JPS611150A - Ringing preventing circuit - Google Patents

Abstract

PURPOSE:To improve the performance of noise immunity by introducing a low resistance to the input side of a reception circuit to adjust the sum of resistance value between the effective resistance of an input clamp diode and the low resistance. CONSTITUTION:The input impedance of a reception circuit Q2 is substituted equivalently by a series circuit comprising a high resistor R and a diode D1 and an input clamp diode D2. Thus, since the input impedance when the input to the circuit Q2 is a negative potential is expressed by the diode D2, the low resistance R0 is inserted to the input side to adjust the sum of resistance value between the effective resistance of the diode D2 and the low resistor R0, causing a reflection signal to be reflected into the final stable state. Since the reflection signal is produced respectively once at the reception and transmission side and converged, a reception input voltage is not larger temporarily from the final stable state and the performance of noise immunity is improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a communication device that performs digital transmission, and particularly relates to a communication device that performs digital transmission.
The present invention relates to transmission between communication devices that configure a transmitting circuit and a receiving circuit using TL elements.

[Conventional technology]

FIG. 5 is a schematic diagram showing an example of a transmission path between a communication device using conventional TTL elements to configure a transmitting circuit and a receiving circuit. The same symbols represent the same objects throughout all the figures below.

Ql in Figure 5 is a transmitting circuit using TTL elements, Q2
is a receiving circuit that similarly uses TTL elements, and the transmitting circuit and receiving circuit are metallically connected by a cable CABLE.

In a circuit configuration like this, a pulse signal is applied to the input of the transmitting circuit output Q1 to turn the output of the transmitting circuit Q1 on or off, and a pulse signal is taken from between the coreftal ground of the output of the transmitting circuit Q1. .

The output of the transmitter circuit Q1 sends the pulse signal to the cable CABLE
When outputted to , the pulse signal propagates within the cable CABLE and reaches the input end of the receiving circuit Q2 on the receiving side.

FIG. 6 is a graph explaining the characteristics of the transmitting circuit Q1 and receiving circuit Q2 used in the above circuit.

In Figure 6, the vertical axis represents voltage and the horizontal axis represents current, Vo
h indicates the voltage-current characteristic when the transmitting circuit output is "High," Vol indicates the voltage-current characteristic when the transmitting circuit output is "Low," and Iin indicates the human power characteristic of the receiving circuit.

Furthermore, the direction of current flowing from the transmitting circuit Q1 to the receiving circuit Q2 is defined as positive.

When the transmitting circuit output is "High", the output impedance is High impedance when there is no load, and as the voltage-current characteristic Voh shows, the voltage value when no load current is taken is about 3.5V at maximum (this point is H), and as the load is increased, the output voltage gradually decreases until the current reaches 70mA.
This is abbreviated as oV.

On the other hand, when the multiplier circuit output is 'Low', the output impedance is Low impedance, and the voltage-current characteristic Vol.
As shown, the voltage value when no load current is taken is about 0.3V at the lowest, and as the load is taken, the output voltage increases sequentially, and when the current exceeds 80 mA, it becomes saturated and the voltage suddenly rises.

Next, as shown in the figure, the input characteristic Iin of the receiving circuit is that the input impedance at the normal TTL level is H4gh impedance, but changes to Low impedance when the potential becomes less than a negative potential.

The circuit in Figure 5 is a cable CABLE with a characteristic impedance Z that becomes the output of a transmitting circuit Q1 with such characteristics.
The input end of the cable CABLE is connected to the terminal end of the cable CABLE, and the input of the receiving circuit Q2 having similar characteristics is connected to the terminal end of the cable CABLE.

In this case, the characteristic impedance Z of the cable CABLE at the input end of the cable CABLE does not necessarily match the output impedance of the transmitting circuit Q1;
At the termination of BLE, the characteristic impedance Z of the cable CABLE does not necessarily match the input impedance of the receiving circuit Q2.

For this reason, a reflection phenomenon occurs at the input and output ends of the cable CABLE.

Now, the output of the transmitting circuit Q1 is in the “High” state, but the output is “Lo”.
Let us consider the case where the state changes to w''.

When the output of the transmitting circuit Q1 is in a "High" state with no load, the transmitting circuit Q1 is located at point H in FIG.

As shown in Figure 7+a), when the output of the transmitting circuit Q1 changes to the "Low" state at time T1, as shown in Figure 6, the line ■ representing the characteristic/impedance 2 of the cable CABLE and the characteristic Vol. The state moves to intersection A.

On the receiving side, as shown in Figure 7 (bl), this change occurs after the propagation time τ determined by the characteristics of the cable CABLE has elapsed, and the input impedance of the input of the receiving circuit Q2 at the normal TTL level becomes High impedance. Therefore, it does not match the characteristic impedance Z of the cable CABLE, a reflection phenomenon occurs, the input impedance of the receiving circuit Q2 becomes Low impedance, and after the propagation time τ has elapsed, it returns to the transmitting circuit Ql side again.Therefore, the transmitting circuit Q1
The state moves from point A to point C.

This change in the state of the transmitting circuit Q1 from point A to point C is also transmitted to the receiving side after the propagation time τ has elapsed, causing a reflection phenomenon again.

While repeating this action, move from point H to point B to point C.
The operating points change one by one from E to F to 'G, and a stable state is reached.

However, if the degree of mismatch is large and the propagation time τ is long, the input voltage on the receiving side may temporarily become larger than the input voltage in the final stable state, as shown by F in FIG. 7(b). That is, in the case of overshooting, a ringing waveform is generated, the noise margin at the "Loiy" level is reduced, and the noise resistance is deteriorated.

[Problem that the invention seeks to solve]

The object of the present invention is to eliminate the above-mentioned drawbacks and provide a method for ensuring that no overshoot phenomenon occurs on the receiving side, that is, at least a state of critical damping.

[Means for solving problems]

A means to solve the problem is to construct a transmitting circuit and a receiving circuit using TTL elements, and when both circuits are connected by a metallic line, ringing is performed by inserting a series resistor into the input of the receiving circuit. This is achieved by a prevention circuit.

[Effect]

According to the present invention, the input impedance of the receiving circuit is Lo-
When the impedance is matched with the characteristic impedance of the cable, the reflection phenomenon is extremely small, and the input voltage on the receiving side does not temporarily become higher than the input terminal in the final stable state, resulting in the effect that noise resistance does not deteriorate.

〔Example〕

FIG. 1 is a block diagram showing one embodiment of a ringing prevention circuit according to the present invention.

FIG. 2 is a diagram explaining the present invention in detail.

The input impedance of the receiving circuit Q2 is represented by the equivalent circuit shown in FIG. That is, it is replaced by a series circuit of a high resistance R and a diode D1, and an input clamp diode D2. Note that Vcc is a power supply voltage, and IN is an input terminal.

Therefore, when the input of the receiving circuit Q2 is at a negative potential, the input impedance is expressed only by the input clamp diode D2, so a low resistance Ro is inserted on the input side as shown in Figure 1. If the sum of the effective resistance and low resistance Ra is adjusted to reflect the reflected signal to the final stable state of the signal, the reflected signal can be converged by regenerating on the receiving and transmitting sides, respectively, so the receiving input Noise resistance is improved because the voltage does not temporarily become higher than the final stable state.

FIG. 3 (al shows the transmitted output waveform according to the present invention, and (b) shows the received input waveform, respectively.

FIG. 4 is a diagram illustrating the operation of the ringing prevention circuit according to the present invention.

As shown in Figure 4, by inserting a low resistance RO on the input side, the input characteristic 1in of the receiving circuit Q2 is changed as shown by the dotted line, and the characteristic line from point A and the characteristic line from final stable point C are changed. If the corrected input characteristic Iin is made to pass through the intersection B, it is possible to move from the point B to the final stable point C without performing an Oats λ-shoot.

The transmitted output waveform in this case is shown in FIG. 3 (al), and the received input waveform is shown in FIG. 3 tb.

〔Effect of the invention〕

As explained in detail above, according to the present invention, by preventing the overshoot phenomenon from occurring on the receiving side, the "LO"
This has the great effect of preventing a reduction in the noise margin on the W'' level side.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of a ringing prevention circuit according to the present invention. FIG. 2 is a diagram explaining the present invention in detail. Fig. 3 (fa) shows the transmission output waveform according to the present invention, and (bl shows the reception input waveform). Fig. 4 is a diagram explaining the operation of the ringing prevention circuit according to the invention. Fig. 5 shows the conventional ringing prevention circuit. FIG. 6 is a schematic diagram showing an example of a transmission path between communication devices that configure a transmitting circuit and a receiving circuit using TTL elements. FIG. 6 is a graph illustrating the input and output characteristics of the TTL element. Fig. 7 is a diagram explaining the operation of Fig. 6. In the figure, Ql is a transmitting circuit using TTL elements, Ql is a receiving circuit using TTL elements, CABLE is a cable, Ro, R are each a resistor, and Dl and D2 are diodes.

Claims (1)

[Claims] 1. A ringing prevention circuit comprising a transmitting circuit and a receiving circuit using TTL elements, and when the two circuits are connected by a metallic line, a series resistor is inserted into the input of the receiving circuit.