JPH04284600A - Signal transmitter - Google Patents

Abstract

PURPOSE:To prevent a power supply voltage to be supplied to a transmitter from being decreased by potential drop caused by receiving resistance and to easily design an electronic circuit on the side of a receiver by obtaining a signal transmitted through a two-wire transmission line to the side of a reception counter by detecting a current value. CONSTITUTION:A transmitter 1 is operated by power supplied through a two- wire transmission line L, changes the value of a current flowing in the transmission line L and transmits a signal corresponding to a process amount, which is measured on the side of the transmitter 1, to the side of a reception counter 2. The reception counter 2 is composed of a power supply part Es to output a power supply voltage E between one terminal T1 of the two-wire transmission line L and a ground line, virtual grounding means 21 to virtually ground another terminal T2 of the transmission line L, and current value detecting means 22 to obtain the signal transmitted through the transmission line L to the side of the reception counter 2 by detecting the current value. Therefore, the power source Es is equivalently turned to a form directly connected to the both terminals of the two-wire transmission line L.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a signal transmission device used for transmitting process signals in the process industry.
More specifically, the present invention relates to a signal transmission device that supplies operating power to a transmitter through a two-wire transmission path, and transmits a signal from the transmitter as a current signal to a receiving instrument via the same two-wire transmission path.

0002

[Prior Art] In the process industry, the process amount (
A transmitter (e.g., pressure transmitter, electromagnetic flow meter, etc.) for detecting temperature, pressure, flow rate, etc.) is installed in the process, and the measured process quantity (measurement signal) is transmitted through the transmission line at a current of 4 to 20 mA. A unified signal is being transmitted to remote locations. FIG. 6 is a conceptual configuration diagram showing the relationship between a transmitter (transmitter) and a receiving instrument that receives measurement signals from the transmitter via a two-wire transmission line. Transmission line L formed by two-wire system
A transmitter 1 installed in the process, such as a pressure transmitter, an electromagnetic flowmeter, or a thermometer, is connected to one end of the transmitter. As this transmitter, one that is equipped with, for example, a microprocessor and has a communication function is generally used. The other end of the transmission line L is connected to a receiving instrument 2 that receives a signal sent from the transmitter 1 in the form of a current signal iL. This receiving instrument 2
Inside, there is a 24V constant voltage power supply Es for operating the transmitter 1 via the transmission line L, and a power supply inserted in series with the transmission line to extract the current signal iL transmitted via the transmission line as a voltage signal. A receiving resistor (voltage drop element) R, a transmitting driver DR that transmits a digital signal D1 to the transmitter 1, for example, to check its operating state or make adjustments, and a transmission line L. A signal separation circuit FL is provided which separates a transmission signal A transmitted as a current signal iL from a digital signal D1 carried on a transmission line L. Note that if there is no need to send a digital signal to the transmitter side, the transmission driver DR is not necessary.

0003

[Problems to be Solved by the Invention] In the conventional signal transmission device configured as described above, since a receiving resistor (voltage drop element) R is inserted and connected in series to the two-wire transmission line L, the transmitter 1 side Power supply voltage supplied to (voltage vL across transmitter 1)
will decrease accordingly. That is, the power supply voltage vL supplied to the oscillator 1 side becomes vL=Es-(iL×R), where Es is the output voltage of the constant voltage power supply. Note that the voltage drop in the transmission line is ignored. For this reason, on the transmitter 1 side, the transmission current iL is set to the maximum value, for example 2
It is necessary to design each electronic circuit so that normal operation is maintained even when 0 mA flows. Furthermore, when a signal is sent to the receiving instrument side and the transmitter side (when bidirectional communication is performed), a separation circuit is required to separate the signal D1 sent from the receiving instrument side and the signal sent from the transmitter side. The present invention has been made in view of these points, and its purpose is to
It is an object of the present invention to provide a signal transmission device that prevents the power supply voltage supplied to an oscillator 1 from decreasing due to a voltage drop due to a receiving resistor, and facilitates the design of an electronic circuit on the oscillator side. Another object of the present invention is to provide a signal transmission device that does not require a special signal separation circuit on the receiving instrument side even when performing bidirectional communication, and can simplify the configuration of the receiving instrument.

0004

[Means for Solving the Problems] The present invention achieves the above object in a signal transmission device in which a current value supplied via a two-wire transmission line is changed by a transmitter and transmitted to a receiving instrument side. The receiving instrument includes a power supply unit that outputs a supply voltage between one end of the two-wire transmission line and a ground line, and a virtual grounding means that virtually grounds the other end of the transmission line, and the receiving instrument receives data via the transmission line. This is a signal transmission device characterized in that a signal transmitted to a meter side is obtained by detecting a current value.

[0005]

[Operation] One end of the two-wire transmission line on the receiving instrument side is grounded via virtual grounding means, and the power supply is directly connected to both ends of the two-wire transmission line for equalization. Therefore, the power supply voltage received by the oscillator is not affected by the voltage drop that occurs in the receiving resistor, making it easier to design the electronic circuit.

[0006]

Embodiments Hereinafter, embodiments of the present invention will be explained in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention. This embodiment assumes an apparatus in which only a signal corresponding to a process amount is transmitted via a transmission path. In the figure, 1 is a transmitter installed on the process side, and 2 is a receiving instrument that receives signals transmitted from the transmitter 1 via a two-wire transmission line. The receiving instrument here generally refers to devices that receive transmission signals, and includes, for example, signal converters and signal conditioners that convert received signals into signals that can be handled by host computers and output them. shall be taken as a thing. The transmitter 1 is operated by power supplied via the two-wire transmission line L, changes the value of the current flowing through the transmission line, and sends a signal (corresponding to the process amount measured by the transmitter 1) to the receiving instrument 2 side transmits analog signals (including DC components). In the receiving instrument 2, Es is one end T1 of the two-wire transmission line L.
21 is a virtual grounding means for virtually grounding the other end T2 of the transmission line L, 22
is a current value detecting means configured to detect the current value of the signal transmitted to the receiving instrument 2 side via the transmission line L and obtain it.

FIG. 2 is a block diagram showing another embodiment of the present invention. In this embodiment, a function is provided to receive a signal corresponding to the process amount from the transmitter 1 side, and to send a digital signal from the receiving instrument 2 side to the transmitter 1 side, for example, for adjusting the transmitter. The structure is such that two-way communication can be performed between the receiver and the receiver. That is, in the receiving instrument 2, 23 is an addition means that adds the digital signal D1 sent to the transmitter 1 side and the voltage signal Eo that determines the output voltage value of the power supply section Es, and uses the added signal to calculate the output voltage from the power supply section Es. It controls the output voltage. Further, on the oscillator 1 side, a voltage change extraction means 11 is provided that extracts a voltage change in the power supply voltage vL supplied via the transmission line L and takes out a digital signal D1.

FIG. 3 is a diagram showing a specific example of the circuit shown in FIG. 2. In this embodiment, the power supply section Es is composed of transistors Q1 and Q2 whose collectors are connected to the power supply Vcc and ground, respectively, and whose emitters are connected to one terminal T1 of the transmission line L. Additionally, the adding means 23
An operational amplifier A1 is used as an input terminal, and a digital signal D1 is applied to one input terminal, and a voltage signal Eo is applied to the other input terminal.
It is configured to apply . The virtual grounding means 21 and the current value detection means 22 have one input end (+) grounded, the other input end (-) connected to the other terminal T2 of the two-wire transmission line L, and the input end (-) It consists of an operational amplifier A2 with a feedback resistor R connected between its output terminals, and a transmission signal A transmitted to the receiving instrument 2 side via a transmission line L is obtained from the output terminal of the operational amplifier A2. The virtual grounding means 21 and the current value detection means 22, which are composed of the operational amplifier A2, do not have parts such as capacitors that create a time constant in this circuit, so they can easily be configured as an IC (the electronic circuit can be integrated into an IC). (If the capacitor has a large capacity, the chip area must be increased.)

[0009] The operation of the apparatus configured as described above will be explained next. FIG. 4 is a waveform diagram of voltage or current signals at each portion of FIG. 3. The oscillator 1 generates a current i flowing through the transmission path L.
As shown in (a), L is varied, for example, in the range of 4 to 20 mA depending on the process amount (measurement signal). Although such a current iL flows through the transmission line L, the potential of the terminal T2 is
Regardless of the magnitude of this transmission current, it is maintained at zero potential by the operation of the virtual grounding means 21. On the other hand, a current of the same magnitude as iL flows through the feedback resistor R of the operational amplifier A2, which constitutes the virtual grounding means, so as to cancel the transmission current iL, and the output terminal of the amplifier A2 is as shown in (b). Then, a voltage signal vb corresponding to the change in the transmission current iL is generated. This voltage signal vb can be expressed by equation (1). vb=-iL×R
...(1) On the other hand, the power supply voltage (voltage at both ends) vL supplied to the transmitter 1 via the 2-wire transmission line L is the voltage here because one end T2 of the 2-wire transmission line L is virtually grounded. No drop occurs, and as shown in (c), the voltage E output from the power supply section Es in the receiving instrument 2
is equal to

When a digital signal D1 is sent from the receiving instrument 2 side to the transmitter 1 side, a voltage signal va whose amplitude changes in accordance with the digital signal D1 is sent to the input terminal of the amplifier A1 as shown in (d). Apply. The output voltage E of the variable power supply Es changes thereby and is sent to the oscillator 1 side. In the transmitter 1, the voltage change extraction means 11 is connected to the transmission line L.
A voltage change is extracted from the power supply voltage vL supplied through the circuit, and a digital signal D1 is obtained as shown in (c). The power supply voltage vL supplied to the transmitter 1 side via the two-wire transmission line L changes according to the digital signal D1, but the transmission current iL sent from the transmitter 1 side to the receiving instrument 2 side (transmission signal va (corresponding) has no effect, and therefore, on the receiving instrument 2 side, there is no need for a signal separation circuit for separating the digital signal D1 sent from the receiving instrument 2 side and the transmission signal va.

FIG. 5 is a block diagram showing one example of application of the device according to the present invention. In this application, one 2
A plurality of transmitters 1 are connected to a line transmission line L. The receiving instrument 2 causes the digital signal D1 to include an address signal, thereby specifying one of the plurality of transmitters, and
Request transmission of transmission signal A from the corresponding transmitter. The transmitter 1 side specified by the address signal sends the transmission signal A to the receiving instrument 2 side using the two-wire transmission line L. At this time, transmission signals are not sent to the two-wire transmission line L from undesignated transmitters. According to such an application example, the transmission line L can be used as an analog multiplexer. Further, for example, the transmitter 1 may be provided with a function that allows the transmitter side to output an interrupt signal to, for example, the receiving instrument 2 as a transmission signal. In this case, it is also possible to arbitrarily dedicate the two-wire transmission line L to the transmitter that outputs the interrupt signal.

[0012] In the above embodiment, the signal transmitted from the receiving instrument 2 side is a digital signal, but an analog signal may also be used. Furthermore, even when using a digital signal, in addition to a signal that changes between positive and negative polarities, a signal that changes only in one polarity or a pulse width signal may be used.

[0013]

[Effects of the Invention] As explained in detail above, according to the present invention, the power supply voltage supplied to the oscillator does not change due to the transmission current, making it easy to design the electronic circuit on the oscillator side. be able to. In addition, the electronic circuit on the receiving instrument side can be
It is possible to reduce costs by converting to C. Furthermore, when a digital signal is transmitted from the receiving instrument side, a circuit for separating the transmitted signal from the received signal sent from the transmitter side is theoretically unnecessary, and the circuit configuration can be simplified.

[Brief explanation of the drawing]

FIG. 1 is a configuration block diagram showing one embodiment of the present invention.

FIG. 2 is a configuration block diagram showing another embodiment of the present invention.

FIG. 3 is a diagram showing a specific example of the circuit shown in FIG. 2;

FIG. 4 is a waveform diagram of a voltage or current signal at each part in FIG. 3;

FIG. 5 is a block diagram showing one application example of the device according to the present invention.

FIG. 6 is a conceptual configuration diagram showing the relationship between a transmitter (transmitter) and a receiving instrument that receives a measurement signal via a two-wire transmission line.

[Explanation of symbols]

1 Transmitter 2 Receiving instrument L 2-wire transmission line Es　Power supply part 21 Virtual grounding means 22 Current value detection means

Claims (3)

[Claims] Claim 1: The current value supplied via the two-wire transmission line is
In a signal transmission device in which a signal is changed by a transmitter and transmitted to a receiving instrument, the receiving instrument includes a power supply section that outputs a supply voltage between one end of a two-wire transmission line and a ground line, and a virtual grounding section that connects the other end of the transmission line to a ground line. virtual grounding means, wherein the signal transmitted to the receiving instrument side via the two-wire transmission line is obtained by detecting a current value. Claim 2: The virtual grounding means includes an operational amplifier having one input end grounded, the other input end connected to the other end of the two-wire transmission line, and a feedback resistor connected between the one input end and the output end. 2. The signal transmission device according to claim 1, wherein the signal transmitted from the output end of the operational amplifier to the receiving instrument side via the transmission path is obtained. Claim 3: A claim in which a variable power source is used as the power source, the value of the power output voltage output between one end of the transmission line and the ground line is changed by a digital signal, and the digital signal is transmitted to the transmitter side. Item 1. The signal transmission device according to item 1.