JPH06237253A - Signal transmission circuit - Google Patents

Abstract

(57) [Summary] [Purpose] To reduce costs by reducing the number of parts. A C-point of a transmission circuit 1 for generating a current change having a waveform corresponding to a signal voltage waveform at a C-point and transmitting a signal to the outside, and a tristate which is turned on / off in response to signal transmission / non-transmission. By inserting a circuit in which another resistor 19 is connected in parallel to the series circuit of the diode 17 and the resistor 18 between the gate 15 and turning on and off the diode 17 according to the high level and low level of the signal. , The voltage at point C is changed. That is, conventionally, two tristate gates were used to change the voltage at the point C. In this embodiment, one of them is a diode 17 and a resistor 1.
By substituting the circuits of Nos. 8 and 19, the number of parts is reduced and the cost is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is currently based on the IEC (Int
international Electro-techni
Cal Commission (International Electrotechnical Commission) and other organizations are promoting standardization, and signal transmission circuits equipped with terminals (field devices) that support the "fieldbus" communication standard for industrial measurement and control, in particular improvements Regarding

[0002]

2. Description of the Related Art FIG. 3 is a waveform diagram for explaining a signal transmission method previously proposed by the applicant (see Japanese Patent Application No. 3-174983: hereinafter also referred to as a proposed method). FIG. 9A shows a current waveform drawn by the terminal (transmission circuit), and FIG. 9B shows an example of a voltage waveform on the transmission line at that time. That is, as shown in (a) of the figure, 4 m from the transmission line when not transmitting.
The current of A is drawn, and the current drawn during transmission is 8 mA.
15 mAp (pp: peak-to-pea
k), and superimposed on DC as shown in FIG.
A signal voltage of 75 Vpp is generated on the transmission line.

In this way, by maintaining the current drawn during non-transmission smaller than the average current drawn during transmission, it becomes possible to increase the number of terminals connectable to the transmission path. This is an important factor to be particularly considered in cases such as intrinsically safe explosion-proof where the energy given to the transmission path is limited. FIG. 4 is a circuit diagram (hereinafter, also referred to as a proposed circuit) showing a specific example of a transmission circuit that creates a current signal waveform as shown in FIG. In the figure, 1
1, 14 is an operational amplifier, 12 is a voltage reference, 13
Is a resistor, 15 and 16 are tristate gates, and T
XEN indicates a control signal that becomes high (H) during transmission and low (L) during non-transmission, and TXD indicates a data signal.

That is, the voltage at the point A is stabilized by the voltage reference 12, the operational amplifier 11, etc., to be, for example, 4.0 V, and becomes a power supply inside the terminal. The voltage at the point B is stabilized to a certain voltage by the operational amplifier 14 or the like according to the voltage at the point C. This voltage is 4.4 V (when not transmitting), 4.05 V (when signal H), 5.55 V (signal L)
Since it is connected to the point A of 4.0 V constant through the resistor 13 (100Ω), the current flowing through the resistor 13 (= current drawn from the transmission line by the terminal) is 4 respectively.
mA (when not transmitting), 0.5 mA (when signal is H), 15.5
It becomes mA (at the time of signal L), and 15 mApp can be realized at the time of transmission.

The voltage at the point C, which serves as a reference for the voltage at the point B, is generated by the voltage reference 12, the tristate gates 15 and 16, etc. according to the data signal TXD and the control signal TXEN. That is, during non-transmission (TXEN = L), the output of the tri-state gate 15 is high impedance, the output of the tri-state gate 16 is L, and C
A predetermined voltage is generated at the point. On the other hand, during transmission (TXEN =
In (H), the output of the tri-state gate 16 becomes high impedance, and the output of the tri-state gate 15 becomes H or L according to TXD, and the voltage at the point C is changed around the voltage of 2.0 V created by the voltage reference 12. .

Table 1 summarizes the relationship between each signal, voltage and current during non-transmission and during transmission. In addition, the state of is a state when it is assumed that both the tri-state gates 15 and 16 have high impedance,
It can be said that it does not actually occur.

[Table 1]

[0007]

By the way, in the proposed circuit of FIG. 4, the point C has three voltages of high and low at the time of non-transmission and at the time of transmission. Since I have two tristate gates, I have a problem that the number of parts is large. In the proposed circuit of FIG. 4, the digital signal (TX
The change in (D, TXEN) causes a change in the voltage at the points C and B, and finally a change in the current to the outside. Since this change is steep, there is a possibility that a failure such as crosstalk will occur in the transmission path. Therefore, there is also a problem that the change in current must be made gradual. Therefore, an object of the present invention is to reduce the number of parts of the signal transmission circuit,
Alternatively, it is to moderate the current change during signal transmission.

[0008]

In order to solve such a problem, in the first invention, a current change having a waveform corresponding to a signal voltage waveform at a certain reference point inside is generated to send a signal to the outside. In the signal transmission circuit, a circuit in which another resistor is connected in parallel to a series circuit of a diode and a resistor is inserted between the tri-state element that is turned on / off in response to signal transmission or non-transmission and the reference point. However, the voltage at the reference point is changed by turning on and off the diode according to the high level and the low level of the signal.

In the second aspect of the present invention, a capacitor is inserted at the reference point in a signal transmission circuit that generates a current change of a waveform corresponding to a signal voltage waveform at a certain reference point inside and sends a signal to the outside. Is characterized by grading the current change.

[0010]

The number of components is reduced by replacing one of the two tri-state gates, which has been conventionally provided, with a circuit including a diode and a resistor. Also, by inserting a capacitor at the reference point, the change in current is moderated, making it less susceptible to crosstalk and the like.

[0011]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.
The feature is that a parallel circuit in which a resistor 19 is connected in parallel to a series circuit of a diode 17 and a resistor 18 is inserted between the tristate gate 15 and a point C, and the other points are almost the same as those in FIG. Then, only differences from FIG. 3 will be mainly described. The voltage at the point A is applied to the tristate gate 15 as a power supply voltage.

That is, during non-transmission (TXEN = L), the output of the tristate gate 15 becomes high impedance, and the voltage at the point C is 2V generated by the voltage reference 12.
become. When transmitting (TXEN = H), TXD =
When the voltage is H, the output of the tristate gate 15 is 0V, and a voltage obtained by dividing 2V by the resistors 20 and 19 is applied to the point C. At this time, since the diode 17 is off, the resistor 18 does not work.

On the other hand, when TXD = L, the output of the tristate gate 15 becomes 4V and the diode 17 is turned on, so that the voltage at the point C is the resistance 20 and the resistances 18 and 1.
The voltage is divided by 9 (to be precise, the ON voltage of the diode 17 is also added). As a result, FIG.
A current waveform similar to that of (a) can be realized. Table 2 shows the relationship among each signal, voltage and current in this case.

[Table 2]

That is, the high level of the signal with reference to the voltage at the point C in Table 1 which does not actually occur in the past,
While the low level voltage is determined, in this embodiment, the high level voltage and the low level voltage of the signal are determined based on the voltage at the point C in the case of Table 2. Thus, the digital signal (TX
The change of (D, TXEN) becomes the change of the voltage at the points C and B, and finally the change of the current to the outside.

However, the change is abrupt. Therefore, in this embodiment, by inserting the capacitor 21 at the point C, the voltage change (rise, fall) at the point C is made gentle and the signal change in the transmission line is made gentle as shown in FIG. ing. As a result, it is possible to reduce the influence of crosstalk on the transmission path. In addition,
Since the capacitor 21 is only added inside, the external impedance does not change. Further, the capacitor 21 can obtain the same effect as the above not only when the proposed circuit shown in FIG. 4 is improved, but also when applied to the proposed circuit itself.

[0016]

According to the present invention, one of the two tri-state gates which has been conventionally provided is replaced with a circuit including a resistor and a diode, so that the number of parts can be reduced as compared with the conventional one. Benefits are obtained. In addition, there is an advantage that the signal change in the transmission line can be moderated only by inserting a capacitor at the reference point, and the influence of crosstalk in the transmission line can be reduced.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram for explaining another embodiment of the present invention.

FIG. 3 is a waveform diagram for explaining the proposed method.

FIG. 4 is a circuit diagram showing a proposed circuit.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Transmission circuit, 11, 14 ... Operational amplifier, 12 ... Voltage reference, 13, 18, 19, 20, 23, 24, 2
5 ... Resistor, 15, 16 ... Tristate gate, 17 ...
Diode, 21 ... Capacitor.

Claims (2)

[Claims] 1. A signal transmission circuit for generating a current change having a waveform corresponding to a signal voltage waveform at a certain reference point inside and transmitting the signal to the outside, wherein the signal is turned on or off depending on whether the signal is transmitted or not. Between the tri-state element and the reference point, a circuit in which another resistor is connected in parallel to a series circuit of a diode and a resistor is inserted,
A signal transmission circuit characterized in that the voltage at the reference point is changed by turning on and off the diode according to a high level and a low level of the signal. 2. In a signal transmission circuit for transmitting a signal to the outside by causing a current change of a waveform corresponding to a signal voltage waveform at a certain reference point inside, a capacitor is connected to the reference point, and a signal at that point is connected. A signal transmission circuit characterized by gradual change in voltage waveform.