JPH0641400Y2 - Signal transmission device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a signal transmission device for converting a physical quantity such as pressure or differential pressure into an electric signal and transmitting the electric current to a load through a transmission line, and more particularly to this The present invention relates to a signal transmission device capable of digital communication via a transmission line.

<Prior Art> First, a conventional signal transmission device having a communication function that enables remote setting from outside the transmitter (Japanese Patent Laid-Open No. 58-85649).
No., name "data communication device and method for alternately communicating digital data and analog data") is shown in FIG.
The main points will be described.

Reference numeral 10 is a transmitter that converts a process variable or the like into an electric signal and transmits the electric signal, and electric power is supplied from a direct current 11 through a resistor 12. The electric signal is transmitted as a current signal by the transmission lines l ₁ and l ₂ , and the voltage generated across the resistor 12 is detected to know the process variable. The current signal is converted into a unified current of 4 to 20 mA and transmitted by the transmitter 10 set to a range corresponding to the pressure in the pipe, for example. In this case, it would be convenient if the transmitter 10 can be operated from outside when changing or monitoring the pressure range, for example. Therefore, the communication device 13 connecting line l ₁ to the outside of the transmitter 10 a ^-, l ₂ ^- is placed through the transmitter 10
It has a data communication function.

Next, the operation of the data communication thus configured will be described below.

When the communication device 13 starts communication with the transmitter 10, first,
The communicator 13 draws an extra current of 16 mA from the transmission lines l ₁ and l _{2 for} only one digital bit time, and notifies the transmitter 10 of the start of communication. When the transmitter 10 detects this current change, it waits for one bit time and fixes its output current at 4 mA.

After that, the communicator 13 sends a change of 4 mA to 20 mA to each of the transmission lines l ₁ and l ₂ for each digital bit including the start bit and the parity bit. This current change is
And each digital bit is detected. When the transmitter 10 detects the end of communication indicated by a steady current of 4 mA lasting the specified time t, the transmitter 10 returns its transmitted current to the previous process variable level in the range of 4-20 mA.

On the other hand, in digital communication from the transmitter 10 to the communicator 13, the transmitter 10 increases its transmission current from the process variable level 4-20 mA to 36 mA. The transmitter 10 maintains its current level for one bit time and then reduces the current level to 4 mA. This 4mA current level is also 1
It is maintained for a bit time, after which time the "start" bit starts the transmission of information. Transmitter
Digital communication from 10 to the communicator 13 is maintained and the transfer current is varied between 4 and 20 mA for each digital bit until the end of the communication operation. The communication operation ends after the transmission current is stably maintained at 4 mA for a predetermined time t. After this communication operation is complete, transmitter 10 adjusts the transmit current to bring it back to the previous process variable level of 4-20 mA.

However, in such a 2-wire type signal transmission device, 4 mA and 20 mA are made to correspond to digital "0" and "1" by using an analog signal transmission line, so that the amplitude of the communication signal is large and it is strong against external noise. However, there is a drawback that digital communication and analog signal transmission cannot be performed simultaneously.

Therefore, in order to solve this drawback, the present applicant has filed Japanese Patent Application No. 69-
264172 (Japanese Patent Laid-Open No. 61-142828) was proposed to solve this problem.

The outline will be described below with reference to FIG. Transmission device 14
Is connected to the sensor 15, the analog / digital converter (A / D) 16 connected to the sensor 15, the microprocessor (CPU) 17 connected to the analog / digital converter 16, and the microprocessor 17. Random accelerator memory (RAM) 18, read only memory (ROM) 19, digital / analog converter (D / A) 20, analog communicator 21 connected to this digital / analog converter 20, and microprocessor 17 And a digital communicator 24 having a modulation / demodulation circuit 23 connected to the universal asynchronous receiver (UART) 22.

The analog communication device 21 and the modulation / demodulation circuit 23 are both connected in parallel to the two-wire type transmission lines l ₁ and l ₂ .

On the other hand, the receiving apparatus 25, the transmission line l _1, l ₂ DC power supply 26 connected in series with, an analog communication unit 27, to which the transmission line l _1, l ₂ to be connected in parallel modulation demodulation circuit 28 A digital communicator 30 including a connected universal asynchronous transmitter / receiver 29,
Controller connected to this universal asynchronous transceiver 29
31 includes an input device 32, a display device 33, and the like, which are connected to the controller 31, respectively.

In such a configuration, analog transmission is performed as follows.

The microprocessor 17 uses the random access memory 18, read-only memory 19 and the like to perform a predetermined calculation such as linearity correction on the electric signal corresponding to the physical quantity detected by the sensor 15 and converts it into a digital signal. , This is output to the analog communication device 21 via the digital / analog converter 20, and converted into a current signal of 4 to 20 mA or the like, and the transmission line
Send to l ₁ and l ₂ . In this case, the power of the transmission device 14 is supplied from the DC power supply 26 of the reception device 25 via the transmission lines l ₁ and l ₂ . The analog signal transmitted via the transmission lines l ₁ and l ₂ is received by the analog communication device 27.

On the other hand, digital communication is carried out as described below.

Digital communication is used for the purpose of reading the span of the transmission device 14 or transmitting data for changing the subang from the reception device 25.

The information from the microprocessor 17 is sent out via the universal asynchronous transmitter / receiver 22 and is modulated by the modulation / demodulation circuit 23 into a carrier having a constant amplitude frequency f ₁ indicating a mark signal and a carrier having a constant amplitude frequency f ₂ indicating a space signal. Is transmitted to the transmission lines l ₁ and l ₂ . This transmission signal is demodulated by the modulation / demodulation circuit 28, input to the controller 31 via the universal asynchronous transmitter / receiver 29 and processed, and the result is displayed on the display 33.
Is displayed in. On the contrary, the information input by the operator from the input device 32 is sent out via the controller 31 and the universal asynchronous transmitter / receiver 29, and the carrier wave of the frequency f ₃ of constant amplitude and the space signal indicating the mark signal in the modulation / demodulation circuit 28. Is transmitted to the transmission lines l ₁ and l ₂ as a transmission signal modulated into a carrier liquid having a constant amplitude frequency f ₄ . This transmission signal is received by the modulation and demodulation circuit 23, a digital signal is reproduced, and is input to the microprocessor 17 via the universal asynchronous transmitter / receiver 22.

<Problems to be Solved by the Invention> Thus, according to the second conventional proposal, both the analog signal and the digital signal can be simultaneously transmitted, and the drawbacks of the first conventional technology are eliminated. On the other hand, in this case, however, the amplitude of the modulated transmission signal to be superimposed on the analog current signal of 4 to 20 mA is very small, so that it has a drawback that it is weak against external noise.

Therefore, an object of the present invention is to enable both analog and digital transmission at the same time and to increase the amplitude of a transmission signal.

<Means for Solving the Problems> In order to achieve this object, the present invention has a signal processing means for processing an electric signal corresponding to a measured physical quantity, and a DC current for the output of the signal processing means. An analog transmission means for converting and sending out to one end of the transmission line, an analog receiving means for receiving this DC current via the other end of the previous transmission line, and a digital signal modulated by digital information to the previous DC current. In a signal transmission device having a pair of digital communication means that are superposed and transmitted and received via a transmission line of the destination,
At least one of the pair of preceding digital communication means is an amplitude control means for changing the amplitude of the preceding digital signal according to the magnitude of the preceding DC current, and a modulating means for modulating the output of the amplitude control means with the preceding digital information. It is equipped with.

<Operation> The amplitude signal whose amplitude is changed by the amplitude control means in accordance with the magnitude of the analog DC current is modulated with digital information and output to the transmission line.

<Embodiment> An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. The parts having the same functions as those of the conventional signal transmission device shown in FIGS. 5 and 6 are designated by the same reference numerals, and the description thereof will be appropriately omitted.

The transmission device 34 has the same configuration except the portion corresponding to the analog communication device 21 and the modulation / demodulation circuit 23 of the transmission device 14 shown in FIG.

The analog communicator 35 includes an operational amplifier Q ₁ , a transistor Q ₂ ,
It consists of resistors R ₁ and R ₄ .

The inverting input terminal (−) of the operational amplifier Q ₁ is connected to the output terminal of the digital / analog converter 20 via the resistor R ₁ , and its non-inverting input terminal (+) is connected to the common potential point COM via the resistor R ₂ . The output terminals of the transistors are connected to the base of the transistor Q ₂ . The collector of the transistor Q ₂ is connected to the transmission line l ₁ , and the emitter of the transistor Q ₂ is connected to the transmission line l ₂ via the resistor R ₃ together with the common potential point COM.
, Respectively. The inverting input of the operational amplifier Q ₁ voltage across the resistor R ₃ is connected through a resistor R ₄ (-) to be negatively fed back.

The modulation / demodulation circuit 36 includes an amplifier C ₁ , a detection circuit C ₂ , an amplitude control circuit C ₃ , resistors R ₅ , R ₆ , a switch SW and the like.

Of these, the modulation circuit portion is configured as follows. Inverting input of the operational amplifier Q ₁ (-) via a resistor R _5, the non-inverting input terminal (+) via a resistor R _6, it is connected to the switching terminal of the switch SW. Also, the common switching end of the switch SW has a digital / analog converter 20 at one end.
Is connected via the amplitude control circuit C ₃ connected to the output terminal of the common switching terminal, and the common switching terminal is switched by the digital signal of the universal asynchronous transmitter / receiver 22.

Further, the demodulation circuit is configured as follows. Amplifier C ₁
Has its input end connected to the transmission lines l ₁ and l ₂ , its output end connected to the input end of the detection circuit C ₂ , and its output end connected to the universal asynchronous transceiver 22.

The receiving device side may have the same configuration as the receiving device 25 shown in FIG.

This is because the power supply required for operating the transmission device on the side of the transmission device 34 is made by using a current of 4 mA or less supplied from the power supply 26 via the transmission lines l ₁ and l ₂ , so that the power consumption is reduced. Although it is not possible to freely set the amplitude of the digital signal due to the limitation, the power supply 26 is built in the receiving device 25 side, so the amplitude of the digital signal is set to a certain size where the influence of noise can be ignored. This is because it can be set.

Next, the operation of the embodiment shown in FIG. 1 configured as described above will be described with reference to FIGS.

First, analog transmission will be described. The output from the sensor 15 is read by the analog / digital converter 16, converted into a digital signal, and input to the microprocessor 17. The microprocessor 17 uses a random access memory 18 in which changeable parameters and temporarily stored data are stored, or a read-only memory 19 in which arithmetic programs and constants are recorded, so that zero adjustment, span adjustment, or temperature adjustment can be performed. The calculation such as correction is executed and output to the digital / analog converter 20. The analog voltage converted by the digital / analog converter 20 is the operational amplifier Q ₁ , transistor
It is converted to a current output of ₄ to 20mA by Q ₂ and the transmission lines l ₁ and l ₂
Is transmitted to the receiving device 25 via.

Next, digital communication will be described. FIG. 2 is a characteristic diagram showing the input / output characteristics of the digital / analog converter 20. The horizontal axis represents input and the vertical axis represents output. Digital/
The analog converter 20 outputs an analog signal of 1 to 5 V to its output terminal according to the input of 0 to 100%.

FIG. 3 is a characteristic diagram showing the input / output characteristics of the amplitude control circuit C ₃ . The horizontal axis represents input and the vertical axis represents output. The amplitude control circuit C ₃ has analog outputs 1 to 1 of the digital / analog converter 20.
The output is amplitude-restricted to an inverted V shape for 5V, and the maximum 1V is output for an input of 3V, for example, as shown in the figure, and the output of 1V is switched for 1V and 5V inputs in common with the switch SW. Output to the end.

This switch SW is switched by the digital signal from the universal asynchronous transmitter / receiver 22, and the limited output of the amplitude control circuit C ₃ is applied to the inverting input terminal (−) or the non-inverting input terminal (+) of the operational amplifier Q ₁ and the result A digital signal whose amplitude is limited by an analog signal is output to the transmission lines l ₁ and l ₂ .

Next, the operation when the digital signal is superimposed on the analog signal will be described with reference to FIG. Figure 4 (a)
Indicates the state of current output (4 mA to 20 mA) with respect to the input (0% to 100%) of the transistor Q ₂ . Transistor Q
₂ can actually lower the current output up to the lower limit current I _l which is obtained by subtracting the circuit current consumption I _c from 4 mA when the current output is 4 mA, and conversely 20 mA when the current output is 20 mA.
The current output can be increased up to the upper limit current I _m . Therefore, in the conventional case, regardless of whether the input is 0%, 50%, or 100%, as shown in FIG. 4 (b), a constant amplitude ± I _l is uniformly determined by the current consumption I _c of the circuit. However, in the embodiment shown in FIG. 1, two points of 4 mA and 20 mA are the same as in the case of (b) in the embodiment shown in FIG. 1, but in other points. In either case, a large digital signal can be superimposed, and in particular, at the 50% point that is most frequently used, the digital signal having the maximum amplitude I _M can be superimposed. Therefore, the embodiment shown in FIG. 1 is much stronger than the conventional one against external noise.

On the other hand, the information input by the operator from the input device 32 of the receiving device 25, for example, a span read command is sent via the controller 31 and the universal asynchronous transmitter / receiver 29, and the modulation / demodulation circuit 28 has a constant amplitude indicating a mark signal. is transmitted to the transmission line l _1, l ₂ as a transmission signal modulated on a carrier frequency f ₄ of the constant amplitude indicating the carrier and space signal frequency f _3.
This transmission signal is received by the amplifier C ₁ of the modulation / demodulation circuit 23, its output is detected by the detection circuit C ₂ , a digital signal is reproduced, and the digital signal is input to the microprocessor 17 via the universal asynchronous transmitter / receiver 22. The microprocessor 17 decodes this information, outputs the data to the universal asynchronous transmitter / receiver 22, and outputs it as a digital signal to the transmission lines l ₁ and l ₂ via the switch SW, the operational amplifier Q ₁ and the transistor Q ₂ . The modulation / demodulation circuit 28 of the reception device 25 receives this digital signal and displays it on the display 33 via the universal asynchronous transmitter / receiver 29 and the like.

In the description so far, it is described that a digital signal is transmitted from the transmission device 34 and a carrier wave in which the digital signal is modulated is transmitted from the reception device 25, but even if both of them communicate using the carrier wave, Alternatively, communication may be performed using a digital signal.

In the embodiment shown in FIG. 1, the power source of the transmission device 34 is supplied by the current from the power source 26 of the receiving device 25. However, the power source of the transmission device 34 is not limited to this, and the power source of the transmission device 34 may be obtained from another power source. good.

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, the amplitude of the digital signal superimposed on the current output is changed according to the size of the current output. It is possible to realize a signal transmission device that is stronger against external noise than in the conventional case in which the height is constant.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of one embodiment of the present invention, FIG. 2 is a characteristic diagram showing the input / output characteristics of the digital / analog converter shown in FIG. 1, and FIG. 3 is the amplitude shown in FIG. FIG. 4 is a characteristic diagram showing the input / output characteristics of the control circuit, FIG. 4 is an explanatory diagram explaining the effects of the embodiment shown in FIG. 1, and FIG. 5 is a block diagram showing the configuration of a conventional first signal transmission device. Fig. 6 shows the conventional first
3 is a block diagram showing the configuration of the signal transmission device of FIG. 10 ... Transmitter, 11 ... DC power supply, 13 ... Communication device, 14, 34 ... Transmission device, 15 ... Sensor, 17 ... Microprocessor, 18 ... Random access memory, 19 ... Read only memory, 21, 27,
35 ... Analog communicator, 22,29 ... Universal asynchronous transceiver, 23,28,36 ... Modulation and demodulation circuit, 24,30 ... Digital communicator, 25 ... Receiver, controller ... 31,32 ... Input device, 33 ... Display unit, C ₂ ... Detection circuit, C ₃ ... Amplitude control circuit.

Claims (1)

[Scope of utility model registration request] 1. A signal processing means for processing an electric signal corresponding to a measured physical quantity, an analog transmission means for converting an output of the signal processing means into a direct current and sending it to one end of a transmission line, and the direct current. It has an analog receiving means for receiving a current through the other end of the transmission line, and a pair of digital communication means for transmitting and receiving a digital signal modulated by digital information on the direct current and transmitting and receiving it through the transmission line. In the signal transmission device, at least one of the pair of digital communication means has an amplitude control means for changing the amplitude of the digital signal according to the magnitude of the direct current, and a modulation for modulating the output of the amplitude control means with the digital information. A signal transmission device comprising means.