JP2723028B2 - Signal output circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal output circuit, and more particularly to a signal output circuit for outputting a signal to a bus transmission line.

[0002]

2. Description of the Related Art Prior to the description of the prior art, the general structure of a bus transmission line will be described with reference to the drawings. Referring to FIG. 3, a plurality of bus transmission lines 33 are provided on the motherboard 30. Also, on the motherboard 30,
A plurality of connectors 31 are attached. Connector 3
A substrate 32 is connected to 1. The circuit inside the board 32 is connected to the bus transmission line 33 by being connected to the connector 31. Each board 32 transmits and receives signals via a bus transmission line 33.

The characteristic impedance of the bus transmission line 33 is
It depends on various factors. The main factors are a change in the number of boards 32 mounted on the connector 31 and a manufacturing error. Among them, the change in the characteristic impedance depending on the number of mounted substrates 32 will be described with reference to the drawings.

In the structure shown in FIG. 3, the number of boards 32 connected to the bus transmission line 33 is not fixed. For example, FIG.
In (a), the board 32 is connected only to the connectors 31 at both ends. The number of mounted substrates 32 is two. on the other hand,
In FIG. 4B, the boards 32 are connected to all the connectors 31. The number of boards 32 to be mounted is seven.

When the number of mounted boards changes,
The characteristic impedance of the bus transmission line 33 changes. To show this, the characteristic impedance of each structure in FIGS. 4A and 4B is approximately calculated.

In the structure shown in FIG. 3, the distance between the connectors 31 is 1 inch (2.54 cm), and the characteristic impedance of the bus transmission line 33 when the board 32 is not mounted is Z0 = 7.
5Ω, and the propagation delay time is t = 7 ns / m. At this time, the inductance component L0 and the capacitance component C0 are 13.5 nH / inch and 2.3, respectively.
It can be approximately calculated as 6 pF / inch.

Here, 25 pF for one substrate 32
Is increased. Then, the characteristic impedance Z1 of the bus transmission line 33 in the structure of FIG. 4A can be calculated as 35.2Ω. In addition, the characteristic impedance Z2 of the bus transmission line 33 in the structure of FIG.
It can be calculated as 0.5Ω. That is, when five substrates 32 are further mounted on the structure of FIG. 4A to obtain the structure of FIG. 4B, the characteristic impedance is reduced by 14.7Ω.

In the above, for easy explanation, the change in the characteristic impedance was theoretically calculated using approximation. However, in reality, it is difficult to predict a change in characteristic impedance in advance. For example, the characteristic impedance changes depending not only on the number of the boards 32 but also on the place where the boards 32 are mounted.

As described above, the characteristic impedance of the bus transmission line 33 changes due to various factors. Then, with the change in the characteristic impedance, the signal waveform propagating on the bus transmission line 33 also changes.

For example, in the case of a pulse wave, the bus transmission path 33
Is too large, the rise time of the pulse becomes long. If the characteristic impedance is too small, ringing occurs. Ringing is a transient vibration of a waveform caused by a sudden rise of a pulse. Ringing causes a malfunction.

By the way, in order to propagate a signal at a high speed, the signal must be kept in a constant waveform. For this purpose, it is necessary to correct the characteristic impedance of the bus transmission line 33 to a constant value.

FIG. 5 shows an example of a signal output circuit conventionally used to solve such a problem. Referring to FIG. 5, a resistor 51 is connected between the output unit 11 that outputs a signal and the bus transmission line 33. In this structure, by changing the resistance value of the resistor 51, the characteristic impedance can be adjusted, and the signal waveform can be shaped to a desirable one. The resistor 51 is determined according to the characteristic impedance of the bus transmission line 33.

[0013]

As described in the section "Change in characteristic impedance of bus transmission line 33", the characteristic impedance changes depending on the mounting state of the board 32 and the like. Therefore, the resistance value of the resistor 51 must be determined each time according to the status of the bus transmission line 33.

It is difficult to predict the characteristic impedance theoretically. Therefore, the resistance value of the resistor 51 also becomes
Must be determined experimentally.

As described above, the above-described prior art has a problem that operability is poor.

[0016]

In order to solve such a problem, a signal output circuit according to the present invention is provided with an output unit for outputting a signal to a bus transmission line, and provided between the output unit and the bus transmission line. Input a selection signal, and the selection signal has a specific pattern.
Sometimes a constant resistance value is generated, while other patterns are conductive
And a plurality of resistor blocks,
Blocks are connected in series and the resistances of the plurality of resistance blocks
When the lowest one of the values is the lowest resistance value,
Each block has a minimum resistance of 2
It is characterized by a multiplied resistance value .

[0017]

Next, a first embodiment of the present invention will be described with reference to the drawings.

Referring to FIG. 1, the first embodiment of the present invention includes an output unit 11 for outputting a signal. The output signal of the output unit 11 becomes the input signal of the variable resistance unit 12. The variable resistance section 12 also inputs a selection signal 123. The variable resistance section 12 generates a resistance having a resistance value according to the selection signal 123 on a transmission path between the output section 11 and the bus transmission path 33. The signal that has passed through the variable resistance unit 12 is
Sent up.

The variable resistor section 12 includes a resistor 121, a resistor 124, and a selector 122. Resistor 1
The output unit 11 is connected to the resistor 21 and the resistor 124. The signal that has passed through the resistors 121 and 124 becomes the input signal of the selector 122. The selector 122 is
In addition, a selection signal 123 is input. Selector 122
Selects and outputs one of the output of the resistor 121 and the output of the selector 122 in accordance with the resistor 124. The signal output from the selector 122 is the output signal of the variable resistance unit 12.

The resistors 121 and 124 having different resistance values are selected. Specifically, a resistor that corrects the characteristic impedance of the bus transmission line 33 when it is too large is selected as the resistor 121. The resistor 124 is selected to correct the characteristic impedance of the bus transmission line 33 when the characteristic impedance is too small. The resistance values of the resistor 121 and the resistor 124 are determined from the expected change width of the characteristic impedance.

When this signal output circuit is used, one of the resistors 121 and 124 is selected according to the characteristic impedance of the bus transmission line 33. The selection of the resistor is performed by the selection signal 123.

The generation of the selection signal 123 may be performed manually. That is, it may be switched by a switch. Further, the selection signal 123 may be automatically generated. That is, the board 3 connected to the bus transmission path 33
2 may be provided with a circuit for monitoring the mounting status and generating the selection signal 123 accordingly. Here, the mounting status of the board 32 refers to the number of mounted boards 32, the mounting position of the board 32, and the like.

In FIG. 1, two types of resistors are connected to the selection signal 123. However, the scope of the present invention is not limited to this. Any number of resistors can be used. However, each resistor must have a different resistance value.

In the first embodiment, the resistance value between the output unit 11 and the bus transmission line 33 is changed by the selection signal 123, so that the effect of good operability can be achieved.

Next, a second embodiment of the present invention will be described with reference to the drawings.

The feature of the second embodiment resides in the internal structure of the variable resistance section 12, and the other structures are the same as those of the first embodiment.

Referring to FIG. 2, the variable resistance section 12 according to the second embodiment of the present invention comprises resistance blocks 21 to 23. The resistance blocks 21 to 23 are connected in series. The output signal of the output unit 11 is
, And is sent out to the bus transmission line 33 in order.

The structures of the resistance blocks 21 to 23 are the same except for the resistance values of the internal resistors 211, 221 and 231. For example, the internal structure of the resistance block 21 is as follows.

The resistor block 21 includes a resistor 211 and a selector 212. The resistance block 21 receives an output signal of the output unit 11. The received output signal is
The signal is sent to the resistor 211 and the selector 212. Resistor 2
The resistance value of 11 is R211. The signal that has passed through the resistor 211 is input to the selector. In addition, the selector 21
2 inputs the selection signal 213.

The selector 212 selects one of the output signal of the resistor 211 and the output signal of the output unit 11 according to the selection signal 213. The selected signal is sent to the resistance block 22.

As described above, the resistance block 21 can generate and eliminate the resistance value R211 on the transmission line between the output unit 11 and the bus transmission line 33 according to the selection signal 213.

The resistance block 22 and the resistance block 23
Are configured similarly to the resistance block 21. However, the resistance value R of the resistor 221 in the resistance block 22
221 is twice as large as R211. The resistance value R231 of the resistor 231 in the resistance block 23 is R21
4 times 1.

The variable resistance section 12 configured as described above
Can generate a multi-stage resistance value on the transmission path between the output unit 11 and the bus transmission path 33 in accordance with the selection signals 213, 223, and 233. Specifically, 0 to (7
× R211) can be generated in eight stages.

In the second embodiment, in addition to the effects of the first embodiment, it is possible to achieve an effect that more stages of resistance can be generated with a small number of resistors and more flexible adjustment is possible. .

[0035]

As described above, according to the present invention, the resistance value of the resistor inserted between the output section and the bus transmission line is changed by the selection signal, so that the operability is higher than that of the prior art. A good effect can be achieved.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a second embodiment of the present invention.

FIG. 3 is a diagram showing a structure of a bus transmission line 33.

FIG. 4 is a diagram showing that the impedance changes depending on the mounting state of the substrate 32;

FIG. 5 is a diagram showing a conventional technique.

[Explanation of symbols]

 11 output section 12 variable resistance section

Claims (5)

(57) [Claims] An output unit for outputting a signal to a bus transmission line; and a selection signal provided between the output unit and the bus transmission line.
Constant resistance value when input and the selection signal is a specific pattern
Multiple resistors that generate a current and become conductive when other patterns
A plurality of resistor blocks connected in series, and
Make the lowest resistance value of the block the lowest resistance value
When the resistance value of each of the plurality of blocks is
A signal output circuit having a resistance value that is a power of 2 times the resistance value . 2. The resistance block according to claim 1, wherein the plurality of resistance blocks include a first resistance block having the minimum resistance value and a second resistance block having a resistance value twice as large as the minimum resistance value. The signal output circuit according to claim 1. 3. The plurality of resistance blocks, a first resistance block having the minimum resistance value, a second resistance block having a resistance value twice as large as the minimum resistance value, and a resistance value equal to 4 of the minimum resistance value. 2. The signal output circuit according to claim 1, further comprising a third resistance block having a double resistance value. 4. The board is connected to any one of a plurality of connectors connected to the bus transmission line, and the selection signal indicates to which of the plurality of connectors the board is connected. The signal output circuit according to any one of claims 1 to 3 , wherein the signal output circuit is determined according to: 5. The board is connected to one of a plurality of connectors connected to the bus transmission line, and the selection signal is reduced to the number of boards connected to the plurality of connectors. The signal output circuit according to any one of claims 1 to 3 , wherein the signal output circuit is determined according to the condition.