JPH0650555B2 - 4-wire field instrument device and its communication method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention detects physical quantities such as pressure, flow rate, and temperature in various plants and transmits the signals to a host instrument.
The present invention relates to a wire-type field instrument device and a communication method thereof.

[Conventional technology]

The instruments called field instruments have a built-in sensor to detect the physical quantity of the process, convert the value into an electric signal, and transmit it to the host receiving instrument. The transmission of this electric signal is standardized so that the field instrument outputs an analog current signal of 4 to 20 mA to the signal transmission line, and the host receiver instrument receives the analog current signal. .

In recent years, field instruments with a built-in microprocessor have been developed and put into practical use due to improvements in semiconductor integrated circuit technology. This is a system in which bidirectional digital signal communication is performed in addition to unidirectional analog signal communication on a two-wire type transmission line so that range setting and self-diagnosis of field instruments can be remotely operated. .

Such field instruments are classified into a two-wire field instrument and a four-wire field instrument depending on the installation location of the external power supply.

FIG. 3 shows a device configuration example of a two-wire field instrument. In the figure, the field instrument 1A outputs an analog signal as a constant current source for flowing a current corresponding to the detected physical quantity to the signal transmission line 5 by the voltage supplied from the external power source 4, and the upper receiver instrument 3 The analog signal is received and used as an indication value of the field instrument 1A. The upper communication instrument 2 includes the field instrument 1A and the upper reception instrument 3,
It is connected to the external power source 4 and communicates with the field instrument 1A by a digital signal.

Further, FIG. 4 shows a device configuration example of a 4-wire type field instrument. In the figure, the field instrument 1B operates as a constant current source that causes a current corresponding to the detected physical quantity to flow through the signal transmission line 5. The operations of the upper receiving instrument 3 and the upper communication instrument 2 are the same as in the case of FIG. An external power supply 40 is connected to the field instrument 1B via a transmission line 50.

As a method of transmitting an analog signal and a digital signal to a signal transmission line, a technique has been proposed in which a digital signal is placed on the analog signal so that the digital signal can be output without affecting the analog signal value. . For example, in Japanese Patent Application Laid-Open No. 58-48198 (hereinafter referred to as Conventional Example 1), the analog signal value is corrected according to the contents of the digital signal pattern when the digital signal is output. Further, in Japanese Patent Laid-Open No. 59-201535 (hereinafter referred to as Conventional Example 2), the digital signal pattern is set to a positive or negative pattern.

[Problems to be Solved by the Invention]

However, in Conventional Example 1 of the above-mentioned conventional techniques, when the communication speed is slow or when digital signals are continuously communicated, the analog signal is affected by the digital signal, and the error of the analog signal value becomes large. There is.

Further, in Conventional Example 2, only the 2-wire field instrument is targeted, and the 4-wire field instrument is not considered. That is, if the line voltage of the signal transmission line is low, there is a problem that the digital signal cannot be superimposed on the analog signal on the signal transmission line from the upper communication device side for communication.

An object of the present invention is to provide a four-wire type field instrument device and a communication method therefor capable of performing digital communication on a signal transmission line without significantly affecting an analog signal value.

[Means for Solving the Problems]

In order to achieve the above object, the present invention detects a physical quantity of a process and outputs a field instrument that outputs an analog signal corresponding to the detected value, an upper receiver instrument that receives the analog signal, and a transmission line. An external power supply for supplying electric power to the field instrument and the transmission line are provided separately, and a signal transmission line connecting the field instrument and a higher-order reception instrument, and connected in the middle of the signal transmission line, A four-wire field instrument device comprising a host communication instrument for outputting a digital signal for controlling the field instrument, wherein a line voltage of the signal transmission line is higher than a maximum value of a voltage level of the analog signal. The means for keeping the height high is arranged on the signal transmission line.

In addition, the present invention receives an analog signal from a field instrument that is operated by being supplied with power through a transmission line, while receiving the analog signal from a higher-order reception instrument through a signal transmission line provided separately from the transmission line, In a communication method of a 4-wire field instrument device, which transmits a digital signal from a host communication instrument to the field instrument using the signal transmission line, and controls the field instrument, a line voltage of the signal transmission line. Is kept higher than the maximum value of the voltage level of the analog signal, and the analog signal and the digital signal are transmitted for communication.

[Action]

According to the present invention, the analog signal flowing through the signal transmission line of the 4-wire type field instrument is 4 to 20 which is internationally unified.
Since it is a current signal of mA, and the resistance connected in series to the transmission line in the upper receiver instrument is usually 250Ω, the voltage applied to the upper receiver instrument is in the range of 1 to 5V. Therefore, in the communication method according to the conventional example, the line voltage of the signal transmission line is as low as 1 to 5 V, so that the upper communication instrument is connected to the signal transmission line and the digital signal is transmitted from the upper communication instrument. Is difficult.

However, in the communication system according to the present invention, if a constant voltage source of 24 V is used as the external power source, as in the case of the two-wire type field instrument, the line voltage of the transmission line is 19 to 23 V.
Since the voltage becomes relatively high in the range, the upper communication instrument can easily transmit the digital signal.

〔Example〕

 An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the overall configuration of a 4-wire field instrument device according to the present invention. In the figure, a field instrument 1 is for detecting physical quantities such as pressure, flow rate, and temperature in various plants, and is operated by electric power supplied from an external power source 40 via a transmission line 50 to generate a current corresponding to the physical quantity. It flows to the two-wire type signal transmission line 5. The upper receiving instrument 3 is
Due to the current flowing through the signal transmission line 5, the field instrument 1
The physical quantity detected by is received. The upper communication instrument 2 is connected between the field instrument 1 and the upper reception instrument 3 and the external power supply 41 on the signal transmission line 5, communicates with the field instrument 1 by a digital signal, and sets the range of the field instrument 1 Perform processing such as diagnosis.

In the inside of the field instrument 1, the entire instrument is processed by the microprocessor 10 by the processing programmed in the ROM 103.
Controlled by 1. The composite sensor 108 includes a plurality of sensors such as a flow sensor and a temperature sensor,
The respective sensor output signals are introduced to the multiplexer 109, and the sensor signal to be sent to the A / D converter 105 is selected by the input selection signal of the I / O interface 106. The microprocessor 101 is an A / D
The output of the composite sensor, which is sequentially sent from the converter 105, is compared with various correction coefficients stored in the ROM 103 or the RAM 102 to perform a correction to obtain a true value, and the output range set in the RAM 102 in advance. Thus, the normalized output value is output to the D / A converter 107.

The output value of the D / A converter 107 is added to the digital output signal of the modulation circuit 112 in the adder 110 and sent to the V / I converter 111. V / I converter 11
In 1, the current corresponding to the input signal (usually 4 ~ 20m
The range (A) is controlled so as to flow to the signal transmission line 5. When digital signal communication is performed, the transmission data output from the transmission / reception circuit (UART) 104 is transmitted by the modulation circuit 112 to two types of frequencies corresponding to “1” and “0” of the digital signal, such as frequency modulation. Converted to a signal. This signal is added to the adder 110 as described above.
Is added to the analog signal output value at the V / I converter 111
The digital signal is transmitted to the signal transmission line 5 in a form of being superimposed on the analog signal. Here, the modulation circuit 11
If the output signal of 2 is a square wave signal or a sine wave signal of the same amplitude in the positive and negative directions, even if a digital signal is output, only an instantaneous change in the output current value of the V / I converter 111 will occur. It has almost no effect on the signal. Further, when receiving a digital signal, the transmission signals from the upper receiving instrument 2 and the upper receiving instrument 3 are digital signals similar to the above-mentioned modulated current signal. Here, the voltage value of the external power supply 41 as a means for increasing the line voltage of the signal transmission line 5 is always constant, and when the value of the current flowing through the signal transmission line 5 changes, the analog signal of the upper receiving instrument 3 is changed. Since the voltage across the detection resistor 30 also changes accordingly, the voltage applied to the field instrument 1 inevitably undergoes a voltage change opposite to the voltage change. The demodulation circuit 113 captures this voltage change and demodulates it to obtain "1",
The transmission / reception circuit 104 can receive the digital signal as “0” digital signal. Similarly, when the above-described field instrument 1 transmits a digital signal, the value of the current flowing through the signal transmission line 5 changes, so that the signal transmitted by itself can be received through the demodulation circuit 113.

Next, the operation of the upper receiving instrument 3 will be described. The resistance 30 connected in series to the signal transmission line 5 is normally 25
The value of 0Ω is used, and the voltage across the resistor is taken out by the amplifier 31 to convert the analog current signal (4 to 20 mA) flowing through the signal transmission line 5 to 1 to 5 V.
Converted to and transmitted to the host system. The communication device 32 is composed of the same circuit as the transmission / reception circuit and the modulation / demodulation circuit in the field instrument 1. The communication device 32 transmits a digital signal by passing a current signal through the signal transmission line 5, and transmits the voltage across the resistor 30. The change receives a digital signal. here,
Similarly to the above, the higher-order receiving instrument 3 can also receive the signal transmitted by itself.

An internal block diagram of the host communication instrument 2 is shown in FIG. The upper communication instrument 2 is controlled by the microprocessor 201, and I
The current state is displayed on the display device 206 through the / O interface 205, and a digital signal is communicated with the field instrument 1 in response to an operation command input from the input device 207. The communication unit includes a transmission / reception circuit 204, a modulation circuit 209, a demodulation circuit 212, and a V / I converter 211, and operates in the same manner as the field instrument 1. Here, the output of the modulation circuit 209 is a signal having the same positive and negative amplitude as described above, and is input to the V / I converter 211.
The V / I converter 211 communicates with the field instrument 1 by superimposing a current corresponding to the modulated digital signal on the current flowing through the signal transmission line 5.

As a characteristic of the V / I converter 211, since a current signal corresponding to an input signal is output, when the constant current circuit is composed of a transistor or the like, the voltage between the collector and the emitter of one transistor is 1V. The above is necessary, and at least a few of them are necessary for the circuit to operate normally.
Double is needed.

Therefore, when the line voltage of the signal transmission line 5 is low, in order for the host communication instrument 2 to output a digital signal, the V / I converter 211 must be configured with a complicated constant current circuit. In the embodiment, since the line voltage of the signal transmission line 5 can be kept sufficiently higher than the external power source 41, the circuit of the host communication instrument 2 can perform digital signal communication with an inexpensive and simple configuration. There is an effect.

In addition, the device configuration on the higher-level instrument side other than the field instrument 1 can be made the same as that of the two-wire type field instrument, so that there is an effect that it can be shared.

[Effects of the Invention] As described above, according to the present invention, in the 4-wire type field instrument device, the line voltage of the signal transmission line between the field instrument and the host receiver instrument is set to the maximum value of the voltage level of the analog signal. Since it can be held higher than that, digital communication can be performed on the signal transmission line without affecting the analog signal.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of a 4-wire type field instrument device of the present invention, FIG. 2 is an internal block diagram of a host communication instrument shown in FIG. 1, and FIG. 3 is an overall configuration diagram of a 2-wire type field instrument device.
FIG. 4 is an overall configuration diagram of a conventional 4-wire field instrument device. DESCRIPTION OF SYMBOLS 1 ... Field instrument, 2 ... Host communication instrument, 3 ... Host receiver instrument, 5 ... Signal transmission line, 40, 41 ... External power supply, 50 ... Transmission line.

Claims (6)

[Claims] 1. A field instrument that detects a physical quantity of a process and outputs an analog signal corresponding to the detected value, a host receiver instrument that receives the analog signal, and power is supplied to the field instrument through a transmission line. External power supply,
A signal transmission line which is provided separately from the transmission line and which connects the field instrument and the upper reception instrument, and a digital signal which is connected in the middle of the signal transmission line and controls the field instrument In the four-wire field instrument device, the means for holding the line voltage of the signal transmission line higher than the maximum value of the voltage level of the analog signal is arranged on the signal transmission line. A 4-wire field instrument device characterized by being installed. 2. A four-wire field measuring device according to claim 1, wherein said means is arranged on a signal transmission line on the side of said upper receiving measuring device. 3. The four-wire field measuring device according to claim 1, wherein the voltage held by said means is 19 to 23V. 4. A four-wire field measuring device according to claim 1, wherein the means is arranged on the high potential side of the signal transmission line. 5. A four-wire field measuring device according to claim 1, wherein said means is arranged on the low potential side of said signal transmission line. 6. An upper-level receiving instrument receives an analog signal from a field instrument that is operated by being supplied with power via a transmission line, via a signal transmission line provided separately from the transmission line, and at the same time, an upper level instrument. In a communication method of a 4-wire field instrument device for transmitting a digital signal from a communication instrument to the field instrument using the signal transmission line and controlling the field instrument, the line voltage of the signal transmission line is A communication method for a four-wire field instrument device, characterized in that the voltage level of the analog signal is kept higher than the maximum value and the analog signal and the digital signal are transmitted for communication.