JP2001007741A - Disconnection detecting circuit for differential transmission path - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a disconnection detecting circuit suitable for monolithic integration without using photocouplers. SOLUTION: This circuit is provided with two-wire transmission lines 3 and 4 and constituted so that a signal can be transmitted to a line receiver 2 by a line driver 1. In this case, this circuit is provided with a diode bridge 11 connected between the transmission lines 3 and 4 and a terminator resistor 5 to which a voltage full-wave rectified by the diode bridge 11 is impressed. Moreover, this circuit is provided with a comparator 14 for comparing the voltage impressed to the terminator resistance 5 with a prescribed value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for detecting disconnection of an operation transmission line.

[0002]

2. Description of the Related Art As a conventional disconnection detection circuit for an operation transmission line, for example, a current flowing through a terminator resistor bridged between two transmission lines as described in Japanese Utility Model Publication No. 7-15219 is disclosed. Was detected by a photocoupler. FIG. 6 is a circuit diagram of a disconnection detection circuit invented in Japanese Utility Model Publication No. Hei 7-15219. In the figure, 1 is a line driver and 2 is a line receiver. Line driver 1
The first transmission line 3 and the second transmission line 4 are connected between the first line 2 and the line receiver 2. Pulses in both positive and negative directions are respectively transmitted to the first transmission line 3 and the second transmission line 4 in combination. Reference numeral 5 denotes a terminator resistor bridged between the first transmission line 3 and the second transmission line 4, and a photocoupler 6 intervenes between the terminator resistor 5 and the first transmission line 3 so that the terminator resistance is provided. 5 is detected. In a normal state, a current always flows in either direction through the terminator resistor 5, so that the photocoupler 6 is always in the ON state. When the photocoupler 6 is in the ON state, the output terminal of the photocoupler 6 is kept at the L level because the output terminal of the photocoupler 6 is connected in series with the resistor 7 on the power supply side. When the first transmission line 3 or the second transmission line 4 is disconnected, the photocoupler 6 is turned off, and the output signal a goes high.

[0003]

However, since the conventional disconnection detection circuit uses a photocoupler, the CTR
There is a problem of deterioration. In addition, since the manufacturing process of the photocoupler is different from the normal IC manufacturing process, there is a problem that not all of the disconnection detection circuit can be realized on the IC. Accordingly, an object of the present invention is to provide a disconnection detection circuit suitable for monolithic operation without using a photocoupler.

[0004]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a disconnection detection circuit for a differential transmission line having a two-wire transmission line and transmitting a signal from a line driver to a line receiver. A diode bridge connected between the transmission lines, a terminator resistor to which a voltage that is full-wave rectified by the diode bridge is applied, and a comparator that compares a predetermined voltage with a voltage applied to the terminator resistor. is there. A first transistor having a terminator resistor and a base connected to one end of the terminator resistor and having an emitter connected to one end of the transmission line; a first transistor having a base connected to the other end of the terminator resistor and having an emitter connected to the other end of the transmission line; , A bypass means for flowing a reverse current from the respective emitters of the first and second transistors to the base, and a current detection for detecting a current flowing to the collectors of the first and second transistors Means. Also,
The bypass means is a diode. Further, the bypass means is a third and a fourth transistor having a reverse junction to the first and the second transistors, wherein the base and the emitter of the first and the third transistors are connected. The bases and emitters of the second and fourth transistors are connected, and the collectors of the third and fourth transistors are grounded via resistors. Also, a first terminator resistor having one end connected to one end of the transmission line, and a base connected to the other end of the first terminator resistor, and an emitter commonly connected to the other end of the transmission line. First and second transistors connected in a reverse junction relationship to each other;
A second terminator resistor having one end connected to the collector of the transistor, a third transistor having a base connected to the other end of the second terminator resistor, a collector of the first transistor, and the third transistor And a current detecting means for detecting a current flowing through the collector. Also, a first terminator resistor having one end connected to one end of the transmission line, a first transistor having a base connected to the other end of the first terminator resistor, and a second transistor having an emitter connected. A transistor; a second terminator resistor having one end commonly connected to the emitter of the first transistor and the base of the second transistor and the other end connected to the other end of the transmission line; Current detecting means for detecting a current flowing through the collectors of the two transistors.

[0005]

FIG. 1 is a circuit diagram of a disconnection detecting circuit according to a first embodiment of the present invention. Hereinafter, the features common to the prior art will be omitted, and the features of the present invention will be described. In the figure, reference numeral 11 denotes a diode bridge composed of four diodes. The diode bridge 11 is connected to the first transmission line 3 and the second transmission line 4 and has a terminator resistor 5 connected thereto.
Is a full-wave rectifier circuit that always applies a voltage in a fixed direction.
Reference numeral 12 denotes a pull-up resistor interposed between one end of the terminator resistor 5 and the power supply, and reference numeral 13 denotes a pull-down resistor interposed between the other end of the terminator resistor 5 and ground. Reference numeral 14 denotes a comparator for comparing the voltages at both ends of the terminator resistor. Next, the function of the disconnection detecting circuit of the first embodiment will be described. When there is no disconnection between the first transmission line 3 and the second transmission line 4 and a signal is transmitted normally, the first transmission line 3
And the second transmission line 4 always has reverse positive and negative pulses, so that a circulating current always flows through the terminator resistor 5. Therefore, at this time, a constant voltage is generated at both ends of the terminator resistor 5, so that the output signal a of the comparator 14 maintains the H level. When the first signal line 3 or the second signal line 4 is disconnected, the potentials of the first signal line 3 and the second signal line 4 become equal. Becomes the L level. In this way, a disconnection is detected. Note that the pull-up resistor 12 and the pull-down resistor 13 are required depending on the characteristics of the comparator 14, and are unnecessary if a comparator that does not require pull-up and pull-down is selected.

FIG. 2 is a circuit diagram of a disconnection detecting circuit according to a second embodiment of the present invention. In the figure, reference numeral 21 denotes a first transistor. The base of the first transistor 21 is connected to one end of the terminator resistor 5, the emitter is connected to the second transmission line 4, and the collector is connected to a current detecting means 25 described later. It is connected. Reference numeral 22 denotes a second transistor. The base of the second transistor 22 is connected to the other end of the terminator resistor 5, the emitter is connected to the first transmission line 3, and the collector is connected to current detection means 25 described later. ing. Diodes 23 and 24 are bypass means for flowing a reverse current from the emitter to the base of the first transistor 21 and the second transistor 22, respectively. Reference numeral 25 denotes current detection means for detecting a current flowing through the collectors of the first transistor 21 and the second transistor 22. The current detecting means 25 includes a pull-up resistor 25a having one end connected to a power supply, and a pull-up resistor 25a.
a, the collector of the first transistor 21 and the collector of the second transistor 22, respectively, diodes 25c and 25d, and the other end of the pull-up resistor 25a via the diode 25e to the base thereof. It comprises a Darlington transistor 25f connected thereto and a capacitor 25g inserted between the other end of the pull-up resistor 25a and ground. Next, the function of the disconnection detecting circuit according to the second embodiment will be described. When there is no disconnection between the first transmission line 3 and the second transmission line 4 and a signal is transmitted normally, a current flows through the terminator resistor 5 in either the right or left direction. For example, when the signal of the first transmission line 3 is at the H level,
When the signal on the second transmission line 4 is at the L level, a current flows from the first transmission line 3 through the diode 24 and the terminator resistor 5 to the base of the first transistor 21,
Transistor 21 is turned on. In the opposite case, the second transistor 22 is turned on. As described above, when there is no disconnection, either the first transistor 21 or the second transistor 22 is turned on. Therefore, the first transistor 21 or the second transistor 22 is supplied from the power supply through the pull-up resistor 25a. A current flows through the collector of the transistor 22.
When a current flows through the pull-up resistor 25a, the voltage at the other end of the pull-up resistor 25a, that is, the voltage at point P in the drawing decreases, so that the Darlington transistor 25f is kept off. When the first transmission line 3 or the second transmission line 4 is disconnected, no current flows through the terminator resistor 5, so that both the first transistor 21 and the second transistor 22 are turned off, and the pull-up resistor 25a , The voltage at point P rises, the Darlington transistor 25f is turned on, and current flows through the collector of the Darlington transistor 25f. The current flowing through this collector functions as a disconnection detection signal. Note that the capacitor 25g is for preventing so-called whisker, which is a pulse-like noise generated when the first transistor 21 and the second transistor 22 are turned ON / OFF.

In the second embodiment, the sink current increases by the amount of current flowing to the base of the Darlington transistor 25f, so that the current flowing to one transmission line increases.
There is a problem that the balance of the current flowing through the transmission lines is very small, but broken. The third embodiment of the present invention realizes the perfect balance of the transmission line. FIG. 3 shows a third embodiment of the present invention.
FIG. 3 is a circuit diagram of a disconnection detection circuit showing the embodiment of FIG. Hereinafter, the third embodiment will be described in comparison with the second embodiment. In the figure, reference numeral 26 denotes a third transistor, which is a bypass unit in place of the diode 23 in the second embodiment. The third transistor 26 is a PNP transistor and has a reverse junction with the first transistor 21 which is an NPN transistor. The emitters of the third transistor 26 and the first transistor 21 are both connected to the second transmission line 4, and the third transistor 26
And the base of the first transistor 21 are both connected to one end of the terminator resistor 5. Reference numeral 27 denotes a fourth transistor, which is also a bypass unit which replaces the diode 24 in the second embodiment. The fourth transistor 27 is also a PNP transistor and has a reverse junction with the second transistor 22 which is an NPN transistor. The emitters of the fourth transistor 27 and the second transistor 22 are both connected to the first transmission line 3,
The bases of the fourth transistor 27 and the second transistor 22 are both connected to one end of the terminator resistor 5. The collector of the third transistor 26 and the collector of the fourth transistor 27 are connected to one end of a resistor 30 via a diode 28 and a diode 29, respectively.
The other end of 0 is grounded. Next, the function of the disconnection detecting circuit according to the third embodiment will be described. When there is no disconnection between the first transmission line 3 and the second transmission line 4 and a signal is normally transmitted, the third transistor 26 is connected to the terminator resistor 5.
Alternatively, a current flows through the fourth transistor 27,
Second transistor 22 or first transistor 2
Since 1 is kept ON, current flows in either the right or left direction. For example, when the signal of the first transmission line 3 is at the H level,
When the signal on the second transmission line 4 is at an L level, a current flows from the first transmission line 3 to the base of the first transistor 21 through the fourth transistor 27 and the terminator resistor 5, and the first transistor 21 turns ON. In the opposite case, the second transistor 22 is turned on. The collectors of the first transistor 21 and the second transistor 22 are connected to the same current detecting means 25 as in the second embodiment, and the current detecting means 25 comprises the first transistor 21 and the second transistor 22. At the same time, the disconnection detection signal is detected.

The second embodiment also has a problem that the sum of the voltage drop between the base and the emitter of the Darlington transistor 25f and the voltage drop of the diode 25e is large. The fourth and third embodiments described below aim at reducing this voltage drop. FIG. 4 is a circuit diagram of a disconnection detecting circuit according to a fourth embodiment of the present invention. Less than,
The fourth embodiment will be described. In the figure, 31 is the first
And the first terminator resistor 3
One end is connected to the second transmission line 4 and the other end is commonly connected to the bases of the first transistor 32 and the second transistor 33. The emitters of the first transistor 32 and the second transistor 33 are commonly connected to the first transmission line 3. The first transistor 32 is an NPN transistor, and the second transistor 33 is a PNP transistor.
Type transistor. That is, the first transistor 3
The second transistor 33 and the second transistor 33 are in a reverse connection relationship with each other. The collector of the second transistor 33 is a diode 3
4 is connected to one end of a second terminator resistor 35, the other end of the second terminator resistor 35 is connected to the base of a third transistor 36,
The emitter of No. 6 is grounded. The diode 34
Is an element for preventing backflow, and may be omitted. The collector of the first transistor 31 and the collector of the third transistor 36 are respectively connected to the current detecting means 25 similar to the second embodiment, and the current detecting means 25 is connected to the first transistor 32 and the third transistor. When the transistor 36 is turned off at the same time, a disconnection detection signal is detected.

FIG. 5 is a circuit diagram of a disconnection detecting circuit according to a fifth embodiment of the present invention. Hereinafter, the fifth embodiment will be described. In the figure, reference numeral 41 denotes a first terminator resistor, one end of the first terminator resistor 41 is connected to the second transmission line 4, and the other ends are the base of the first transistor 42 and the emitter of the second transistor 43. Are connected in common. The emitter of the first transistor 42 and the base of the second transistor 43 are connected to a second terminator resistor 44.
Of the second terminator resistor 44
Is connected to the first transmission line 3. The collectors of the first transistor 42 and the second transistor 43 are connected to the same current detecting means 25 as in the second embodiment.
And the current detecting means 25 is connected to the first transistor 4
When the second and second transistors 43 are simultaneously turned off, a disconnection detection signal is detected.

[0010]

As described above, according to the present invention, the disconnection detection circuit of the operating transmission path can be constituted only by discrete components such as transistors and diodes, so that it is easy to make it into an IC, that is, to make it monolithic, and to improve reliability. This is advantageous in that a disconnection detection circuit with high reliability can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a disconnection detection circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram of a disconnection detection circuit according to a second embodiment of the present invention.

FIG. 3 is a circuit diagram of a disconnection detection circuit according to a third embodiment of the present invention.

FIG. 4 is a circuit diagram of a disconnection detection circuit showing a fourth embodiment of the present invention.

FIG. 5 is a circuit diagram of a disconnection detection circuit according to a fifth embodiment of the present invention.

FIG. 6 is a circuit diagram of a disconnection detection circuit according to the related art.

[Description of Signs] 1: Line driver 2: Line receiver 3: First transmission line 4: Second transmission line 5: Terminator resistor 6: Photocoupler 7: Resistor 11: Diode bridge 12: Pull-up resistor 1
3: Pull-down resistor 14: Comparator 21: First transistor 22: Second transistor 23, 24: Diode 25: Current detecting means 25a: Pull-up resistor 25c, 25d, 25e: Diode 25f: Darlington transistor 25g: Capacitor 26: Third transistor 27: Fourth transistor 28, 29: Diode 30: Resistance 31: First terminator resistance 32: First transistor 33: Second transistor 34: Diode 35:
Second terminator resistance 36: Third transistor 41: First terminator resistance 42: First transistor 43: Second transistor 44: Second terminator resistance

Claims (6)

[Claims] 1. A disconnection detection circuit for a differential transmission line including a two-wire transmission line and transmitting a signal from a line driver to a line receiver, wherein: a diode bridge connected between the transmission lines; 1. A disconnection detection circuit for a differential transmission line, comprising: a terminator resistor to which a full-wave rectified voltage is applied, and a comparator that compares a voltage applied to the terminator resistor with a predetermined voltage. 2. A disconnection detection circuit for a differential transmission line comprising a two-wire transmission line and transmitting a signal from a line driver to a line receiver, comprising a terminator resistor and a base connected to one end of the terminator resistor. A first transistor having an emitter connected to one end of the transmission line, a second transistor having a base connected to the other end of the terminator resistor, and an emitter connected to the other end of the transmission line; and the first and second transistors And a current detecting means for detecting a current flowing through the collectors of the first and second transistors. . 3. The circuit according to claim 2, wherein said bypass means is a diode. 4. The first and third transistors, wherein the bypass means is a third and fourth transistor having a reverse junction with respect to the first and second transistors, wherein the base and the emitter of the first and third transistors are connected to each other. The bases and the emitters of the second and fourth transistors are connected to each other, and the third and fourth transistors are connected to each other.
3. The circuit for detecting disconnection of a differential transmission line according to claim 2, wherein the collector of said transistor is grounded via a resistor. 5. A disconnection detection circuit for a differential transmission line, comprising a two-wire transmission line and transmitting a signal from a line driver to a line receiver, wherein a first end of which is connected to one of the transmission lines. A first and a second terminator resistance, and a first and a second connection in which the respective bases are commonly connected to the other end of the first terminator resistance and the respective emitters are commonly connected to the other end of the transmission line. And the second transistor
A second terminator resistor having one end connected to the collector of the third transistor, a third transistor having a base connected to the other end of the second terminator resistor, a collector of the first transistor and a third transistor connected to the collector of the third transistor. A disconnection detection circuit for a differential transmission line, comprising a current detection means for detecting a current flowing through a collector. 6. A disconnection detection circuit for a differential transmission line having a two-wire transmission line and transmitting a signal from a line driver to a line receiver, wherein a first end of which is connected to one end of the transmission line. A terminator resistor, a first transistor having a base connected to the other end of the first terminator resistor, a second transistor having an emitter connected, an emitter of the first transistor and a second transistor having an emitter connected to the second transistor; A second terminator resistor having one end commonly connected to the base and the other end connected to the other end of the transmission line, and a current for detecting a current flowing through the collector of the first transistor and the collector of the second transistor A disconnection detection circuit for a differential transmission line, comprising a detection unit.