JP3099689B2 - Fieldbus interface circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field bus interface circuit (hereinafter, referred to as a "bus") for performing bidirectional transmission of a digital signal to and from a field bus via two signal lines.
In particular, the present invention relates to an improvement in an interface circuit that can be driven with a low current.

[0002]

2. Description of the Related Art FIG. 6 shows an example of a configuration including a conventional interface circuit and its vicinity. In this case, an example is shown in which the interface circuit is provided at the output stage of a field device such as a differential pressure transmitter.

[0003] The field device 11 generally includes a sensor unit 12 for detecting a differential pressure or the like therein, and a transmission unit 13 for performing signal processing of an electric signal output from the sensor unit 12, and the like. It is equipped with a microcomputer or the like and is made intelligent. In the subsequent stage, an interface circuit 16 connected to the field bus 14 and the signal line 15 via the terminals T ₁ and T ₂ is arranged.

The field bus 14 is composed of two transmission lines, and is connected to a higher-level device such as a control unit (not shown).
A digital signal for performing bidirectional digital communication with a higher-level control device or another field device is superimposed on a power supply that supplies electric power required for the operation of (1).

For this reason, the interface circuit 16
Field bus 14 according to a predetermined field bus standard
A power supply unit that takes in a power supply current from itself to generate a power supply voltage V _C1 to itself or the transmission unit 13, a reception unit that receives a digital reception signal R _XS from the field bus 14, and a digital transmission signal T _XS from the transmission unit 13 , And a transmission unit 17 for transmitting a transmission control signal T _XE for controlling the same.

FIG. 7 shows the configuration of the transmission unit 17 among the various functions of the interface circuit 16. The transmission unit 17 includes a transmission controller 18, the deviation amplifier 1
9, a reference power supply 20 having a reference voltage V _S1 , a feedback resistor R _f ,
Resistors _{R 1, R 2, R 3} , R 4, R 5, transistor T _r1,
Tr2 , a constant voltage element Z and the like.

The transmission controller 18 receives the digital transmission signal T _XS and the transmission control signal T _XE , _{combines them,} and outputs the result as an output signal V ₀₁ to its output terminal. This output signal V ₀₁ is _divided by the resistors R ₁ and R ₂ to form a deviation amplifier 19.
Are input to the inverting input terminal (−).

The signal line 15 is connected to the non-inverting input terminal (+).
The sum of the feedback voltage V _f generated in the feedback resistor R _f by the transmission current I ₀₁ flowing through the feedback resistor R _f and the reference voltage V _S1 is referred to as the feedback resistor R _f ,
A divided voltage divided by the resistors R ₃ and R ₄ is applied.

The deviation amplifier 19 controls the collector current via the base current of the transistor Tr1 so that the _divided voltages at the inverting input terminal (-) and the non-inverting input terminal (+) become equal. The base current of the transistor _Tr2 is controlled by the collector current to control the transmission current _I01 flowing through the feedback resistor _Rf via the constant voltage element Z or the like.

For the above points, the waveform diagram shown in FIG.
And will be described in more detail. Input of transmission controller 18
The transmission end includes a transmission control signal T shown in FIG._XEAnd FIG.
The digital transmission signal T shown in (b)_XSIs entered
You. These signals are transmitted inside the transmission controller 18.
The signal is synthesized and the output signal shown in FIG.
V ₀₁Get.

[0011] Figure 8 the output signal V ₀₁ is the corresponding as a current signal by deviation amplifier 19, the transistor T _r1, T _r2
It is converted to the transmission current I ₀₁ shown in (d) of being sent to the field bus 14.

At this time, the signal amplitude V _SP1 of the output signal V ₀₁ is 0.75 V _{pp to} 1.0 V in the field bus standard.
_pp and the impedance of the field bus viewed from the interface circuit 16 is 50Ω, so that the signal amplitude I _SP1 of the transmission current I ₀₁ is 15 mA _{pp to} 20 mA.
_pp (FIG. 8D) is required, for example, I _SP1 = 16 m
A _pp .

At this time, the interface circuit 16 draws a DC base current I drawn from the field bus 14.
_b1 (FIG. 8 (d)) always requires 7.5 mA or more,
For example, it is set to 10 mA.

In this case, the base current I _b1 of the transmission current I ₀₁ supplied from the field bus 14 is I _b1 = V _S1 / R _f [R ₄ / R ₃ -R ₂ / R ₃ (R ₃ + R ₄ ) /
(R ₁ + R ₂ )].

Further, the signal amplitude I of the transmission current I _{01
SP1 is, I SP1 = V SP1 / R} f [R 2 / R 3 (R 3 + R 4) / (R 1 + R
₂ )].

Therefore, when designing a circuit,
Their base current I_b1And signal amplitude I _SP1Is the resistance R₁~ R
_FourAnd feedback resistor R_fAnd make an appropriate choice.
Will be determined.

[0017] However, the smaller the consumption current consumed within the field device, more by reducing the transmission current I ₀₁ from field bus 14 during non-transmission, it is possible to connect more field devices.

For this purpose, it is _conceivable to _adopt a low current drive system in which the base current I _rb2 supplied from the field bus 14 during non-transmission and the base current I _Tb2 supplied from the field bus 14 during transmission are changed. Can be In order to make such a system, the configuration of the transmission unit shown in FIG.
For example, it is conceivable to improve the configuration of the transmission unit as shown in FIG.

The circuit of the transmitting section 21 shown in FIG. 9 adds a resistor R ₅ shown by a dotted line to the circuit shown in FIG. 7, and transmits the transmission control signal T _XE via the resistor R ₅ to the non-inverting of the deviation amplifier 19. The base current I applied to the input terminal (+) during transmission and non-transmission
_In this configuration, the size of _b1 is changed.

The operation of this configuration will be further described with reference to a waveform diagram shown in FIG. In this case, the reference voltage V _{S1 having} a value smaller than the reference voltage V _S1 is used as the reference power supply 22.
_S2 is set, and the base signal of the output signal _V02 output from the transmission controller 23 at the time of non-transmission is set small (FIG. 1).
0 (c)).

Accordingly, the base current I _rb2 (FIG. 1) supplied from the field bus 14 when the transmission current I ₀₂ is not transmitted.
0 (d)) is smaller than that of FIG.
Although a mA, transmission time increases the base current is applied to the non-inverting input terminal of the error amplifier 19 a transmission control signal T _XE through the resistor R ₅ (+), the base current I shown in FIG. 8
base current I _Tb2 (= 10 mA) equal to _b1 (= 10 mA)
To enable signal transmission to the signal amplitude I _{S P2} = 16mA _pp as A).

[0022]

However, according to the configuration shown in FIG. 9, the base current I _Tb2 , the base current I _rb2 , the signal amplitude I _SP2 (= I _SP1 ) and the resistance R ₁
To R ₅ and must be determined by the feedback resistor R _f.

Therefore, these three parameters I
_Tb2 , I _rb2 , and I _SP2 cannot independently determine constants, and interfere with each other, causing a problem that constant design becomes extremely difficult.

[0024]

According to the present invention, as a main structure for solving the above-mentioned problems, two signal lines (15) are provided.
A transmission control signal ( _TXE ) and transmission contents for controlling transmission of data to be transmitted in a fieldbus interface circuit (16) for performing bidirectional transmission of digital signals to and from a fieldbus (14) via the digital transmission signal (T _XS) and transmitting the base current at the time of transmission through the signal line with is input _{(I Tb3, I T b3 '} ) non-transmission time and the first setting signal for setting the (S _C2) indicating the The non-transmission base current (I _rb3 , I
_rb3 '), and a second setting signal (S _C3 ) for setting the transmission base current, the non-transmission base current, and the amplitude (I _SP3 ,
I _SP3 ′) corresponding to the base signals (I ₁ , I ₂ , I ₃ ,
V ₁ , V ₂ , and V ₃ ) are independently set, and transmission controllers (23, 27, and 28) for selectively adding (25, 29) and sending out output signals (V ₀₃ , V ₀₃ ′) are provided. It is characterized by the following.

[0025]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an overall configuration of a transmitting unit according to an embodiment of the present invention. In the following description, portions having the same functions as those of the conventional configuration shown in FIGS. 6 to 10 are denoted by the same reference numerals, and the description thereof will be appropriately omitted.

The transmitter 22 transmits the controller 23, the deviation amplifier 19, a reference power supply 24 with a reference voltage V _S3, the feedback resistor R _f, resistors _{R 6, R 7, R 8} , R 9, transistor T _r1, T _{r 2.} It is composed of a constant voltage element Z and the like.

A setting signal S for setting the digital transmission signal T _XS , the transmission control signal T _XE , and the non-transmission base current I _Tb3 when the transmission current I ₀₃ is not transmitted is transmitted to the transmission controller 23.
_C3 , the transmission base current I _Tb3 when transmitting the transmission current I ₀₃
Is input setting signal S _C2 for setting, these combined to output as the output signal V ₀₃ to its output. The output signal V ₀₃ is _divided by the resistors R ₆ and R ₇ and input to the inverting input terminal (−) of the deviation amplifier 19.

The signal line 15 is connected to the non-inverting input terminal (+).
A _divided voltage obtained by dividing the sum of the feedback voltage V _f3 generated in the feedback resistor R _f and the reference voltage V _S3 by the feedback resistor R _f , the resistors R ₈ and R ₉ by the transmission current I ₀₃ flowing through the feedback resistor R _f is applied. ing.

The deviation amplifier 19 controls the collector current via the base current of the transistor Tr1 so that the _divided voltages at the inverting input terminal (-) and the non-inverting input terminal (+) become equal. By controlling the base current of the transistor _Tr2 by the collector current, the constant voltage element Z
To control the transmission current I ₀₃ flowing through the feedback resistor R _f through the control circuit.

For the above points, the waveform diagram shown in FIG.
And will be described in more detail. Input of transmission controller 23
The transmission end has a transmission control signal T shown in FIG._XEAnd Figure 2
The digital transmission signal T shown in (b)_XSIs entered
You. Then, these signals are transmitted inside the transmission controller 23.
The signal is synthesized and the output signal shown in FIG.
V ₀₃Get.

When the transmission control signal T _XE is at the L-level non-transmission state (FIG. 2A), a voltage smaller than the reference voltage V _S3 , which is a calculation reference of the circuit, by V _rx (FIG. 2B) ), And the non-transmission base current I _{rb3 of the} transmission current I ₀₃ (FIG. 2)
(C)) is set to a small value of, for example, 4 mA.

Next, when the transmission control signal T _XE is transmitting at the H level (FIG. 2A), a voltage smaller by V _Tx than the reference voltage V _S3 which is a calculation reference of the circuit (FIG. 2A). 2
(B)) and outputs the transmission base current I of the transmission current I _03.
Tb3 (FIG. 2D) is set to, for example, 10 mA.

Then, the amplitude V _SP3 (FIG. 2D) of the digital transmission signal T _XS at the time of this transmission changes around V _Tx so that the amplitude of the digital signal of the transmission current I ₀₃ is I _Set to _SP3 and transmit current I ₀₃
The parameters I _rb3 , I _Tb3 , and I _SP3 are set independently of each other.

Next, the fact that these parameters can be set independently by the transmission controller 23 shown in FIG. 1 will be described with reference to a specific configuration of the transmission controller 23 shown in FIG.

The transmission controller 23 has an AND gate G
₁ , OR gate G ₂ , constant current sources CC _{1 to} CC ₃ , CC ₄ , C
C _5, the switch _{SW 1, SW 2, SW 3} , the current / voltage converter 25 having a feedback resistor R _10, and is constituted by a reference voltage source 26 having a reference voltage V _S3.

The digital transmission signal T _XS is _supplied to the AND gate G ₁
Negative logic input terminal of the transmission control signal T _{X E} AND gate G
It is input to the positive logic input terminal of the ₁ switches the switch SW ₂ at the logical level of its output.

Next, the logical level of the output terminal of the AND gate G ₁ is input to the positive logical input terminal of the OR gate G ₂ , and the transmission control signal T _XE is input to the negative logical input terminal thereof. switch the SW _1. Furthermore, it switches the switch SW ₃ by transmitting control signals T _XE.

Circuit power supply V_ccAnd current between the circuit potential point
Value I₁Constant current source CC having₁And CC _TwoAnd switch SW₁
Connected in series through the switch SW₁And constant current source C
C₁Is connected to the inverting input terminal of the current / voltage converter 25.
(-).

The output terminal of the current / voltage converter 25 and
A feedback resistor R is connected between the inverting input terminal (-)._TenIs connected,
The non-inverting input terminal (+) has a reference voltage with respect to the circuit potential point.
Pressure V _S3Is applied.

[0040] Further, the circuit between the power supply V _cc and the connection point C, a constant current source CC ₃ having a current value I ₁ and the switch SW ₂
And are connected in series. Also, the transmission base current I
Sends the base signal I ₂ is a current which determines the _Tb3 a constant current source CC ₅ for sending a base signal I ₃ is a current which determines a constant current source CC ₄ the non-transmission base current I _rb3 each switch SW ₃ are connected to the switching end, and the common end is connected to the connection point C.

The setting signal S _C2 is supplied to the constant current source CC _4.
Is set, the base signal I ₂ for determining the transmission base current I _Tb3 is set, and the setting signal S _C3 is set for the constant current source CC ₅ , and the base signal I ₃ for determining the non-transmission base current I _rb3 is set. Is done.

Next, the operation of the transmission controller 23 configured as described above and shown in FIG. 3 will be described. First, in the state where the constant current sources, excluding the constant current source CC ₂ is not connected, by the constant current source CC _2, which is selected to be _{V S3 = I 1 · R 10} , in this state the output Signal V ₀₃
Is the reference potential V _S3 of the circuit (FIG. 2C).

First, the case of non-transmission will be described. In this case, since the transmission control signal T _XE is held at the low level L, the switch SW ₃ is connected to the constant current source CC ₄ , and the base signal I ₂ is supplied to the feedback resistor R ₁₀ by the current / voltage converter 25. the _{output, V rx = -I 2 · R} 10 (1) only reduced output signal V ₀₃ with respect to the reference potential V _{S 3} (FIG. 2 (c)) appears.

Therefore, the non-transmission base current I _rb3 of the transmission current I ₀₃ is I _rb3 = −V _rx / R _f (R ₆ / R ₇ ) (2) is obtained (FIG. 2D). However, for simplicity, R ₆ = R ₈ , R
₇ = there as R _9.

The base current I _{Tb3 for} transmission (FIG. 2
(D)) Since the transmission control signal T _XE is held at the high level H and the base signal I ₃ of the constant current source CC ₅ flows through the feedback resistor R ₁₀ , V _TX = −I ₃ · R ₁₀ (3 ) Appears, the output signal V ₀₃ (FIG. 2 (c)) is reduced.

Therefore, the transmission base current I _Tb3 of the transmission current I ₀₃ is I _Tb3 = −V _Tx / R _f (R ₆ / R ₇ ) (4) (FIG. 2D).

In the case of transmission, the amplitude V _SP3 of the digital transmission signal T _XS appearing as the output signal V ₀₃ (FIG. 2)
(C)) is as follows. When the digital transmit signal T _XS is at the high level H, since the transmission control signal T _XE is held at the high level H, the amplitude control signal V _{A 1} appearing at the output terminal of the AND gate G ₁ is held at a low level L is the constant current source CC ₃ as a result of this is turned off.

Then, the amplitude control signal V _A2 appearing at the output terminal of the OR gate G ₂ is also held at the low level L, and as a result, the constant current source CC ₁ is also turned off. Therefore, the potential increases from the reference potential V _S3 by a voltage corresponding to 2I ₁ · R ₁₀ .

When the digital transmission signal T _XS is at the low level L, the transmission control signal T _XE is held at the high level H, so that the amplitude control signal V _A1 appearing at the output terminal of the AND gate G ₁ is at the high level. It is held at the level H, the constant current source CC ₃ as a result of this is turned on.

Then, the amplitude control signal V _A2 appearing at the output terminal of the OR gate G ₂ is kept at the high level H, and as a result, the constant current source CC ₁ is turned on. Therefore, the reference potential V _{S3 is} reduced by a voltage corresponding to 2I ₁ · R ₁₀ .

Therefore, taking the above into consideration, when the transmission control signal T _XE is at the high level H, the reference potential V _S3 changes with the high / low level of the digital transmission signal T _XS.
V _SP3 = 2I ₁ · R ₁₀ (5).

On the other hand, when the transmission control signal T _XE is at the high level H, V _TX is reduced by the value shown in the equation (3), so that the output signal V ₀₃ eventually becomes as shown in FIG. V
The change of ± I ₁ · R ₁₀ with respect to _TX is shown.

Therefore, the amplitude I _{SP3 of} the transmission current I ₀₃ with respect to the digital transmission signal T _XS (FIG. 2D) is I _SP3 = V _SP3 / R _f (R ₆ / R ₇ ) (6)

The parameters I _rb3 , I _Tb3 , and I _SP3 can be determined by the base signals I ₂ , I ₃ and the current value I ₁ , respectively. The base signal I ₂ is determined by the setting signal S _C2 , and the base signal I ₃ is determined by the set signal S _C2. Since these can be set by the setting signal S _C3 and the current value I ₁ for determining the amplitude of the digital signal can be set in advance, these parameters can be determined independently of each other.

FIG. 4 is a circuit diagram showing a configuration of a transmission controller according to another embodiment of the embodiment shown in FIG. The transmission controller 27, instead of selecting the transmission control signal T _XE base signals I ₂ and I _3, as shown in FIG. 3, the base signal when the time of transmission of the non-transmission I ₂
Is added to the base signal I ₃ ′ at the time of transmission.

The transmission controller 27 is an AND gate G
_1, OR gate G _2, a constant current source CC ₁ to CC _3, CC _4, the switch SW _1, SW _2, current / voltage converter 25 having a feedback resistor R _10, a reference voltage source 26 having a reference voltage V _S3 and It has the same points as those of the transmission controller 23 shown in FIG.

However, the value of the base signal I ₃ ′ generated by the constant current source CC ₆ set by the setting signal S _C3 ′ and the constant current source CC ₆ only when the transmission control signal T _XE is at the high level H The configuration of the switch SW _{4 to be} connected and the constant current source C
The point that C ₄ is connected to the connection point C without passing through the switch SW ₄ is different from the transmission controller 23 shown in FIG.

In the above configuration, first, in the case of non-transmission, since the transmission control signal T _XE is held at the low level L, the switch SW ₄ is disconnected from the constant current source CC ₆ and the base signal I ₂ to the output terminal of the current / voltage converter 25 to the feedback resistor _{R 10, V rx = -I 2} · R 10 (7) appears the output signal V ₀₃ was reduced by with respect to the reference potential V _S3.

Therefore, the non-transmission base current I _rb3 of the transmission current I ₀₃ is I _rb3 = −V _rx / R _f (R ₆ / R ₇ ) (8) However, also in this case, for simplicity, R ₆ = R ₈ , R ₇
= There as R _9.

The base current I _{Tb3 in} the case of transmission is the same as the constant current source CC with the transmission control signal T _XE held at the high level H.
'Since flows by adding the feedback resistor _{R 10, V TX = - (2} · R 10 + I 3' 6 base signal I ₃ of · R ₁₀₎ (9) appears the output signal V ₀₃ was reduced by.

Therefore, the transmission base current I _Tb3 of the transmission current I ₀₃ is I _Tb3 = −V _Tx / R _f (R ₆ / R ₇ ) (10)

Also, in the case of transmission, the amplitude V _SP3 of the digital transmission signal T _XS appearing as the output signal V ₀₃ is basically the same as that shown in FIG. The amplitude I _{SP3 of the} current I ₀₃ with respect to the digital transmission signal T _XS is also shown in the same manner as the equation (6).

Therefore, each parameter I _rb3 , I _Tb3 ,
I _SP3 can be determined based on the base signals I ₂ and I ₃ ′ and the current value I ₁ , respectively, and can be determined independently of each other as in the case shown in FIG.

FIG. 5 shows a third embodiment for the embodiment shown in FIG.
FIG. 3 is a circuit diagram illustrating a configuration of a transmission controller according to the first embodiment. The transmission controller 28 is configured to add and select a voltage instead of adding and selecting a current as shown in FIG.

The transmission controller 28 has an AND gate G
₁, OR gate G_Two, And gate G _Three, OR gate G_Four,
Constant voltage source CV₁, CV_Two, CV_Three, CV_Four, Switch S
W_Five, SW₆, SW₇, Resistance R₁₁, R₁₂, R₁₃, Feedback resistor
R₁₄Voltage adder 29 having a reference voltage V_S3Group having
It comprises a quasi-voltage source 26 and the like.

Constant voltage source CV_1,CV_TwoIs the voltage value as it
Each base signal V₁And constant voltage source CV _ThreeIs the setting signal S_C2To
The base signal V_TwoAnd constant voltage source CV_FourIs set
Constant signal S_C3The base signal V_TwoTo it
Each is set.

The digital transmission signal T _XS is _supplied to the AND gate G ₁
Negative logic input terminal of the transmission control signal T _{X E} AND gate G
Are input to the positive logic input terminal of the _1, the logic signal of the output terminal to the positive logic input terminal of the OR gate G _2, the transmission control signal T _XE is input to the negative logic input terminal of the OR gate G _2, the OR gate G ₂ The output terminal outputs the amplitude control signal _VA3 .

The digital transmission signal T _XS and the transmission control signal T _XE are both applied to the positive input terminal of the AND gate G ₃ , and the logical signal at the output terminal is applied to the positive logical input terminal of the OR gate G _4. The signal T _XE is OR gate OR gate G ₄
And outputs an amplitude control signal _VA4 to its output terminal.

On the other hand, one end of each of the constant voltage sources CV ₃ and CV ₄ is connected to the H-side and L-side switching terminals of the switch SW ₇ , and the other end is connected to the non-inverting input terminal (+) of the voltage adder 29. It is connected. The common terminal of the switch SW ₇ is the inverted input terminal of the resistance R ₁₁ via a voltage adder 29 - is connected to ().

One end of each of the constant voltage sources CV ₁ and CV ₂ is connected to the H side of each of the switches SW ₅ and SW ₆ with opposite polarities, and the other end is connected to the non-inverting input terminal (+) of the voltage adder 29. It is connected. Then, the switch SW _5, the common end inverting input terminal of the voltage adder 29 via the resistor R _12, R ₁₃ of SW ₆ (-) to be connected, these switches SW _5,
SW ₆ is switched by the amplitude control signal V _A4, V _A3.

The voltage adder 29 has a reference voltage source 25 connected between its non-inverting input terminal (+) and a common potential point, and a feedback resistor R between its inverting input terminal (-) and the output terminal. ₁₄
Are connected, and an output signal V ₀₃ ′ is obtained at the output terminal.

Next, the operation of the embodiment configured as described above will be described. First, when not transmitting, the signal line 1
The non-transmitting base current I _rb3 ′ flowing through 5 will be described. In this case, since the transmission control signal T _XE is at the low level L, the switch SW ₇ is switched to the CV ₃ side, and the base signal V ₂ as a voltage signal is applied to the voltage adder 29 via the resistor R _11. Then, a voltage of V _rx ′ = −V ₂ (11) with respect to the reference voltage V _S3 is output to the output terminal. However, for simplicity, R ₁₁ = R ₁₂ =
It is assumed that R ₁₃ = R ₁₄ .

Accordingly, the non-transmission base current I _rb3 ′ flowing through the signal line 15 is given by I _rb3 ′ = −V _rx ′ / R _f (R ₆ / R ₇ ) (12)

At the time of transmission, the transmission control signal T _XE
Since at a high level H, the switch SW ₇ is switched to the constant voltage source CV ₄ side, the base signal V ₃ is a voltage signal
There at its output is applied to the voltage adder 29 via the resistor R _12, V _Tx '= voltage -V ₃ (13) relative to the reference voltage V _S3 is output.

Therefore, the transmission base current I _Tb3 ′ flowing through the signal line 15 becomes I _Tb3 ′ = −V _Tx ′ / R _f (R ₆ / R ₇ ) (14)

Next, in the case of transmission, that is, the transmission control signal T
The amplitude I _SP3 ′ of the digital transmission signal flowing through the signal line 15 when _XE is at the high level H will be described. First, when the digital transmission signal T _XS is at the high level H, since the amplitude control signal V _A3 appearing at the output of the OR gate G ₂ is is at low level L, the switch SW ₆ is open.

[0077] Further, since the amplitude control signal V _A4 appearing at the output of the OR gate G ₄ are at the high level H, the switch SW ₅ is switched to CV ₁ side, the output end of the the voltage adder 28 V A voltage of _SP3 '(H) = V ₁ (15) appears.

Next, in the case of transmission, the digital transmission signal T
_{When XS} is at the low level L, the amplitude control signal V _A3 is at the high level H and the amplitude control signal V _A4 is at the low level L. Therefore, the switch SW ₆ is connected to the constant voltage source CV ₂ and the switch SW ₅ is It is in an open state, and a voltage of V _SP3 ′ (L) = − V ₁ (16) appears at the output terminal of the voltage adder 29.

Accordingly, the amplitude V _SP3 ′ of the digital transmission signal appearing at the output terminal of the voltage adder 29 is as follows: V _SP3 ′ = V _SP3 ′ (H) −V _SP3 ′ (L) = 2V ₁ (17) The amplitude I of the digital transmission signal flowing on line 15
_{SP3 'is, I SP3' = V SP3 '} / R f (R 6 / R 7) become (18).

Also in this case, each parameter I _rb3 ′, I
_Tb3 ', I _SP3' includes a base signal V _2, V _3, the voltage value V ₁
, And can be determined independently of each other, as in the case shown in FIG.

[0081]

As described above, according to the present invention, according to the present invention, the transmission at the time of transmission supplied from the field bus is performed by the first setting signal and the second setting signal input to the transmission controller. Since the base current and the non-transmission base current having a value smaller than the transmission base current flowing at the time of non-transmission can be set independently, there is an advantage that these adjustments become very easy.

According to the configuration of the third aspect, the constant of the voltage dividing resistor in the output stage is fixed without depending on the transmission base current, the non-transmission base current, and the amplitude of the digital signal at the time of transmission. Therefore, there is an advantage that the design becomes easy. Further, even in the case of an ASIC, since a voltage dividing resistor and a feedback resistor can be built in the IC, a compact configuration can be realized.

[Brief description of the drawings]

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

FIG. 2 is a waveform chart for explaining the operation of the embodiment shown in FIG.

FIG. 3 is a circuit diagram showing a specific circuit of a transmission controller in the embodiment shown in FIG.

FIG. 4 is a circuit diagram showing another modification of the transmission controller shown in FIG. 2;

FIG. 5 is a circuit diagram showing still another modification of the transmission controller shown in FIG. 2;

FIG. 6 shows an example of a configuration including a conventional interface circuit and its vicinity.

FIG. 7 is a circuit diagram showing a configuration of a first transmission unit built in FIG. 6;

FIG. 8 is a waveform chart for explaining the operation of the transmitting section shown in FIG.

FIG. 9 is a circuit diagram illustrating a configuration of a transmission unit obtained by improving the transmission unit illustrated in FIG. 7;

FIG. 10 is a waveform chart illustrating an operation of the transmission unit shown in FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 11 Field device 12 Sensor part 13 Transmission part 14 Field bus 15 Signal line 16 Interface circuit 17, 21 Transmission part 18, 23, 27, 28 Transmission controller 19 Deviation amplifier 24, 26 Reference voltage source 25 Current / voltage converter 29 Voltage Adder T _XE transmission control signal T _XS digital transmission signal S _C2 , S _C3 setting signal I _Tb3 , I _Tb3 ′ transmission base current I _rb3 , I _rb3 ′ non-transmission base current I ₁ , I ₂ , I ₃ , V ₁ , V _2, V ₃ base signal V _03, V ₀₃ 'output signal

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H04L 12/28 G08C 15/00 H04L 25/00

Claims (5)

(57) [Claims] A transmission control signal for controlling transmission of data to be transmitted and a transmission content in a field bus interface circuit for performing bidirectional transmission of digital signals to and from a field bus via two signal lines. And a first setting signal for setting a transmission base current at the time of transmission flowing through the signal line when a digital transmission signal is input and a non-transmission base current of a value smaller than the transmission base current flowing at the time of non-transmission. 2 setting signals are input and used to independently set the transmission base current, the non-transmission base current, and the base signal corresponding to the amplitude of the digital transmission signal at the time of transmission, and selectively add these to output signals. A fieldbus interface circuit, comprising: a transmission controller for transmitting a signal. 2. The transmission controller according to claim 1, wherein said transmission control signal switches between a first transmission power supply and a second transmission power supply to generate first and second base signals corresponding to a transmission base current and a non-transmission base current. 2. The field bus interface circuit according to claim 1, wherein a third transmission power source is switched from the transmission control signal and the digital transmission signal to generate an amplitude control signal for controlling the amplitude of the digital transmission signal. 3. The transmission current so that the output signal matches a divided voltage obtained by dividing a sum of a feedback voltage corresponding to a transmission current flowing through the signal line and a reference voltage for determining a reference potential of a circuit. 2. The field bus interface circuit according to claim 1, further comprising: a deviation amplifying means for controlling the field bus. 4. A field bus interface circuit according to claim 2, wherein a constant current source set to a predetermined value is used as said first transmission power source, said second transmission power source, and said third transmission power source. 5. The field bus interface circuit according to claim 2, wherein a constant voltage source set to a predetermined value is used as said first transmission power source, said second transmission power source, and said third transmission power source.