JP2006323661A - Two-wire type filed equipment and abnormality notification control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To secure an operation power source even when superimposing a transmission signal on a burn-out signal. <P>SOLUTION: In this two-wire type field equipment 10, when limiting a DC current value of a transmission line 40 to an abnormality notification current value Iw (a first current value), and outputting the transmission signal by being superimposed on DC current in a period wherein the burn-out signal is outputted, a transmission control part 17 controls the DC current to an abnormality notification transmission current value (a second current value) Iwsend larger than the abnormality notification current value Iw, wherein an amplitude lower end value Iblow of the transmission signal becomes a minimum operation current value Imin or above necessary for the two-wire type field equipment. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a field device communication technique, and more particularly to a communication technique for notifying an abnormality from a field device of a two-wire transmission system to a host device.

  In process facilities such as plants, any process variables such as flow rate, temperature, pressure, and valve opening are measured by field devices such as transmitters, and the measured values are notified to a host device such as a controller via a communication network. It has become. As a data transmission method used in such a field device, there is a two-wire transmission method in which a measured value of a process variable is converted into an analog direct current in a range of 4 to 20 mA and output to a two-wire transmission line. In a two-wire field device using such a two-wire transmission method, a direct current in the transmission line may be used as an operating power source.

In this type of two-wire transmission system, as an abnormality notification method for notifying a host device of an abnormality detected by a two-wire field device, a direct current consisting of an abnormality notification current value of 4 mA or less is used as a burnout signal. There is a method of notifying the occurrence of an abnormality by outputting from a formula field device. Therefore, in such a case, the operating power supply of the 2-wire field device is reduced from 4 mA.
Conventionally, in a two-wire field device using such an abnormality notification method, when outputting a burnout signal indicating a predetermined abnormality notification current value, by increasing the voltage value of the power supply voltage obtained from the direct current, Techniques have been proposed for securing an operating power supply for a two-wire field device with an abnormality notification current value (see, for example, Patent Document 1).

On the other hand, in the 2-wire transmission method, a communicator or the like in which various data are connected to the 2-wire transmission line from the 2-wire field device by superimposing a transmission signal on the direct current output from the 2-wire field device. In some cases, a digital transmission method for transmitting to a communication terminal is also used (see, for example, Non-Patent Document 1).
In this type of digital transmission system, desired transmission data is frequency modulated (FSK: frequency shift keying) with a two-wire field device, and the obtained transmission signal is superimposed on a direct current in a current mode and transmitted to a communication terminal. From the communication terminal, it is superimposed on the direct current in the voltage mode and transmitted to the two-wire field device.

JP-A-6-60287 http://www.hartcomm.org/technical/doclist.html, HART Communication Foundation

  However, in such a conventional technique, when the digital transmission method is used in the two-wire field device, there is a possibility that the transmission signal is superimposed and output on the burnout signal that notifies the host device of the abnormality. In this case, the transmission signal from the higher-level device or communication terminal to the 2-wire field device is superimposed on the burnout signal in the voltage mode, so this is not a problem, but the transmission signal from the 2-wire field device to the communication terminal is Since it is superimposed on the burnout signal in the current mode, the current that can be used as the operating power source of the 2-wire field device further decreases according to the amplitude of the transmission signal, and the operation of the 2-wire field device becomes insufficient due to the shortage of the power supply current. There was a problem that became unstable.

  As shown in FIG. 4, in the two-wire field device, during normal operation, a direct current 51 is output to the controller with a current value Iout in a range of 4 to 20 mA corresponding to the measured value. When digital transmission is performed, the transmission signal 52 that has been frequency-modulated is superimposed on the direct current 51 and output as shown at times T11 and T12. On the other hand, when an abnormality is detected at time T13, the two-wire field device reduces the current value Iout of the direct current 51 to a predetermined abnormality notification current value Iw and outputs it. When digital transmission is performed in this state, the transmission signal 52 is superimposed on the direct current 51 and output as shown at time T14.

Here, since the transmission signal 52 has a prescribed amplitude, when the DC current Iout is output from the two-wire field device at the abnormality notification current value Iw, the amplitude lower limit value Iblow of the transmission signal 52 is There is a possibility that a phenomenon 53 of lower than the minimum current value necessary for stable operation of the two-wire field device may occur. In such a case, the power source current is insufficient and the operation of the two-wire field device is not effective. It becomes stable. In particular, when the digital transmission method is used, the circuit scale of the two-wire field device also increases, so it is difficult to reduce the current consumption, and the possibility of the phenomenon 53 occurring increases.
The present invention is intended to solve such a problem, and an object of the present invention is to provide a two-wire field device and an abnormality notification control method capable of securing an operating power supply even when a transmission signal is superimposed on a burnout signal. Yes.

  In order to achieve such an object, a two-wire field device according to the present invention is connected to a host device and a communication terminal via a transmission line, and generates an operating power supply for the device from a direct current flowing in the transmission line. And controlling the direct current to a current value corresponding to the desired measurement value to notify the host device of the measurement value and superimposing the transmission signal generated by modulating the desired data on the direct current A two-wire field device including a transmission control unit that notifies the host device or communication terminal of the data by outputting the first current value below a current range used for notification of a measurement value by the transmission control unit When the abnormality is notified to the host device by limiting the direct current to the first apparatus and the transmission signal is superimposed on the direct current and output during the period in which the direct current is limited to the first current value, the first Amplitude lower value of larger and transmit signal from the flow is obtained by so controlling the DC current to the second current value as a minimum operating current value or more necessary for the two-wire field device.

  At this time, a current value obtained by adding 1/2 of the amplitude value of the transmission signal to the minimum operating current value may be used as the second current value.

  Also, the abnormality notification control method according to the present invention is connected to a host device and a communication terminal via a transmission line, and generates a power supply unit that generates an operating power supply of the device from a direct current flowing through the transmission line. The current value corresponding to the measured value is controlled to notify the host device of the measured value, and the transmission signal generated by modulating the desired data is superimposed on the direct current to output the data. Alternatively, an abnormality notification control method used in a two-wire field device including a transmission control unit that notifies a communication terminal, wherein the transmission control unit causes a direct current to be applied to a first current value that is lower than a current range used for measurement value notification. The step of notifying the host device of the occurrence of an abnormality by limiting the current and the transmission control unit superimposing the transmission signal on the DC current during the period in which the DC current is limited to the first current value. And outputting the direct current to a second current value larger than the first current and having a lower amplitude value of the transmission signal equal to or greater than the minimum operating current value required for the two-wire field device; It has.

  At this time, a current value obtained by adding 1/2 of the amplitude value of the transmission signal to the minimum operating current value may be used as the second current value.

  According to the present invention, when a transmission signal is superimposed on a direct current and output in a period in which a direct current value is limited to the first current value and a burnout signal is output in a two-wire field device, transmission is performed. Since the control unit controls the direct current to a second current value that is larger than the first current value and whose amplitude lower limit value of the transmission signal is equal to or greater than the minimum operating current value required for the two-wire field device, Even when a transmission signal is output at the time of occurrence notification, the current value at the lower end of the amplitude of the transmission signal does not fall below the minimum operating current value required for the 2-wire field device, and stable operation of the 2-wire field device can be realized. .

Next, embodiments of the present invention will be described with reference to the drawings.
[2-wire field device]
First, a two-wire field device according to the first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the configuration of the two-wire field device according to the first embodiment of the present invention.
The two-wire field device 10 is used as a transmitter in a process facility such as a plant, and is an apparatus that measures an arbitrary process variable such as a flow rate, temperature, pressure, and valve opening.

  The two-wire field device 10 is a communication device controlled by a computer as a whole, and is connected to a host device 20 such as a controller via a transmission path 40 including two transmission lines, and is transmitted from the host device 20. It operates with an operating power source obtained from a DC current supplied via the path 40, and controls the DC current of the transmission path 40 to a current value corresponding to a measured value of an arbitrary process variable, thereby the measured value is transferred to the host device 20. To notify. Similarly, the two-wire field device 10 is connected to a communication terminal 30 such as a communicator via the transmission line 40, and uses a transmission signal generated by modulating arbitrary data such as setting data as a direct current in the transmission line 40. The data is notified to the communication terminal 30 by being superimposed on the data.

  In the present embodiment, by limiting the direct current to the first current value lower than the current range used for the notification of the measurement value, the abnormality detected by the two-wire field device 10 is notified to the host device 20 and the direct current is detected. When a transmission signal is superimposed on a direct current and output during a period in which the current is limited to the first current value, the amplitude lower limit value Iblow of the transmission signal is larger than the first current and the 2-wire field device The direct current is controlled to a second current value that is equal to or greater than the required minimum operating current value.

Next, the configuration of the two-wire field device 10 according to the first embodiment of the present invention will be described in detail with reference to FIG.
The two-wire field device 10 includes a two-wire transmission interface unit (hereinafter referred to as a two-wire I / F unit) 11, an analog signal processing unit 12, a digital signal processing unit 13, a power supply unit 14, a measurement unit 15, A storage unit 16 and a transmission control unit 17 are provided.

  The two-wire transmission I / F unit 11 is connected in series with the power supply unit 14 to a loop composed of two transmission lines of the transmission line 40, and performs data communication based on the two-wire transmission method and the digital transmission method. DC circuit (loop current) flowing in the transmission line 40 based on the analog output signal from the analog signal processing unit 12 based on the two-wire transmission system to a predetermined current value within a range of 4 to 20 mA. The control signal (constant current source) and the transmission signal generated by the digital signal processing unit 13 based on the digital transmission method are superimposed on the direct current of the transmission line 40 and output, and from the host device 20 or the communication terminal 30 It has a function of receiving a transmission signal output by being superimposed on a direct current of the transmission line 40 and outputting it to the digital signal processing unit 13.

The analog signal processing unit 12 includes a circuit unit that performs analog signal processing including D / A conversion. The analog signal processing unit 12 converts the measurement value (digital value) output from the transmission control unit 17 into the measurement value by performing D / A conversion. This is a circuit unit that generates a corresponding analog output signal and outputs it to the two-wire transmission I / F unit 11.
The digital signal processing unit 13 includes a circuit unit that performs digital signal processing including modulation / demodulation, and generates a transmission signal by frequency-modulating data (digital value) output from the transmission control unit 17 to generate a two-wire type. A function to be output to the transmission I / F unit 11 and a signal transmitted from the host device 20 or the communication terminal 30 by demodulating a signal from the host device 20 or the communication terminal 30 output from the two-wire transmission I / F unit 11. And a function of generating data including various commands and outputting the data to the transmission control unit 17.

The power supply unit 14 is connected in series with the two-wire transmission I / F unit 11 to the loop composed of the two transmission lines of the transmission line 40, and from the direct current supplied from the host device 20 and flowing through the transmission line 40. It has a function of generating operation power for the device and supplying it to each unit.
The measurement unit 15 includes an interface circuit that controls an arbitrary sensor, and has a function of acquiring a measurement value of a desired process variable measured by the sensor and outputting the measurement value to the transmission control unit 17.
The storage unit 16 includes a storage device such as a memory, and has a function of storing various types of information such as measurement values, setting data, and programs used for processing in the transmission control unit 17. These pieces of information are read from the host device 20, the communication terminal 30, or a recording medium via an input / output interface such as the two-wire transmission I / F unit 11, or acquired from a sensor via the measuring unit 15 and stored in the storage unit. 16 is stored.

The transmission control unit 17 includes a microprocessor such as a CPU and its peripheral circuits, and reads and executes a program from a storage device such as a memory included in the peripheral circuit, thereby causing the hardware and the program to cooperate with each other. This is a functional unit that implements data communication based on the two-wire transmission method and the digital transmission method in the two-wire field device 10.
The transmission control unit 17 outputs a measurement value output from the measurement unit 15 to the analog signal processing unit 12 to notify the higher-level device 20 of the measurement value, and a storage device such as a memory included in the peripheral circuit. A function of notifying the communication terminal 30 of the predetermined data such as setting data read from the digital signal processing unit 13 and outputting the data to the digital signal processing unit 13, and processing the data from the communication terminal 30 received from the digital signal processing unit 13 or A function of storing in the storage unit 16.

  Further, the transmission control unit 17 restricts the DC current value Iout to an abnormal notification current value (first current value) Iw that is lower than the current range of 4 to 20 mA used for measurement value notification (burnout signal). A function of notifying the host device 20 of the occurrence of an abnormality detected by the two-wire field device 10 and a case where a transmission signal is superimposed on a direct current and output during a period in which the direct current value Iout is limited to the abnormal notification current value Iw Is a direct current to the abnormality notification transmission current value (second current value) Iwsend that is larger than the abnormality notification current value Iw and the amplitude lower limit value Iblow of the transmission signal is equal to or greater than the minimum operating current value Imin required for the two-wire field device. It has a function of controlling the current value Iout.

[Operation of 2-wire field device]
Next, the operation of the two-wire field device according to one embodiment of the present invention will be described with reference to FIGS. FIG. 2 is a flowchart showing abnormality notification processing of the two-wire field device according to the embodiment of the present invention. FIG. 3 is a signal waveform diagram showing an abnormality notification operation of the two-wire field device according to the embodiment of the present invention.

  The transmission control unit 17 of the two-wire field device 10 sequentially outputs the measurement values (digital values) detected by the measurement unit 15 to the analog signal processing unit 12 during normal operation where no abnormality has occurred. The analog signal processing unit 12 performs D / A conversion on the measurement value, generates an analog output signal corresponding to the measurement value, and outputs the analog output signal to the two-wire transmission I / F unit 11. The two-wire transmission I / F unit 11 controls the DC current value Iout flowing through the transmission line 40 based on the analog output signal from the analog signal processing unit 12 based on the two-wire transmission method. As a result, a direct current 51 having a direct current value Iout corresponding to the measured value is output from the two-wire field device 10 to the transmission line 40.

  The host device 20 has a DC power supply (24VDC) and a load resistor R connected in series to a loop composed of two transmission lines of the transmission line 40, and has a two-wire transmission I / F unit 11 side. The DC current value Iout of the transmission line 40 is detected based on the function of supplying a direct current corresponding to the constant current source realized by the two-wire transmission I / F unit 11 and the voltage generated at both ends of the load resistor R. A function, a function of acquiring a measured value notified from the two-wire field device 10 based on the DC current value Iout, and a two-wire when the detected DC current value Iout is less than or equal to a predetermined abnormality notification current reference value It has a function of detecting that an abnormality has occurred in the expression field device 10.

  Accordingly, the direct current output from the two-wire field device 10 to the transmission line 40 changes its direct current value Iout according to the measured value. This DC current value Iout is detected as a voltage across the load resistance R of the host device 20, and is received by the host device 20 as a measured value from the two-wire field device 10 based on the DC current value Iout obtained from the voltage across the terminal. .

  Further, when the transmission control unit 17 of the two-wire field device 10 transmits data to the host device 20 or the communication terminal 30 in response to a request command from the host device 20 or the communication terminal 30 or spontaneously, The data is output to the digital signal processing unit 13. The digital signal processing unit 13 generates a transmission signal by frequency-modulating the data (digital value) output from the transmission control unit 17 based on the digital transmission method, and outputs the transmission signal to the two-wire transmission I / F unit 11. . The two-wire transmission I / F unit 11 superimposes the transmission signal generated by the digital signal processing unit 13 on the direct current of the transmission path 40 and outputs the superimposed signal.

The communication terminal 30 is connected to the transmission line 40, receives a transmission signal superimposed on a direct current flowing through the transmission line 40, and demodulates the received transmission signal based on a digital transmission method to restore original data. A function of acquiring, a function of generating a transmission signal by modulating the frequency of arbitrary data based on a digital transmission method, and a function of outputting the generated transmission signal by superimposing it on a direct current flowing through the transmission line 40 Yes.
Therefore, the transmission signal superimposed on the direct current flowing through the transmission line 40 from the two-wire field device 10 is received and decoded by the communication terminal 30 and received by the communication terminal 30 as data from the two-wire field device 10. .

  In this way, during normal operation in which no abnormality has occurred, as shown in the period before time T1 in FIG. 3, the measured value indicating the process variable obtained by the two-wire field device 10 is The host device 20 is constantly notified using the DC current value Iout. If necessary, the transmission signal is superimposed on the direct current of the transmission line 40, so that the desired data is notified from the two-wire field device 10 to the communication terminal 30.

When the transmission control unit 17 of the two-wire field device 10 detects an abnormality determined from the measurement value detected by the measurement unit 15 or an abnormality in the two-wire field device 10 at time T1, the abnormality occurs. Is notified to the higher-level device 20, the abnormality notification process shown in FIG. 2 is started.
First, instead of the measurement value output to the analog signal processing unit 12, the transmission control unit 17 corresponds to the abnormality notification current value (first current value) Iw that is lower than the current range of 4 to 20 mA used for notification of the measurement value. The data to be output is output to the analog signal processing unit 12 (step 100). As a result, a DC current (burnout signal) of the abnormality notification current value Iw is output from the two-wire transmission I / F unit 11 to the transmission line 40, and the host device 20 detects the abnormality notification current value Iw. To recognize that an abnormality has occurred in the two-wire field device 10.

Thereafter, the transmission control unit 17 determines whether or not to start transmission of a transmission signal used for digital transmission with the communication terminal 30 (step 101), and when it is not necessary to transmit the transmission signal (step 101: NO), detection is performed. It is determined whether or not the abnormality is completed (step 105). If the abnormality has not ended (step 105: NO), the process returns to step 101 to continue the abnormality notification state.
On the other hand, when the detected abnormality ends in step 105 (step 105: YES), the transmission control unit 17 sequentially outputs the measurement value (digital value) detected by the measurement unit 15 to the analog signal processing unit 12. (Step 106), a series of abnormality notification processing ends.

Further, when a transmission signal used for digital transmission with the communication terminal 30 is transmitted in a state where the abnormality notification current value Iw is being output, as shown at time T2 in FIG. It is determined that transmission of a transmission signal used for digital transmission is started (step 101: YES).
In this case, the transmission control unit 17 first replaces the abnormality notification current value Iw with a minimum operating current value Imin that is larger than the abnormality notification current value Iw and the transmission signal amplitude lower limit value Iblow is necessary for the two-wire field device. Data corresponding to the abnormality notification transmission current value (second current value) Iwsend as described above is output to the analog signal processing unit 12 (step 102), and data to be transmitted to the communication terminal 30 is output to the digital signal processing unit 13. (Step 103).

As a result, a DC current having an abnormality notification transmission current value Iwsend is output from the two-wire transmission I / F unit 11 to the transmission path 40, and a transmission signal is superimposed on the DC current and output.
Therefore, as shown in FIG. 3, during the period from time T2 to time T3 when the transmission signal 52 is output, the DC current value of the transmission line 40 is controlled to the abnormality notification transmission current value Iwsend that is larger than the abnormality notification current value Iw. Therefore, the lower-end amplitude value Iblow of the transmission signal 52 does not fall below the minimum operating current value Imin necessary for the two-wire field device.

  Thereafter, the transmission control unit 17 returns to step 100 after the transmission of the transmission signal is completed (step 104: YES), and outputs data corresponding to the abnormality notification current value Iw to the analog signal processing unit 12. As a result, a DC current having an abnormality notification current value Iw is output from the two-wire transmission I / F unit 11 to the transmission line 40.

  As described above, in the present embodiment, when an abnormality is detected in the two-wire field device 10, the direct current value is added to the abnormality notification current value (first current value) Iw that is lower than the current range used for the measurement value notification. By restricting lout, the detected abnormality occurrence is notified to the host device 20, and the transmission signal is superimposed on the direct current during the period when the direct current value is restricted to the abnormal notification current value Iw. The direct current is controlled to an abnormality notification transmission current value (second current value) Iwsend that is larger than the notification current value Iw and the amplitude lower limit value Iblow of the transmission signal is equal to or greater than the minimum operating current value necessary for the two-wire field device. .

  Therefore, even when a transmission signal is output at the time of notification of occurrence of abnormality, the lower limit amplitude value Iblow of the transmission signal does not fall below the minimum operating current value Imin required for the two-wire field device 10. Stable operation can be realized.

  In FIG. 3, the minimum operating current value Imin, the abnormality notification current value Iw, and the transmission signal amplitude Ibpp are actually fixed values that are determined in advance when the two-wire field device 10 is used. Therefore, in order to control the amplitude lower limit value Iblow of the transmission signal 52 to be equal to or higher than the minimum operating current value Imin, the abnormality notification transmission current value Iwsend may be set to at least Imin + Ibpp / 2 (Iwsend ≧ Imin + Ibpp / 2). Even when a transmission signal is output at the time of occurrence of an abnormality notification, it is necessary to continue to notify the host device 20 of the occurrence of the abnormality. Therefore, the abnormality notification transmission current value Iwsend needs to be equal to or lower than the abnormality notification upper limit current value Iw0. is there. Also, the actual transmission signal amplitude output from the two-wire transmission I / F unit 11 due to a decrease in the direct current flowing in the transmission line 40 and the condition of the stabilizing capacitor provided in the power supply unit 11 is normal. The value may be smaller than the value Ibpp. In such a case, the abnormality notification transmission current value Iwsend is also smaller than the normal value Imin + Ibpp / 2. Therefore, the actual abnormality notification transmission current value Iwsend is not a fixed value and varies according to various conditions.

Since the general host device 20 is provided with the abnormality notification upper limit current value Iw0 as a reference for detecting the burnout signal from the two-wire field device 10, the abnormality notification upper limit current value Iw0 is used as the abnormality notification current value Iw. Is used, even when a transmission signal is output at the time of notification of occurrence of abnormality, there is little possibility that the lower-end amplitude value Iblow of the transmission signal is lower than the minimum operating current value Imin required for the two-wire field device 10.
However, the abnormality notification upper limit current value Iw0 varies depending on the installation environment such as the length of the host device 20 and the transmission path 40. Therefore, when designing the two-wire field device 10 that can be connected to various host devices, the abnormality notification current value Iw of the two-wire field device 10 is set low so that many host devices 20 can detect accurately. Must be set.

  As in the present embodiment, when the direct current value is limited to the abnormality notification current value Iw and the burnout signal is output and the transmission signal is superimposed on the direct current and output, the abnormality notification current value Iw By controlling the direct current to an abnormality notification transmission current value (second current value) Iwsend that is larger and the amplitude lower limit value Iblow of the transmission signal is equal to or greater than the minimum operating current value necessary for the two-wire field device, The notification current value Iw can be set to a current value lower than the abnormality notification upper limit current value Iw0, and even if there is a variation in the abnormality notification upper limit current value Iw0, this can be dealt with in many cases, and it is extremely versatile. Formula field devices can be realized.

  When the difference between the abnormality notification upper limit current value Iw0 and the minimum operating current value Imin is larger than the transmission signal amplitude Ibpp / 2, a slight margin Im is secured, and the abnormality notification transmission current value is set to Iwsend = Imin + Ibpp / 2 + Im. Well, stable operation can be realized more reliably.

1 is a block diagram showing a configuration of a two-wire field device according to an embodiment of the present invention. It is a flowchart which shows the abnormality notification process of the 2-wire type field apparatus concerning one embodiment of this invention. It is a signal waveform diagram which shows the abnormality notification operation | movement of the 2-wire type field apparatus concerning one embodiment of this invention. It is a signal waveform diagram which shows the abnormality notification operation | movement of the conventional 2-wire field apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Two-wire field apparatus, 11 ... Two-wire transmission I / F part, 12 ... Analog signal processing part, 13 ... Digital signal processing part, 14 ... Power supply part, 15 ... Measurement part, 16 ... Memory | storage part, 17 ... Transmission control unit, 20 ... host device, 30 ... communication terminal, 40 ... transmission line, 51 ... DC current, 52 ... transmission signal, Iout ... DC current value, Imin ... minimum operating current value, Iw ... abnormality notification current value (first) 1), Iwsend ... abnormal notification transmission current value (second current value), Iw0 ... abnormal notification upper limit current value, Ibpp ... transmission signal amplitude, Iblow ... amplitude lower end value.

Claims (4)

A power supply unit that is connected to a host device and a communication terminal via a transmission line and generates an operating power supply for the device from a direct current flowing in the transmission line, and controls the direct current to a current value corresponding to a desired measurement value In this way, the measured value is notified to the host device, and the transmission signal generated by modulating desired data is superimposed on the direct current to output the data to the host device or the communication terminal. A two-wire field device comprising a transmission control unit,
The transmission control unit
Notifying the host device of the occurrence of abnormality by limiting the direct current to a first current value that is lower than the current range used for notification of the measurement value,
When the transmission signal is superimposed on the direct current and output during the period in which the direct current is limited to the first current value, the lower end amplitude value of the transmission signal is larger than the first current The two-wire field device is characterized in that the direct current is controlled to a second current value that is equal to or higher than a minimum operating current value required for the two-wire field device. The two-wire field device according to claim 1,
The two-wire field device characterized in that the second current value is a current value obtained by adding 1/2 of the amplitude value of the transmission signal to the minimum operating current value. A power supply unit that is connected to a host device and a communication terminal via a transmission line and generates an operating power supply for the device from a direct current flowing in the transmission line, and controls the direct current to a current value corresponding to a desired measurement value In this way, the measured value is notified to the host device, and the transmission signal generated by modulating desired data is superimposed on the direct current to output the data to the host device or the communication terminal. An abnormality notification control method used in a two-wire field device including a transmission control unit,
Notifying the host device of the occurrence of abnormality by limiting the direct current to a first current value lower than a current range used for notification of the measurement value by the transmission control unit;
When the transmission control unit outputs the transmission signal superimposed on the DC current during a period in which the DC current is limited to the first current value, the transmission signal is larger than the first current and the transmission signal And a step of controlling the direct current to a second current value at which a lower end amplitude value of the current becomes equal to or greater than a minimum operating current value necessary for the two-wire field device. In the abnormality notification control method according to claim 3,
The abnormality notification control method, wherein the second current value is a current value obtained by adding 1/2 of the amplitude value of the transmission signal to the minimum operating current value.

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