JPH0916882A - Two-wire signal reception equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To separate an analog signal and a digital signal flowing through a two-wire transmission line to insulate them from each other with a simple circuit constitution and to eliminate an influence of the digital communication signal upon the analog signal. SOLUTION: A two-wire field device 1 is connected to a two-wire signal reception equipment through a two-wire transmission line 3 and uses the same transmission line to receive/supply the operating power, transmit the analog current signal, and perform the communication with the digital signal. The two-wire signal reception equipment transmits the insulated analog signal to a higher-order device 4 through a switched capacitor circuit and a sampling and holding circuit. The digital communication signal is insulated by a communication interface 26 and is communicated with the higher-order device through a transmission/reception circuit 27. The analog signal is held by the sampling and holding circuit during reception of the digital communication signal, thereby eliminating an influence of the digital communication signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal receiving device for receiving an analog signal and performing digital communication on a two-wire type transmitter, and particularly to prevent the received analog signal from being disturbed by digital communication. The present invention relates to a suitable signal receiving device.

[0002]

2. Description of the Related Art A signal transmission device called a so-called two-wire type field device operates by electric power supplied from a higher-order signal reception device side far away via a two-wire type transmission line, and is used in various plants. It is usual to detect a physical quantity such as pressure, temperature, flow rate, etc., convert the value into an electric signal, and transmit the signal to the signal receiving device through the same two-wire transmission path.

The transmission of the electric signal is standardized, and the field device is connected to the 2-wire type transmission line for 4 to 20 m.
The signal receiving device installed on the upper side outputs the analog current signal of A and receives the analog current signal. Further, generally, one-way communication of an analog signal from a field device to a signal receiving device has been performed.

However, in recent years, with the improvement of semiconductor integrated circuit technology, a two-wire field device having a built-in microprocessor is easily developed and put into practical use. According to this,
In addition to one-way analog signal communication on the transmission line,
Communicating with bi-directional digital signal communication such as range setting and self-diagnosis of 2-wire field devices.
It is becoming possible to operate remotely from higher-level devices.

The communication of this digital signal is carried out according to the above 4 to 2
This is done by superimposing it on an analog current signal of 0 mA or by switching between the analog current signal and the digital communication signal.

[0006] The signal receiving device is provided with a 4
Receive an analog current signal of ~ 20mA, for example 1 ~ 5
After converting to the V signal, the signal is isolated and transmitted to the host system.

[0007]

However, in the above-mentioned prior art, no consideration is given to the fact that the analog signal is disturbed when communicating with a digital signal, and the disturbed analog signal is There was a problem that it was transmitted to the host system as it was.

In the method of superimposing the digital current signal on the analog current signal of 4 to 20 mA, the superposed signal is made uniform in the vertical direction and the signal amplitude is reduced to reduce the influence value. However, the influence cannot be completely eliminated, and the problem that the reliability of digital signal communication decreases when the signal amplitude is reduced,
Since the digital signal is blunted and smoothed by the low-pass filter, the response of the output analog signal is deteriorated.

According to the present invention, when an analog signal from a field device is received and output using a 2-wire type transmission line,
By holding the analog signal only during the reception period of the digital signal, the above problems are solved, and further, a two-wire type signal receiving device capable of insulating the analog signal transmitted to the host device with a simple circuit configuration is provided. The purpose is to

[0010]

To achieve the above object, the first feature of the present invention is to supply the operating power of the field device between the field device and the signal receiving device, and between the devices. Transmission of analog current signal of 4 to 20 mA and communication of digital signal are performed by the same two-wire type transmission line 2
In the line type signal receiving device, sample holding means is provided in the analog signal receiving part of the signal receiving device connected to the two wire type transmitter, and the sample holding means is operated to hold when the signal of digital communication is received, and the analog That is, the output of the current signal is held at the value immediately before the signal reception of the digital communication. A second feature of the present invention is that between the field device and the signal receiving device,
Supply of operating power for field devices and 4 to 2 between devices
2-wire signal reception in which the 0 mA analog current signal is transmitted and the digital signal is communicated through the same 2-wire transmission line, and the digital signal is superimposed on the analog current signal for communication during digital communication. The apparatus is provided with means for separating an analog signal and a digital communication signal in the two-wire type signal receiving apparatus, and means for insulating each signal and transmitting it to a host device.

A third feature of the present invention is that the operating power of the field device is supplied between the field device and the signal receiving device, the analog current signal of 4 to 20 mA is transmitted between the devices, and the digital signal is transmitted. Signal communication is performed on the same two-wire transmission line, and during digital communication, the analog current signal and the digital signal are switched to perform communication 2
The line type signal receiving device is provided with a means for separating the analog signal and the digital communication signal in the two line type signal receiving device, and a means for insulating each signal and transmitting it to a host device.

[0012]

In the two-wire type signal receiving apparatus according to the present invention, the current flowing on the two-wire type transmission line connecting the field device and the signal receiving apparatus is an analog current signal of 4 to 20 mA and digital communication superposed on or switched to the analog current signal. The signal is a signal, and the sample-and-hold circuit built in the signal receiving device performs a holding operation on the received signal during digital communication.

Therefore, during digital communication, 2
The output of the line-type signal receiving device will hold the analog signal value before digital communication, and the analog signal disturbed by the digital communication signal during that period will be cut off. It is possible to prevent the influence from appearing in the output of the two-wire type signal receiving device.

[0014]

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

In FIG. 1, the output of the 2-wire field device 1 is composed of an analog current signal of 4 to 20 mA, and a digital signal superimposed on this analog current signal is used to communicate with the host device 4 via the signal receiving device 2. The block diagram in case of performing communication by is shown.

A schematic device configuration will be described with reference to FIG.
The two-wire field device 1 has a composite sensor, which detects physical quantities such as pressure, temperature, and flow rate in various plants, and operates by electric power supplied from a power supply 21 in the signal receiving device. ing. Then, the output from the sensor is subjected to various processes in the two-wire type field device 1, and the processed signal is output to the signal receiving device 2 via the two-wire type transmission line 3. It has become. The signal receiving device 2 includes a load resistor 22 connected to a transmission loop of the analog current signal of the transmission line 3, and detects a voltage between the load resistors 22 to detect the voltage from the 2-wire field device 1. It is designed to receive physical quantities. Further, a communication interface 26 and a transmission / reception circuit 27 are further incorporated, and the digital communication signal is used to communicate between the two-wire field device 1 and the host device 4 via a signal receiving device,
For example, processing such as reading self-diagnosis information and changing the range value is performed on the upper device 4 side.

Next, a method of transmitting a digital signal will be described. The communication interface 26 insulates the communication path between the two-wire transmission path 3 and the higher-level equipment 4, and is composed of parts such as the transformer shown in FIG. The communication interface 26 is connected in series to the transmission loop of the two-wire type transmitter 3 and is connected to the two-wire type transmission line 3 through two lines.
Output signal of the wire field device 1 (4 to 20 mA DC)
By superimposing a digital signal on, the digital signal is transmitted to the two-wire type field device 1, and conversely, the change in the current signal flowing through the two-wire type transmission line 3 is detected and the digital signal is received. ing. As a method of performing digital communication, there are a method of superimposing a digital current signal on an analog current signal of 4 to 20 mA DC and a method of switching the analog current signal and the digital current signal for communication. In this embodiment, the former superposition method will be described later.

Two-wire field device 1 and signal receiving device 2
The digital signal used for communication with the digital signal is a method of modulating two types of frequency signals or a method of modulating two types of amplitude signals, corresponding to the digital signals "1" and "0".
A method of modulating two types of phase signals is used.

During communication between the two-wire field device 1 and the higher-level device 4, the transmitter / receiver circuit 27 performs processing such as modulation and demodulation of the modulated signal, and a period during which a digital signal flows through the two-wire transmission line. A signal (R _BUSY ) indicating that the digital signal is being received is output therein.

Next, a method of transmitting an analog signal will be described.

The load resistor 22 is usually a resistor having a value of about 250Ω and is connected to a current loop of a current signal (4 to 20 mA DC) which is an output signal of the two-wire field device 1 to detect the current signal. . The current signal is converted from the load resistor 22 through the amplifier 23 into a voltage signal,
It is transmitted to the analog output circuit 24.

The analog output circuit 24 is composed of a circuit in which a switched capacitor type insulation circuit and a sample hold circuit are combined. The analog output circuit 24 performs the above-mentioned various operations according to the control signals (SW1, SW2) from the sequence circuit 29, and the upper order An analog signal (1 to 5 VDC) insulated from the two-wire transmission line 3 is output to the device 4. Further, in order to strengthen the withstand voltage, a photomos relay or the like is usually used as the analog switch.

Each of these circuits in the signal receiving device 2 is
Power supply circuit 2 based on the power (24V) supplied from the outside
It operates on the power supplied from the isolation voltage (V _DD ) made in 5.

Next, detailed operations of the sequence circuit 29 and the analog output circuit 24 will be described.

FIG. 2 shows an embodiment of the sequence circuit 29. The sequence circuit 29 produces signals (SW1, SW2) for controlling the sample hold circuit of the switched capacitor circuit of the analog output circuit 24 with reference to the clock signal (clock) output from the oscillator 28.

The flip-flops 292 and 293 divide the frequency of the clock signal and output the respective signals and the logic circuit 2.
Output to 94. The logic circuit 294 creates control signals (SW1, SW2) from the collection signal. The reset circuit 291 is created based on a signal (R _BUSY ) indicating that the digital signal output from the transmission / reception circuit 27 is being received, and by incorporating a delay circuit, the digital signal is being received and the transmission line is thereafter transmitted. The flip-flops 292 and 293 are reset until a certain period of time when the influence of the digital signal completely disappears.

FIG. 3 shows the timing chart.
Since the control signals (SW1, SW2) do not turn ON at the same time and alternately repeat ON / OFF, in the analog output circuit 24 of FIG.
It realizes both signal transmission and signal insulation by charging / discharging the built-in capacitor. Here, the control signal (SW
When 2) is OFF, the built-in output amplifier and the capacitor attached to the preceding stage constitute a sample hold circuit to hold the analog output value.

Therefore, both the control signals (SW1, SW2) are not turned ON during the reception of the digital signal and thereafter for a certain period, so that the analog output holds the value before the digital communication, and Since the analog signal affected by the digital communication signal is separated from the circuit, the effect of the digital signal on the two-wire transmission line 3 can be removed as shown in FIG.

Further, when a small signal of a square wave or a sine wave having the same amplitude in the positive and negative directions is adopted as the digital signal on the two-wire type transmission line 3 and is modulated, a low pass filter is used to modulate the digital signal. Although the influence can be reduced, it cannot be eliminated completely, and the built-in low-pass filter causes a problem that the response of the analog output signal becomes dull. According to the present embodiment, since it is not necessary to incorporate a low-pass filter that cuts a low frequency so as to remove the influence by blunting the digital signal,
There is also an effect that the signal can be insulated while the high-speed response is maintained and the influence of the digital signal can be completely removed.

[0030]

As is apparent from the above description, according to the two-wire type signal receiving apparatus of the present invention, the analog signal and the digital communication signal are separated by a simple circuit configuration, and each signal is separated. After being insulated, it is possible to transmit to the host device and remove the influence of the digital communication signal on the analog signal.

[0031]

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a two-wire signal receiving device according to the present invention, and a device configuration diagram.

FIG. 2 is a circuit diagram of a sequence circuit of the signal receiving device according to the present invention.

FIG. 3 is a timing chart of the signal receiving device according to the present invention.

FIG. 4 is a diagram of an input current signal in which a digital signal is superimposed on an analog signal and an output signal separated into an analog signal and a digital signal.

[Explanation of Codes] 1 ... 2-wire type field device, 2 ... 2-wire type signal receiving device,
3 ... 2-wire type transmission line, 4 ... Host device, 21 ... Power supply power source, 22 ... Load resistance, 23 ... Amplifier, 24 ... Analog output circuit, 25 ... Power supply circuit, 26 ... Communication interface, 27 ... Transceiver circuit, 28 … Oscillator, 29… Sequence circuit

Claims (8)

[Claims] 1. A field device and a signal receiving device,
Supply of operating power for field devices and 4 to 2 between devices
In a two-wire type signal receiving device that performs transmission of a 0 mA analog current signal and communication with a digital signal on the same two-wire type transmission line, an analog signal receiving section of the signal receiving device connected to a two-wire type transmitter A two-wire system characterized in that a sample-hold means is provided to the sample-hold means and the sample-hold means is operated to hold at the time of digital communication signal reception, and the output of the analog current signal is held at a value immediately before the reception of the digital communication signal. Signal receiving device. 2. Between the field device and the signal receiving device,
Supply of operating power for field devices and 4 to 2 between devices
2-wire signal reception in which the 0 mA analog current signal is transmitted and the digital signal is communicated through the same 2-wire transmission line, and during digital communication, the digital signal is superimposed on the analog current signal for communication. A two-wire signal, characterized in that it comprises means for separating an analog signal and a digital communication signal in the two-wire signal receiving apparatus, and means for insulating each signal and transmitting it to a host device. Receiver. 3. The two-wire signal receiving apparatus according to claim 2, wherein the means for separating the analog signal and the digital communication signal is such that the analog current signal is received immediately before the digital communication signal is received when the digital communication signal is received. A two-wire type signal receiving device, characterized in that it is held at the value of and the analog current signal from which the influence of digital is removed is output. 4. A field device and a signal receiving device,
Supply of operating power for field devices and 4 to 2 between devices
A two-wire signal receiving apparatus that performs transmission of an analog current signal of 0 mA and communication by a digital signal on the same two-wire transmission line, and performs communication by switching the analog current signal and the digital signal during digital communication. In the above-mentioned two-wire type signal receiving device, there is provided means for separating an analog signal and a digital communication signal, and a means for insulating each signal and transmitting it to a host device. apparatus. 5. The two-wire type signal receiving device according to claim 4, wherein the means for separating the analog signal and the digital communication signal receives the analog current signal immediately before the digital communication signal is received when the digital communication signal is received. A two-wire type signal receiving device, characterized in that it is held at the value of and the analog current signal from which the influence of digital is removed is output. 6. The two-wire type signal receiving device according to claim 1, 2, 3, 4, or 5, wherein a digital signal for a transmission line is transmitted after receiving the digital communication signal and before canceling sample hold. A two-wire signal receiving device, characterized in that a fixed delay period is provided until the influence disappears. 7. A two-wire type signal receiving device according to claim 1, 2, 3, 4, 5 or 6, wherein a signal insulating means of a switched capacitor circuit is provided before a sample hold circuit of said signal receiving device. A two-wire type signal receiving apparatus comprising means for opening an input switch of the switched capacitor circuit and performing a sample hold operation during a reception period of the digital communication signal. 8. The two-wire signal receiving device according to claim 1, wherein the switch for signal insulation of the switched capacitor circuit also functions as a switch of a sample hold circuit. Receiver.