JPH03186020A - Terminating circuit - Google Patents

Abstract

PURPOSE:To obtain a terminating circuit which controls the level of a reflected wave by making a connection with a reception terminal so that the forward voltage by a rectifying element for termination is canceled with the other rectifying element. CONSTITUTION:The rectifying element (terminating diodes D1 and D2) for termination is connected to the reception terminal 4 so that the forward voltage by the element is canceled with the other rectifying element (diodes D3 and D4). The forward voltage by the rectifying element (terminating diodes D1 and D2) for termination is canceled with the other rectifying element (diodes D3 and D4), so the ringing level of the signal at the reception terminal 4 is lowered. Consequently, the level of the reflected wave can be controlled.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a termination circuit for a transmission line in electronic equipment and the like.

[Conventional technology]

FIG. 2 is an electrical circuit diagram of a conventional termination circuit, and this termination circuit is composed of diodes. Second
In the figure, DI and D2 are terminating diodes, 1 is a driver circuit that drives a signal to the transmission line 3, and 2 is a receiver circuit at the receiving end 4 of the transmission line 3. Diode Di
are connected in the opposite direction between the power supply Vcc and the transmission line 3, and the diode D2 is connected in the opposite direction between the ground GND and the transmission line 3.

FIG. 3 is an electrical circuit diagram of another conventional termination circuit, which is constructed of resistors. In FIG. 3, a terminating resistor R1 is connected between the power supply VCC and the transmission line 3, and a terminating resistor R2 is connected between the ground GND and the transmission line 3.

For reference, FIG. 4 shows the waveform of the transmission signal at the receiving end 4 in the case of an unterminated system in which the diodes D1°D2 shown in FIG. 2 and the resistors R1 and R2 shown in FIG. 3 are not provided.

Next, the operations of these conventional examples will be explained.

When driving a long transmission line 3, if its termination is not impedance matched, reflected waves will be generated and various noises will be generated (see FIG. 4). In particular, the reflected waves become a primary cause of noise, causing unexpected troubles such as the receiver circuit 2, which is the receiving end 4, inverting its operation many times. In order to prevent ringing that occurs in an unterminated transmission line, termination with a characteristic impedance is usually performed. Figure 3 shows that the parallel resistance value (R= ) of resistor n1・81 R1 and R2 is approximately R−↑
R1 is selected so that it matches the characteristic impedance Z and the DC potential is slightly above the "H" level of the circuit logic. This can prevent ringing of the transmission signal, but the transmission line 3
For example, the characteristic impedance of the cable constituting the
Since the power consumption by the resistors R1 and R2 is large since the resistors are small (approximately 0Ω to 200Ω) (the current also flows through R1↑Ill when the driver is off), the diode DI is connected as shown in FIG.

A circuit is used that uses D2- to suppress reflection on the transmission line 3. In this case, terminating diodes DI and D2 are connected to the receiving end toward the power supply Vcc and the ground GND. This is based on the principle that the diode D1 suppresses the portion where the transmission signal potential is higher than the voltage of the power supply Vcc, and the diode D2 prevents the portion where the transmission signal potential is lower than the ground GND, so that no reflection occurs. In other words, although characteristic impedance matching is not considered, no current flows when the driver is off, which has the effect of reducing power consumption.

[Problem to be solved by the invention]

Since the conventional termination circuit is configured as described above, it is impossible to adjust the ringing level, and therefore there is a problem in that the level of the reflected wave cannot be controlled. That is, if the forward voltage of the diode is V, the signal potential S
can only be suppressed within the range of Vcc+v or less and G,,-v or more.

In other words, the forward voltage v (=0.7V) in a digital transmission line with little noise margin cannot be ignored.

The present invention was made to solve the above-mentioned problems, and an object of the present invention is to obtain a termination circuit that can control the level of reflected waves.

[Means to solve the problem]

The termination circuit according to the present invention is characterized in that it is connected to the receiving end 4 so that the forward voltage caused by the termination rectifier (termination diodes DI, D2) is canceled out by other rectifiers (diodes D3, D4). It is something to do.

[Effect]

The forward voltage due to the termination rectifier (termination diodes DI, D2) is - rectifier (diode D3', D4)
offset by Therefore, the ringing level of the signal at the receiving end 4 is reduced.

〔Example〕

FIG. 1 is an electrical circuit diagram of a termination circuit according to an embodiment of the present invention. In FIG. 1, components corresponding to those shown in FIG. 2 are designated by the same reference numerals, and their explanations will be omitted. In FIG. 1, D3 and D4 are terminating diodes (terminating rectifying elements) Dl.

A voltage adjusting diode (another rectifying element) for canceling the forward voltage caused by D2, R3 is a resistor, and C is a capacitor. A series circuit including a diode D3, a resistor R3, and a diode D4 is connected between the power supply Vcc and the ground GND. Diode D3 and diode D4 are connected in the forward direction.

Next, the operation of this embodiment will be explained. Now assume that the forward voltage of the diode DI-D4 is V. “H” of transmission signal
When the ringing at the level exceeds the potential (Vcc-v+v), the diode p1 becomes conductive, so that the current flowing through it charges the capacitor C. In other words, the signal level is Vc
Ringing is suppressed at a potential that does not deviate from the c level.

On the other hand, when the ringing (undershoot) at the "L" level of the transmission signal exceeds the potential (GND+v-v), the diode D2 becomes conductive, and the current flowing through it charges the stray capacitance of the transmission line 3. That is, the signal level is kept at a potential that does not deviate from the GND level to suppress ringing. In addition, since the diode termination method does not take into account the characteristic impedance of the transmission line 3 and the matching of the termination, the resistance R3 can be increased, and the consumption when the driver is off is 1ifi ("-""'-v(") Ham view i"
tk 4 *3 In this way, the termination circuit of the above embodiment includes voltage adjustment diodes D3. The power supply potential is lowered or raised by D4, and the clamp level of the terminating diodes DI and D2 is adjusted.

In the above embodiment, a case has been described in which the number of diodes D1 and D4 is one, but the ringing (overshoot and undershoot) level can be controlled by connecting a plurality of diodes in series. Further, although FIGS. 1 to 3 illustrate the case where there is only one transmission line, the transmission line is usually composed of a plurality of lines. For example, data bus
The address bus, timing bus, etc. are composed of multiple signal lines. Furthermore, the diodes D1 to D4 are not limited to diodes, and any nonlinear element having the same rectification function can produce the same effect.

〔Effect of the invention〕

As described above, according to the present invention, the terminating rectifying element is connected to the receiving end so as to cancel out the forward voltage caused by the other rectifying element, so that the clamp level of the terminating rectifying element is different from that of the other rectifying element. It is now possible to adjust using the forward voltage drop of This has the effect of providing good transmission characteristics.

[Brief explanation of drawings]

FIG. 1 is an electric circuit diagram of a termination circuit according to an embodiment of the present invention, FIGS. 2 and 3 are electric circuit diagrams of a conventional termination circuit,
FIG. 4 is a waveform diagram of a transmission signal at the receiving end in the case of no termination. 3... Transmission line, 4... Receiving end, Dl. D2... Termination diode (termination rectifier), D3
．． D4...Voltage adjustment diode (other rectifying element).

Claims (1)

[Claims] In a terminating circuit in which a terminating rectifying element is connected to the receiving end of a transmission line to suppress reflection on the transmission line, another rectifying element is connected to the receiving end to cancel the forward voltage caused by the terminating rectifying element. A termination circuit characterized by being connected.