limiting the maximum value of the dark current flowing through the external wiring.

4. A digital input module as claimed in claim 1 or 2, comprising: a shunt resistor through which the dark current flows; and a second power supply unit for applying a single voltage for measuring the dark current flowing through the shunt resistor, wherein the contact detection unit detects the contact by comparing a determination voltage obtained by converting the dark current into a voltage by the shunt resistor with a preset reference value.

5. The digital input module according to claim 4, further comprising a bias voltage application unit for applying a bias voltage to the determination voltage, wherein the bias voltage is set so that the determination voltage becomes a positive voltage regardless of the contact detection voltage.

6. The digital input module according to claim 5, wherein the bias voltage is half the maximum value of the determination voltage.

7. A digital input module as claimed in claim 1 or 2, comprising a leakage current control section for controlling leakage current flowing into or out of the external input terminal between the digital input module and other digital input modules when multiplexed and connected to the external device.

8. The digital input module of claim 7, wherein the leakage current regulation unit includes: a first rectifier circuit for regulating the leakage current flowing in a first direction; a second rectifier circuit for regulating the leakage current flowing in a second direction opposite to the first direction; and a switching circuit configured to enable one of the first rectifier circuit and the second rectifier circuit and cut off the other in accordance with the contact detection voltage.

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