JPH02177622A - Transmission circuit - Google Patents

Abstract

PURPOSE:To prevent the communication impossibility of a transmission system due to the short-circuit of respective transmission lines by changing the bias state of the transmission line when the short-circuit is detected and changing a circuit state transmittable. CONSTITUTION:When transmission lines A and B are short-circuited, the voltage of the transmission line A is the partial voltage of bias resistors R1 and R2 to a power source voltage Vcc and boosted up to Vcc/2. Then, the base voltage of a transistor Tr 11 is made high and the transistor Tr 11 is turned on. As a result, a transistor Tr 12 is turned off and a switching circuit 5 is opened. Accordingly, when the transmission line of a transmission circuit is short- circuited, such a condition is detected by a short-circuit detecting means 6 and a bias converting means is operated. Then, the bias state of the transmission line is changed. On the other hand, the transmission/reception circuit to be connected to the transmission lines A and B detects the change of the bias state and changes the circuit state transmittable. Thus, even when the transmission lines A and B are short-circuited, the transmission can be executed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a transmission circuit connected to a data transmission system that requires high reliability.

(Prior Art) In an electrical serial data communication system between multiple electronic units, failures in the transmission medium (hereinafter referred to as transmission lines) may occur, for example, disconnection of the connector terminal of the transmission line, grounding of the transmission line, or disconnection of the power supply voltage. If a short circuit occurs, all units communicating via that transmission line will be unable to transmit to each other. Therefore, depending on the quality and quantity of data to be communicated, transmission may be possible even in such cases. It is necessary to design a highly reliable transmission system that achieves this.

Conventionally, there is a transmission system with high reliability as shown in FIG. The transmission lines A and B of this transmission system are twisted pair wires or similar balanced communication transmission lines, and the output signal D of the digital circuit (FIG. 8(a)) is transmitted by the drive transistors Tri and Tr2 of the transmission circuit 1. When the transmission line A1B of the transmission circuit 4 is driven to have an opposite phase and current flows through the bias resistors R1 and R2, the waveforms of these transmission lines A and B become as shown in FIG. These transmission lines A,
The transmission waveform generated at B passes through the DC component cut capacitors CI and C2 of the receiving circuit 2, and compares and outputs the variation of the differential voltage of each transmission line from the time of no signal as a voltage threshold, and outputs it as a digital signal. The signal is input to a comparator 3 which converts it into a signal. This comparator can receive signals even if one side of the transmission line is shorted to ground.

As a result, for example, if the switch St in FIG. 8 is closed and the transmission line A is connected to ground, the transmission line AA
, B are as shown in FIG. 10(b), and the amplitude of the voltage difference between transmission lines A and B (FIG. 10(C)) is greater than that when switch S1 is open. It becomes half. Here, by setting the threshold voltage of the comparator 3 lower than the amplitude of the waveform of one transmission line as shown by the dotted line in Figure 1O (C), the waveform of the digital signal is applied to the output terminal of the comparator 3. It is output and reproduced as a digital signal P (FIG. 10(d)). That is, it becomes possible to transmit a digital signal from the transmitting circuit 1 to the receiving circuit 2.

(Problems to be Solved by the Invention) However, the conventional transmission circuit described above can transmit when one transmission line is shorted to ground or a constant voltage, but when transmission lines A and B are shorted together, All units communicating via these transmission lines A and B may not be able to transmit to each other.Depending on the quality and quantity of the data to be communicated, there is a high level of reliability that allows transmission even in such cases. A sexual transmission system is desired. In particular, when twisted pair wires are used in a transmission circuit, since the wires are intertwined with each other, there is a high probability of short-circuiting due to external force, which is a major problem in ensuring high reliability of the transmission system.

The present invention has been made in view of the above points, and even if balanced transmission lines such as twisted pair lines are short-circuited, all units communicating via the transmission lines can perform mutual transmission. The purpose is to provide a transmission circuit that achieves the following.

(i!Means for Solving the Problem) According to the invention, in order to achieve the above object, a transmission line of the transmission circuit of an electrical serial data communication system using a balanced transmission line is provided. A short-circuit detection means for detecting a short-circuit, and a bias change means for changing a bias state of a transmission line under the control of the short-circuit detection means, and the transmission line can detect a change in the bias state and transmit data. When the circuit status changes
It is configured to connect a receiving circuit.

(Operation) The short-circuit detection means detects when the transmission line of the transmission circuit is short-circuited and activates the bias changing means to change the bias state of the transmission line.

On the other hand, the transmitting/receiving circuit connected to the transmission line detects a change in the bias state and changes the circuit state to enable transmission.

This allows transmission even if the transmission line is short-circuited.

(Example) An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

Note that circuit elements that are the same as those in FIG. 8 are given the same reference numerals.

In FIG. 1, the base of the transistor Tri of the transmitting circuit 1 is connected to the input terminal 1a through the resistor R3 and the inverter IN, and the power source Vcc is connected through the resistor R5, and the emitter is connected to the power source fiVcc through the resistor R7. The base of the transistor Tr2 is connected to the input terminal 1a via a resistor R4, the ground via a resistor R6, and the emitter to ground via a resistor R8. And the collectors of transistors Tri and Tr2 are output terminals t
b.

connected to lc.

One end of each of the transmission lines A, B of the transmission circuit 4 is connected to the output terminals 1b, lc of the transmitting circuit 1, respectively, and the other ends thereof are connected to the input terminals 2a, 2b of the receiving circuit 2. The receiving circuit 2 is
A comparator 3 is provided which can transmit data even if one of the transmission lines A or B is grounded. One input terminal (-) of this comparator 3 is connected to input terminal 2a through capacitor C1 and resistor R9, and the other input terminal (+) is connected to input terminal 2b through capacitor C2 and resistor RIO.
It is connected to the. The transmission line B4 is connected to the power supply Vcc through the resistor R1, and the transmission line A is connected to the ground through the resistor R2 and the switch circuit 5.

Between the transmission lines A and B, there is a transmission line interconnection on the input side, that is, on the output terminal 1b, lc side of the transmitting circuit 1, which detects a short circuit between the transmission lines A and B and opens the switch circuit 5. Short circuit detection circuit (hereinafter referred to as short circuit detection circuit) 6
is connected.

Further, a switch S2 is connected between the transmission lines A and B on the output side, that is, on the input terminals 2a and 2b of the receiving circuit 2.

Further, in the receiving circuit 2, a transmission line abnormal voltage detection circuit (hereinafter simply referred to as an abnormal voltage detection circuit) 7 is connected to the transmission line A on the side to which the switch circuit 5 is connected. A switch circuit 8 is connected between the connection point between the resistor R9 and the capacitor CI of the receiving circuit 2 and the ground for shorting to the ground AC or DC. This switch circuit 8 is controlled by an abnormal voltage detection circuit 7.

The action will be explained below.

When the transmission line ASB is in a normal state with no short circuit, both the short detection circuit 6 and the abnormal voltage detection circuit 7 do not operate, the switch circuit 5 is in a closed state, and the switch circuit 8 is in an open state. It has become. Therefore, in this state, the transmission circuit 4 is the same as the conventional transmission circuit shown in FIG. 8, and operates in the same manner.

Now, for example, when the switch S2 is closed to short-circuit the transmission lines A and B, the short-circuit detection circuit 6 detects the short-circuit and is activated to open the switch circuit 5. As a result,
Transmission line A is connected to power supply Vcc via switch S2 and resistor R1.
The voltage reaches the power supply voltage Vcc. In this state, when the digital signal D (Fig. 2 (a)) is input from the digital circuit to the transmitting circuit 1, the voltage of the transmission line A has increased to near the electric voltage Vcc, so the transistor Tri can be considered as being turned off. On the other hand, the transistor Tr2 is turned on, and the signal waveform of the transmission line B becomes as shown in FIG. Transmission line A is connected to transmission line B via switch S2, so the signal waveforms of transmission lines A and B are close to the normal waveform of transmission line B, as shown in Figure 2(b). .

Furthermore, as the voltage of the transmission line A becomes higher than the normal voltage range Va shown by the dotted line in FIG. 2(b), the abnormal voltage detection circuit 7 connected to the transmission line A is activated and the switch Circuit 8 is closed and transmission line A is grounded. As a result, no signal appears at the input point J of the receiving circuit 2, and the amplitude of the voltage difference between the input point J and K of the receiving circuit 2 (Fig. 2 (C))
is the same as when the transmission line A of the conventional transmission circuit shown in Fig. 8 is shorted to ground (Fig. 10), and the receiving circuit 2 outputs the signal P (Fig. 2 (d)). . Thus, the digital signal transmitted from the digital circuit is
It is reproduced as a signal P, and can be transmitted via the transmission circuit 4.

3 and 4 show specific examples of the switch circuit 5 and the short detection circuit 6 shown in FIG. Piezoresistors R11 and R12, a transistor T whose collector is connected to the power supply Vcc via a resistor R13, whose base is connected to the connection point between the voltage dividing resistors R11 and R12, and whose emitter is connected to ground.
The switch circuit 5 is constituted by a transistor Tr12 whose collector is connected to the bias resistor R2, whose base is connected to the collector of the transistor Trll of the short-circuit detection circuit 6, and whose emitter is connected to ground.

When the voltage to the transmission line is low, the voltage divided by the resistors R11 and R12 cannot turn on the transistor Trll, and the transistor Tr12 is turned on by being supplied with a base current through the resistor R13. That is,
When the transmission lines A and B are normal, the switch circuit 5 is closed.

When transmission lines A and B are short-circuited, the voltage on transmission line A becomes the voltage divided by bias resistors R1 and R2 in FIG.
The base voltage of transistor Tr11 increases.
is turned on, and as a result, the transistor Tr12 is turned off. That is, the switch circuit 5 is opened. Further, this circuit has the effect of preventing overcurrent from flowing through the bias resistor R2 and generating heat in the bias resistor R2 when the transmission line A is short-circuited to a high voltage.

FIG. 4 shows a short circuit detection circuit 6 whose base is a resistor R20.
The switch circuit 5 is composed of a transistor Tr21 whose emitter is connected to the transmission line A through the transmission line A, whose emitter is connected to the transmission line B, and whose collector is connected to the ground through the voltage dividing resistors R21 and R22. is the resistance R21
A transistor Tr22 whose emitter is connected to ground is provided at the connection point between the transistor Tr22 and R22.

When the transmission lines A and B are in a normal state, there is a sufficient potential difference between the transmission lines A and B, and the transistor Tr
21 is turned on, a base current flows through the transistor Tr22, and the transistor Tr22 is turned on. That is, the switch circuit 5 is closed.

However, transmission! When 1i1A is short-circuited across the transmission line, the potential difference between transmission lines A and B becomes 0, transistor Tr21 is turned off, and accordingly, transistor Tr21 is turned off.
22 is turned off. That is, the switch circuit 5 is opened.

5 and 6 show specific examples of the abnormal voltage detection circuit 7 and switch circuit 8 shown in FIG. It is composed of a Zener diode ZD, the cathode is connected to the connection point of the resistor R9 and the capacitor C1, and the anode is connected to the ground. When the voltage of the transmission line A reaches the Zener voltage Vz of the Zener diode ZD or higher, the Zener The diode ZD is made conductive, and the voltages at the three points are fixed at the Zener voltage Vz.

Note that this circuit also functions as a protection circuit against surge voltage input from the transmission line A.

In FIG. 6, the abnormal voltage detection circuit 7 is configured with voltage dividing resistors R31 and R32 connected between the transmission line A and the ground, and the switch circuit 8 is configured with the collector connected to the power source V through the resistor R33.
cc, a transistor Tr31 whose base is connected to the connection point of voltage dividing resistors R31 and R32, and whose emitter is connected to ground.
It is composed of the following. Note that when the voltage of the transmission line A reaches a certain voltage or higher, the voltage divided by the voltage dividing resistors R31 and R32 turns on the transistor Tr31,
Short the 3 points to ground. At this time, if you want to avoid a DC short circuit, connect the three points and the transistor Tr31.
The capacitor C33 (shown in parentheses in the figure) is connected to the collector of the capacitor C33.

Also, as shown in FIG. 7, a short detection circuit 6 is connected to each receiving circuit instead of the abnormal voltage detection circuit 7, and the transmission line A
The switch circuit 8 may be closed when a short circuit between and B is detected.

Note that the switch circuit 5, short detection circuit 6, abnormal voltage detection circuit 7, and switch circuit 8 do not necessarily need to be connected to the transmission line A side, and may be connected to the transmission line B side by changing the polarity of the circuits.

Further, even if a plurality of receiving circuits and bias circuits are connected in parallel on the transmission line of the transmission circuit of the present invention, they can be operated without any problem.

(Effects of the Invention) As explained above, according to the present invention, short circuit detection for detecting a short circuit in the transmission line of the transmission circuit of an electrical serial data communication system using a balanced transmission line is provided. and a bias changing means for changing the bias state of the transmission line under the control of the short detection means, and a transmission line for changing the circuit state to enable transmission by detecting a change in the bias state of the transmission line. By connecting the receiving circuit, in many transmission systems such as those in automobiles, it is possible to prevent the transmission system from being unable to communicate due to short circuits between the transmission lines of the balanced transmission line, and has high reliability. It becomes possible to configure a transmission system.

In addition, the transmission line short detection circuit is connected only to the bias circuit, and when the transmission line is short-circuited, the bias state is changed, and the transmitter and receiver circuits detect the bias state, allowing transmission even when the transmission line is short-circuited. Because the system changes automatically, there is no need for a system that makes decisions and controls using digital logic circuits, and it can be easily configured by adding a minimum number of circuits to the conventional transmission circuit. It has excellent effects.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a transmission system to which the transmission circuit according to the present invention is applied, FIG. 2 is a diagram showing an example of a signal waveform when the circuit in FIG. 1 is abnormal, and FIGS. 4 is a circuit diagram showing specific examples of the short circuit detection circuit and switch circuit shown in FIG. 1, respectively, and FIGS. 5 and 6 are circuit diagrams showing specific examples of the abnormal voltage detection circuit and switch circuit shown in FIG. 1, respectively. Figure 7 is a circuit diagram showing another embodiment of the transmission system in Figure 1, Figure 8 is a circuit diagram of a conventional transmission system, and Figure 9 shows the signal waveform of the circuit in Figure 8 during normal operation. Figure shown, 1st
FIG. 0 is a diagram showing signal waveforms when the circuit of FIG. 8 is abnormal. l... Transmission circuit, 3... Comparator, 4... Transmission circuit, 5.8... Switch circuit, 6... Short circuit detection circuit, 7... Abnormal voltage detection circuit.

Claims (1)

[Claims] A transmission circuit of an electrical serial data communication system using a balanced transmission line includes a short detection means for detecting a short circuit in a transmission line of the transmission circuit, and a bias state of the transmission line controlled by the short detection means. 1. A transmission circuit comprising: bias changing means for changing the bias state, and a transmitting/receiving circuit whose circuit state changes to enable transmission by detecting a change in the bias state is connected to the transmission line.