JPH0369460B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an oscillator that performs transmission by changing the value of current flowing through a two-wire transmission line in an industrial process, etc. The present invention relates to a communication device that is attached to a transmission device having a receiver that receives data based on a value, and is used for transmitting and receiving data with a transmitter as necessary.

[Conventional technology]

Conventionally, in industrial measurement, when transmitting the measurement output of a differential pressure transmitter, electromagnetic flowmeter, etc. to a remote point,
Generally, a unified signal such as 4 to 20 mA is used, and the current value of this analog signal indicates the measured value, and this current value is received by a receiver.

[Problem that the invention seeks to solve]

However, differential pressure transmitters, electromagnetic flowmeters, etc. are generally installed spread over a wide area, and in order to adjust them and check their operating status, staff must patrol and perform maintenance and inspection work. Therefore, there is a growing demand for a means that can make adjustments and check operating conditions by remote control using existing equipment.

[Means for solving problems]

In order to satisfy the above-mentioned demands, the present invention is constructed by the following means.

That is, while adding a function to the transmitter to perform reception based on line voltage changes in the two-wire transmission line, a first variable impedance element inserted in series with one line of the two-wire transmission line, and this a series circuit including a receiving impedance element connected in series with the variable impedance element, a series impedance element and a second variable impedance element connected in parallel to the first variable impedance element, and a first variable impedance element; The impedance of the first variable impedance element is controlled in the direction of making the voltage between the terminals of the variable impedance element and the receiving impedance element constant, and the second variable impedance is controlled in the direction of making the current value flowing through the series impedance element constant. Controlling the impedance of the element and simultaneously controlling each impedance of the first and second variable impedance elements to change the voltage between the terminals while keeping the current value flowing through the series impedance elements constant according to the transmission signal. The control circuit constitutes a communication device.

[Effect]

Therefore, only the current that indicates the signal value passes through the receiving impedance element, which enables reception, and also enables transmission by changing the voltage between the terminals in response to the transmission signal, and the current value While it is possible to simultaneously receive data and transmit data using a voltage between terminals, it is also possible to obtain a local power supply current within the range of a bias component of, for example, 4 mA that passes through a series impedance element.

Therefore, it is possible to give commands, etc. from the communication device to the transmitter, and if the transmitter responds by changing the current value in a pulse form, it is possible to know the response status of the transmitter by receiving this. becomes freely available.

Further, since no change is made to the current flowing through the two-wire transmission line, there is no problem at all with reception of the current value by the receiver.

〔Example〕

Hereinafter, details of the present invention will be explained with reference to figures showing examples.

Fig. 1 is a block diagram showing a communication device, and one line of a two-wire transmission line connected via line terminals t ₁ and t ₂ is a line La on one side and a line Lb on the other side. A first variable impedance element (hereinafter referred to as an element) Z ₁ is inserted into the resistor, and a resistor Rs is connected in series as a receiving impedance element. A series circuit consisting of a resistor Rc and a second element _Z2 is connected.

In addition, a control circuit CNT is connected in parallel with element Z ₂ as a power supply load, and the circuit CNT has an inter-terminal voltage V _CE with respect to the line terminal t ₂ side, and a resistor.
Given the load side voltage V _c of Rc, the terminal voltage V _S of resistor R _S , and the data from keyboard KB,
The control circuit CNT sends out first and second control voltages V _d1 and V _d2 in accordance with the voltages V _CE and V _c to each element.
The impedances of Z ₁ and Z ₂ are controlled to keep the voltage V _CE at a constant value of, for example, 10 V and the voltage V _c at a constant value of, for example, 7 V, and also to control the display part DP of a character display etc. according to the received value based on the voltage V _S. It is designed to send out a display signal.

Here, the current Ic flowing through the resistor Rc is expressed by the following equation.

Ic = V _CE −V _c /Rc ...(1) Therefore, the impedance of element Z ₁ is controlled in the direction of keeping V _CE constant, and the impedance of element Z ₂ is controlled in the direction of keeping V _c constant. By controlling and adjusting the current _I1 flowing to this, the current Ic becomes constant regardless of the load current _I2 , and the following equation holds true.

I _L = I _S + I ₁ + I ₂ = I _S + Ic ∴I _S = I _L - Ic...(2) That is, by setting Ic equal to the bias component, when the line current I _L is, for example, 4 to 20 mA, Since the current I _S flowing through the resistor R _S has only a signal component of 0 to 16 mA, the received value can be detected using the voltage V _S .

Further, when I _L is 4 to 20 mA, it is possible to freely supply a maximum power current of 4 mA.

Note that on the oscillator side, which will be described later, the line current I _L is sent out by a constant current circuit, so even if the impedance on the line side changes, the current value is not affected.

Regarding the above, when transmitting command data etc. to the transmitter, it is possible to simultaneously change the control voltages V _d1 and V _d2 in a pulsed or analog manner while keeping the current Ic constant according to the transmitted signal. , the inter-terminal voltage V _CE changes without changing the line current I _L , and transmission is thereby performed.

Therefore, for example, by raising or lowering the voltage _VCE while keeping the current Ic at 4 mA, it is possible to transmit by changing the voltage while receiving by the current value.

In addition, on the transmitter side, the line voltage of the two-wire transmission line that corresponds to the change in the terminal voltage V _CE is compared with a predetermined reference voltage, and the change is extracted and decoded. It is possible to transmit and receive at the same time.

Furthermore, if changes in the voltage V _c affect the operation of each load circuit, a voltage stabilizing circuit may be inserted into the portion through which the current I ₂ flows.

In addition, the elements Z ₁ and Z ₂ are transistors,
A photocoupler or other device exhibiting controllable variable impedance may be used.

Figure 2 is a block diagram showing the entire configuration of the transmission device.
and power supply B are connected by a two-wire transmission line (hereinafter referred to as transmission line) L consisting of lines L ₁ and L ₂ , and a resistor r is inserted as an impedance element in series with power supply B. The terminal voltage of this is applied to the receiver RX.

Therefore, when the transmitter TX changes the line current I _L flowing to the transmission line L by the power source B within a range of, for example, 4 to 20 mA, a corresponding terminal voltage V _r is generated in the resistor r, and this is applied to the receiver. The signal indicated by the current value is received by the RX.

In addition, the line L ₂ as one line of the transmission line L has
A switch S is inserted in series, and the first
The communication device CE shown in the figure is connected, and switch S is turned on when not transmitting and receiving, but when transmitting and receiving, by turning off switch S, communication device CE and transmitter TX are connected.
Transmission and reception will be carried out between the two.

That is, the transmitter TX changes the line current I _L in an analog manner in a range of 4 to 20 mA depending on the differential pressure or flow rate, etc., but the communication device CE changes the terminal voltage V _CE depending on the transmitted signal. When changed, the current
While I _L does not change, the line voltage V _L changes in response to changes in the voltage V _CE , and the voltage V _L is expressed by the following equation.

V _L = V _B − I _L (r + R _L ) − V _CE ...(3) However, V _B : Voltage of power supply B R _L : Loop resistance of transmission line L Therefore, voltage change from communication device CE can be received by the change in line voltage V _L at the transmitter TX, and at the receiver RX
can continue receiving by the current I _L .

In addition, the transmission from the transmitter TX to the communication device CE can be carried out by superimposing a pulse-like current change that changes at high speed on the analog line current I _L value, and sends it to the input side of the receiver RX. By inserting a waveform generator, an integrating circuit, etc. that removes high-speed changes, the reception status of the receiver RX is completely unaffected.

Note that it is preferable that the pulse-like current change is such that the same change is repeated in the positive and negative directions, and the average value becomes zero.

FIG. 3 is a block diagram of the control circuit CNT in FIG. , digital-to-analog converters (hereinafter referred to as DACs) D/A ₁ and D/A ₂ , and interfaces I/F ₁ and I/F ₂ are arranged around the periphery, and these are connected by a bus bar and are fixed. The processor CPU executes instructions in the memory ROM and performs control operations while accessing predetermined data to the variable memory RAM.

In addition, the voltage V _c shown in Fig. 1 is
It is stabilized in REG and is supplied to each part as local power E.

On the other hand, on the input side of the ADC/A/D, a multiplexer MPX controlled by the processor CPU is provided, whereby each voltage V _CE , V _c , V _S is selected sequentially and repeatedly. After each signal is converted into a digital signal by ADC/A/D, it is sent to the processor CPU.
Depending on these, the processor CPU
In order to provide control data to A ₁ and D/A ₂ , control voltages V _d1 and V _d2 converted into analog signals are sent out.

Figure 4 is a flowchart of the control situation by the processor CPU, in which the voltage "V _CE input" 101 is passed through the multiplexer MPX and the ADC/A/D.
After that, it is compared with the first reference voltage Vr ₁ stored in the fixed memory ROM in advance to determine “V _CE = Vr ₁ ?”102, and this is N(NO).
If the control voltage “V _d1 correction” according to the value of V _CE
103, and repeat this until step 10 becomes Y (YES).

Next, in the same way as step 101, the voltage “V _c capture” is
After performing step 111, as in step 102, it is compared with the second reference voltage Vr ₂ to determine "V _c = Vr ₂ ?" 112. If this is N, as in step 103,
Control voltage "V _d2 correction" 113 is performed until this becomes Y.

After making V _CE and V _c constant as described above, voltage "V _S capture" 121 is carried out in the same way as in step 101, and "conversion calculation" is performed to decode the pulse signal accordingly.
122 is performed, and then "display signal transmission" 123 via the interface I/F ₁ is performed.

FIG. 5 is a flowchart of transmission control,
After performing the “control processing” 201 shown in FIG.
Determine whether or not to transmit the data from the keyboard KB by "Data transmission?" 202. If this is Y, perform "conversion operation" 203 to the transmission data, then assume that the current Ic does not change. The control voltage "V _d1 and V _d2 are changed simultaneously" 204 is performed, transmission is performed thereby, and steps 201 and subsequent steps are repeated.

FIGS. 6 and 7 show a first embodiment showing another embodiment.
This is a block diagram similar to the one shown in the figure. In Figure 6, the resistor Rc is inserted to the line terminal _t2 side, and in Figure 7, the resistor R _S is further inserted to the line terminal _t1 side. is the same as in FIG.

Note that the control circuit CNT needs to select the detection reference potential of each voltage depending on the insertion position of the resistors R _S and Rc, and accordingly, the configuration shown in FIG. 3 may be slightly modified.

Therefore, the purpose can be achieved with only a single control circuit CNT, and since the circuit CNT is entirely composed of digital circuits, the communication situation is stable and miniaturization is easy.

FIG. 8 is a circuit diagram showing an embodiment in which the control circuit is an analog circuit, in which the emitter and collector of the transistor Q ₁ are connected in series as the first variable impedance element to the line terminals t ₁ and t ₂ . At the same time, a resistor R _S is connected in series on the collector side of this as an impedance element for reception, while a resistor is connected in parallel with these.
A voltage divider circuit consisting of R ₁ and R ₂ is connected, and a series circuit consisting of a resistor R ₃ as a series impedance element and the emitter-collector of a transistor Q ₂ used as a second variable impedance element is connected. .

Further, in parallel with the transistor Q ₂ , as a load circuit, a voltage dividing circuit including differential amplifiers A ₁ , A ₂ , resistors R ₄ , R ₅ , DAC・D/A ₁₁ , D/A ₁₂ , ADC・
A/D ₁₁ , arithmetic circuit OP consisting of a microprocessor, memory, etc., interface AI/F ₁₁ ,
I/F ₁₂ , display unit DP is connected, and arithmetic circuit
OP is the reference voltage via DAC/D/A ₁₁ and D/A ₁₂
It is designed to send out Vr ₁ and Vr ₂ .

Here, the resistors R ₁ , R ₂ and the differential amplifier A ₁ constitute a part of the control circuit, and the DAC/D/D/
Based on the reference voltage Vr ₁ from A ₁₁ , the terminal voltage V _CE
According to the voltage V ₁ divided by resistors R ₁ and R ₂ ,
Transistor in the direction of keeping the terminal voltage V _CE constant
The impedance of Q ₁ is controlled, so that the voltage across the terminals V _CE remains constant regardless of the line current I _L
is kept at a constant value of 10V, for example.

In addition, resistors R ₄ and R ₅ and differential amplifier A ₂ also constitute a part of the control circuit, and based on the reference voltage Vr ₂ from DAC/D/A ₁₂ , the load circuit side voltage of resistor R _{3 is}
The impedance of transistor Q ₂ is controlled in the direction of keeping the value of current Ic flowing through resistor R ₃ constant according to voltage V ₂ obtained by dividing V _c by resistors R ₄ and R ₅ , and each load circuit Regardless of the supply current of
The current Ic is maintained at a constant value of 4 mA, for example.

Therefore, the resistors R ₁ and R ₂ are set to high resistance values,
Assuming that the current I ₁ flowing through this can be ignored, the current I _S flowing through the resistor R _S becomes I _S = I _L - Ic, and by setting Ic equal to the bias component, I _S becomes 0 to 16 mA, for example. Therefore, if the terminal voltage V _S of the resistor R _S is converted into a digital signal by the ADC/A/D ₁₁ and given to the arithmetic circuit OP as a received signal, the circuit OP will perform the conversion calculation. The display signal is sent to the interface I/
It is applied to the display unit DP via _F11 to display the received data.

In addition, in FIG. 8, negative feedback is applied to the differential amplifiers A ₁ and A ₂ by the above operation,
Since the conditions are V ₁ ≒Vr ₁ and V ₂ ≒Vr ₂ , the following equation holds true.

V ₁ =V _CE R ₂ /R ₁ +R ₂ =Vr ₁ ∴V _CE =Vr ₁ (1 + R ₁ /R ₂ ) ...(4) V ₂ =V _c R ₅ /R ₄ +R ₅ =Vr ₂ ∴V _c = Vr ₂ (1+R ₄ /R ₅ ) ...(5) Here, Vr ₁ and Vr ₂ are stabilized as long as the data from the arithmetic circuit OP is constant, so V _CE and V _c are also becomes constant, and the following formula is obtained.

Ic=V _CE −V _c /R ₃ ...(6) In other words, Ic is constant.

On the other hand, line current I _L is expressed by the following equation.

I _L = I ₁ + I ₂ + I ₃ + I _S = I ₁ + Ic + I _S ... (7) Here, if I ₁ ≒ 0, I _S = I _L − Ic ... (8) Therefore, I _L For example, if Ic is 4 to 20mA,
=4 mA, I _S =0 to 16 mA, and a maximum of 4 mA of power supply current can be stably supplied to each load circuit without interfering with the reception of the signal indicated by I _S .

On the other hand, when transmitting command data etc. to the transmitter TX, the arithmetic circuit OP changes the reference voltages Vr ₁ and Vr ₂ while keeping the current Ic constant according to the transmission signal. _A11 ,
In order to change the data to D/A ₁₂ in a pulse form, for example, the terminal voltage V _CE changes in a pulse form accordingly, and transmission is performed by this, and the transmitted signal is indicated by a pulse code. It becomes something that can be done.

In other words, to keep the current Ic constant, the numerator of equation (6) should remain unchanged, and if V _CE −V _C is V _R , then
The following equation is obtained from equations (4) and (5).

V _CE −V _c =V _R =Vr ₁ (1+R ₁ /R ₂ )−Vr ₂ (1+R ₄ /R ₅ ) ∴Vr ₂ [Vr ₁ (1+R ₁ /R ₂ )−V _R ]1/(1+R ₄ /
R ₅ ) ...(9) Here, assuming the relationship of the following equation, R ₂ /R ₁ +R ₂ =R ₅ /R ₄ +R ₅ =K ...(10) From equations (9) and (10), The following relationship holds true.

Vr ₂ = (Vr ₁ 1/K - V _R )K = Vr ₁ = V _R・K ... (11) Therefore, while maintaining the relationship of equation (11),
If the data to ADC/A/D ₁₁ and A/D ₁₂ are changed simultaneously, the current Ic remains at 4 mA, for example,
The inter-terminal voltage V _CE can be freely raised or lowered, and it is possible to transmit by changing the voltage while receiving by the current value.

As described above, any data can be sent and received between the transmitter TX without affecting the reception of current values by the receiver RX. You can freely monitor and check the operating status based on commands and responses from the transmitter TX.

However, instead of the resistor R _S , an impedance element such as a diode or a circuit that directly detects the current value may be used, and a constant voltage diode or the like may be used as the resistors Rc and _R3 .

Moreover, in FIG. 8, transistors Q ₁ , Q ₂
Instead, other controllable variable impedance elements such as field effect transistors and photocouplers may be used, and the reference voltages Vr ₁ , Vr are set by constant voltage diodes, etc., regardless of the DAC/D/A ₁₁ and D/A ₁₂ . ₂ may be generated and these may be switched at the time of transmission.

Note that the line current can be modified in various ways, such as by determining the bias component according to the required power supply current of the load circuit, and by determining the change amount indicating the signal according to the situation.

〔Effect of the invention〕

As is clear from the above explanation, according to the present invention, transmission is performed by changing the line voltage by changing the impedance inserted into one line of the transmission path without changing the current value flowing through the two-wire transmission path. Transmission and reception with the transmitter can be freely performed without affecting the reception of the current value by the receiver, and remarkable effects can be obtained in commands to the transmitter, monitoring of the transmitter, and the like.

[Brief explanation of drawings]

The figures show an embodiment of the present invention; FIG. 1 is a block diagram of a communication device, FIG. 2 is a block diagram showing the entire transmission device, third is a block diagram of a control circuit, and FIGS. 4 and 5 are block diagrams of a communication device. Control status flowchart, No. 6
7 and 7 are block diagrams similar to FIG. 1 showing another embodiment, and FIG. 8 is a circuit diagram in which the control circuit is constituted by an analog circuit. L...Two-wire transmission line, _Z1 , _Z2 ...Element (variable impedance element), R _S , Rc...Resistor (impedance element), CNT...Control circuit, CPU...
Processor, ROM...Fixed memory, RAM...
Variable memory, MPX... multiplexer, A/D,
A/D ₁₁ ...ADC (analog-digital converter), D/A ₁ , D/A ₂ , D/A ₁₁ , D/A ₁₂ ...
DAC (digital-to-analog converter), KB...Keyboard, DP...Display section, _Q1 , _Q2 ...Transistor (variable impedance element), R _S , _R3 ...Resistor (impedance element), _R1 , R ₂ , R ₄ , R ₅
...Resistor, A ₁ , A ₂ ... Differential amplifier, OP ... Arithmetic circuit, TX ... Transmitter, RX ... Receiver, CE ...
...communication equipment, I _L ... line current, V _L ... line voltage,
V _CE ...Voltage between terminals, Ic...Current.

Claims (1)

[Claims] 1. Transmission comprising a transmitter that transmits by changing the current value flowing through a two-wire transmission line and receives data by changing the line voltage of the transmission line, and a receiver that receives data by changing the current value of the transmission line. In the apparatus, a first impedance element inserted in series with one line of the transmission path, a receiving impedance element connected in series with the variable impedance element, and the first variable impedance element and the impedance element. In contrast, a series circuit including a series impedance element and a second variable impedance element connected in parallel, and the first variable The impedance of the second variable impedance element is controlled in the direction of controlling the impedance of the impedance element and the value of the current passing through the series impedance element is constant, and the current value passing through the series impedance element is controlled in accordance with the transmission signal. a control circuit that simultaneously controls each impedance of the first and second variable impedance elements and changes the voltage between the terminals while maintaining a constant relationship.