JPH0795221A - Interface circuit - Google Patents

Abstract

PURPOSE:To improve safety by providing both an essential safety explosion- proof function and a transmission current detecting function at a current limiting resistance. CONSTITUTION:A voltage regulator 14 supplies a constant voltage Vc as the circuit power source of a signal processing circuit 10. On the other hand, a digital signal VD corresponding to field information such as a pressure and a differential pressure is outputted from a terminal T5 of the circuit 10. A feedback voltage Vf3 in proportion to transmission currents IS are outputted from a current limiting resistor RS2 and the VD and the Cf3 are inputted to a control circuit 23. The circuit 23 outputs a control voltage VS so that the currents IS can be held constant corresponding to a reference value set inside. Therefore, an interface circuit 22 fetches the constants IS from the field bus 11, supplies the voltage VC to the circuit 10, controls a current control circuit 13 through the circuit 23, and holds the current IS constant. Thus, the current limiter resistor can be provided with the function of a feedback resistor and safety can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interface circuit provided between a field bus for transmitting a power supply and a digital signal through a common transmission line and a signal processing circuit for processing the field signal, and more particularly, The present invention relates to an interface circuit improved so as to obtain a highly safe intrinsically safe function.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing a configuration centering on a conventional interface circuit compatible with a conventional field bus. Reference numeral 10 is a signal processing circuit for processing a field signal, for example, for processing pressure, differential pressure, etc. and outputting as a digital signal.

Reference numeral 11 is a field bus, and a plurality of signal processing circuits 1 on the field side via transmission lines L ₁ and L _2.
0 is supplied with DC power, and at the same time, the signal processing circuit 10 on the field side and the like are connected to a higher-order control device (not shown) and the like to transmit and receive digital signals.

Reference numeral 12 is a conventional interface circuit inserted between the signal processing circuit 10 and the field bus 11. Next, the inside of the interface circuit 12 will be described.

T ₁ and T ₂ are terminals connected to the field bus 11 by connection lines L ₁ and L ₂ , respectively. T ₃ , T ₄ , T
Reference numerals ₅ are terminals connected to the signal processing circuit 10, respectively.

A current control circuit 13 is provided between terminals T ₁ and T _2.
And Tsu voltage regulator 14 which is constituted by a Ena diode and a feedback resistor R _f1 are connected in series, the both ends of the voltage regulator 14 is connected to the power supply terminal of the control circuit 15, generates a constant voltage V _C to the opposite ends I am letting you.

Both ends of the voltage regulator 14 are connected to power source terminals T ₃ and T ₄ of the signal processing circuit 10 and supply a constant voltage V _C as a circuit power source of the signal processing circuit 10. On the other hand, the terminal T ₅ of the signal processing circuit 10 outputs a digital signal V _D corresponding to field information such as pressure and differential pressure.

Further, the transmission current I _S is _fed from the feedback resistor R _f1.
A feedback voltage V _f1 proportional to the output voltage is output, and the digital signal V _D and the feedback voltage V _f1 are input to the control circuit 15 so as to keep the transmission current I _S constant corresponding to a built-in reference value. Output as voltage V _S. Field Bus 11
This transmission current I _S to be held constant, which is taken from
It is set to 10 mA in the SA / SP50 standard.

In the configuration described above, the interface circuit 12 takes in a constant transmission current I _S from the field bus 11, uses this to generate a constant voltage V _C in the voltage regulator 14 and supplies it to the signal processing circuit 10.

On the other hand, the signal processing circuit 10 uses the digital signal V
_D is output to the control circuit 15, signal processing is performed together with the feedback voltage V _f1, and the result is output to the current control circuit 13 as the control voltage V _S to keep the transmission current I _S constant and to digital signal. The signal corresponding to V _D is the field bus 11.
Send to.

In this case, the minimum operating voltage V _MIN (1) seen from the terminals T ₁ and T ₂ is represented by V _MIN (1) = V _C + I _S · R _f1 + V ₁₃ (1). However, V ₁₃ is the minimum operating voltage across the current control circuit 13.

FIG. 3 shows the intrinsic safety corresponding to the conventional unified current.
Focusing on interface circuits that meet explosion-proof standards
It is a block diagram which shows a structure. Terminal T₁And T₂Transmission between
Line L ₁, L₂Direct connection between the DC power source 16 and the receiving resistor 17 via
A column circuit is connected and signal processing is performed via the interface 18.
A voltage is supplied to the processing circuit 19.

Next, the inside of the interface 18 will be described. Between the terminals T ₁ and T ₂ , the current control circuit 2
0, a current limiting resistor R _S1 , a voltage regulator 14 constituted by a zener diode, and a feedback resistor R _f2 are connected in series.

A power supply terminal of the control circuit 21 is connected to both ends of the voltage regulator 14, and a constant voltage V _C is generated at both ends. Current control circuit 20 and current limiting resistor R _S1
Connection point C ₁ , voltage regulator 14 and feedback resistor R _f2
A parallel circuit of diodes D ₁ , D ₂ and D ₃ is connected between the connection point C ₂ and the connection point C ₂ .

The signal processing circuit 19 signal-processes sensor signals such as pressure and differential pressure and outputs them as analog signals V _A to one end of the input of the control circuit 21. In addition, the control circuit 2
Feedback voltage V _f2 generated across the feedback resistor R _f2 is applied to the other end of the first input.

Then, the control circuit 21 controls the current control circuit 20 so that the analog signal V _A and the feedback voltage V _f2 coincide with each other. As a result, the analog signal V _A
And sends the received resistor 17 a current signal I _L that corresponds, for example, as a unified signal 4~20mA to.

In this case, in order to prevent the energy specified in the intrinsically safe explosion-proof standard from being transmitted from the signal processing circuit 19 side to the receiving resistor 17 side, the diodes D ₁ and D connected in parallel with the current limiting resistor R _S1 at the time of abnormality are provided. performing voltage limit at _2, D ₃ group. During normal operation, diodes D ₁ , D ₂ , D ₃
No current flows in the group.

The minimum operating voltage V _MIN (2) seen from the terminals T ₁ and T _{2 in} this case is represented by V _MIN (2) = V _C + I _L (R _S1 + R _f2 ) + V ₂₀ (2). However, V ₂₀ is the minimum operating voltage across the current control circuit 20.

[0019]

However, in the interface circuit 12 for the conventional fieldbus shown in FIG. 2 as described above, the conventional 4 to 20 m as shown in FIG. 3 is used.
If an interface circuit that corresponds to the unified current of A and satisfies the intrinsically safe explosion-proof standard is directly applied, the feedback resistance R _f2
The minimum operating voltage is increased by the amount of voltage drop at. As a result, in view of the intrinsically safe explosion-proof standard, the margin for the minimum operating voltage becomes small, and there is a problem that the safety is deteriorated.

[0020]

SUMMARY OF THE INVENTION The present invention has a structure for solving the above problems, one end of which is connected to one end of a field bus for transmitting both a DC current and a digital signal as a transmission current. The current control circuit for controlling the transmission current of the constant, the voltage limiting element connected between the other end of this current control circuit and the other end of the previous field bus to limit the voltage at both ends to a predetermined value, and the field signal. Current limiting circuit connected in series between the signal processing circuit for processing and transmitting as a digital signal to the other end of the current control circuit, and the other end of the field bus and the current control circuit. The voltage regulator connected between the other end of the voltage regulator, the power supply terminal is connected to both ends of the voltage regulator, the feedback voltage generated in the current limiting resistor and the digital signal are input, and the control signal is input. And control means for outputting, in which so as to have both functions of the detection of intrinsic safety in the previous transmission current ahead of the current limiting resistor.

[0021]

[Operation] A current control circuit, one end of which is connected to one end of a field bus that transmits both a DC current and a digital signal as a transmission current, constantly controls the previous transmission current by a control signal.

The voltage limiting element connected between the other end of the current control circuit and the other end of the above field bus limits the voltage across the current limiting resistor to a predetermined value in cooperation with the current limiting resistor to perform signal processing. Limit the energy flowing out of the circuit and satisfy the intrinsically safe standards.

On the other hand, the feedback voltage generated in the current limiting resistor and the digital signal are input to the control means, and the previous transmission current is made constant by processing these signals and outputting the control signal. To control.

In this way, the current limiting resistor is provided with both the intrinsically safe explosion-proof function and the earlier transmission current detection function to prevent a decrease in the minimum operating voltage and improve the safety for intrinsic safety. Let

[0025]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing the configuration of one embodiment of the present invention. The parts having the same functions as those of the conventional field bus compatible interface circuit shown in FIG. 2 are designated by the same reference numerals, and the description thereof will be appropriately omitted.

A field bus 11 is connected between the terminals T ₁ and T ₂ via transmission lines L ₁ and L ₂ , and the field bus 11 supplies a voltage to the signal processing circuit 10 via an interface 22. ing. A series circuit including a current control circuit 13, a current limiting resistor R _S2, and a voltage regulator 14 including a zener diode is connected between the terminals T ₁ and T ₂ .

A power supply terminal of the control circuit 23 is connected to both ends of the voltage regulator 14, and a constant voltage V _C is generated at both ends. Current control circuit 13 and current limiting resistor R _S2
Between the connection point C ₃ and the terminal T ₂ of the diodes D ₁ , D ₂ ,
A parallel circuit of D ₃ is connected.

Both ends of the voltage regulator 14 are connected to power source terminals T ₃ and T ₄ of the signal processing circuit 10, and the voltage regulator 14 supplies a constant voltage V _C as a circuit power source of the signal processing circuit 10. On the other hand, the terminal T ₅ of the signal processing circuit 10 outputs a digital signal V _D corresponding to field information such as pressure and differential pressure.

A feedback voltage V _f3 proportional to the transmission current I _S is output from the current limiting resistor R _S2 , and the digital signal V _D and the feedback voltage V _f3 are input to the control circuit 23 and incorporated into the reference value. The control voltage V _S for keeping the transmission current I _S constant is output in response to the above.

The transmission current I _S taken from the field bus 11 and to be kept constant is IS as in the case of FIG.
It is 10 mA according to the standard of A / SP50. In the above configuration, the interface circuit 22 takes in a constant transmission current I _S from the field bus 11, uses this to generate a constant voltage V _C in the voltage regulator 14 and supplies it to the signal processing circuit 10.

On the other hand, the signal processing circuit 10 uses the digital signal V
_D is output to the control circuit 23, and the control circuit 23 outputs the feedback voltage V
Signal processing is performed together with _f3, and the result is output as a control voltage V _S to the current control circuit 13 to keep the transmission current I _S constant and send a signal corresponding to the digital signal V _D to the field bus 11. To do.

The minimum operating voltage V _MIN (3) seen from the terminals T ₁ and T _{2 in} this case is represented by the following: V _MIN (3) = V _C + I _S · R _S2 + V ₁₃ (3)

Therefore, as compared with the equation (2) showing the minimum operating voltage in the interface circuit satisfying the conventional intrinsically safe explosion-proof standard shown in FIG. 3, there is a margin in the minimum operating voltage by the voltage drop of the feedback resistor R _f2. You can have it. Therefore, the value of the current limiting resistor R _S2 can be increased by the amount corresponding to this margin, and the safety is improved.

[0034]

As described above in detail with reference to the embodiments, according to the present invention, the current limiting resistor for satisfying the intrinsically safe explosion-proof standard is constituted so as to have the function of the feedback resistor. The voltage can be lowered, and thus the current limiting resistance can be increased, which can contribute to the improvement of safety.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a configuration diagram showing a configuration centering on a conventional interface circuit corresponding to a field bus.

FIG. 3 is a configuration diagram showing a configuration centering on a conventional interface circuit that corresponds to a unified current and satisfies an intrinsically safe explosion-proof standard.

[Explanation of symbols]

10, 19 Signal processing circuit 11 Fieldbus 12, 18, 22 Interface circuit 13 Current control circuit 14 Voltage regulator 15, 21, 23 Control circuit 16 DC power supply 17 Reception resistance R _S1 , R _S2 Current limiting resistance R _f1 , R _f2 Feedback resistor

Claims (1)

[Claims] 1. A current control circuit, one end of which is connected to one end of a field bus for transmitting both a direct current and a digital signal as a transmission current, for controlling the transmission current to a constant level by a control signal, and the other end of the current control circuit. And a second end of the current control circuit, a voltage limiting element connected between the other end of the field bus and a voltage limiting device for limiting the voltage at both ends to a predetermined value, and a signal processing circuit for processing a field signal and transmitting it as the digital signal. And a voltage regulator connected between the other end of the field bus and the other end of the current control circuit, and a power supply end connected to both ends of the voltage regulator. A feedback voltage generated in the current limiting resistor and a control means for receiving the digital signal and outputting the control signal; Interferon chair circuit, characterized in that gave both functions of the detection of the transmission current.