Differential signal transmitting apparatus and differential signal receiving apparatus

A differential signal transmitting apparatus for transmitting a differential signal through two transmission lines. The apparatus includes: transmitting-side board connecting terminals that are provided for each of the transmission lines, and are able to connect a transmitting circuit for transmitting a differential signal; receiving circuit connecting terminals that are provided at a far end of a daisy-chain connection extending from the transmitting-side board connecting terminals provided for each of the transmission lines, and connect the receiving circuit for receiving the differential signal; and an undefined-logic preventing circuit that is connected at a near end of the daisy-chain connection and outputs a prescribed potential difference to the receiving circuit connecting terminals when the transmitting circuit is not connected to the transmitting circuit connecting terminals.

BACKGROUND ART

In order to transmit high-frequency signals such as clock signals, the differential signal transmission is utilized, in which two signals of different polarities are transmitted.

FIG. 7is a circuit diagram showing an example of the conventional differential signal transmitting apparatus. The differential signal transmitting apparatus has a transmitting-side board10and a receiving-side board11. The transmitting-side board10has a transmitting circuit1, a transmission line4a(positive side), and a transmission line4b(negative side). The receiving-side board11has a receiving circuit2, a transmission line5a(positive side), a transmission line5b(negative side), a matching-termination circuit6, a transmitting-side board connecting terminal7a(positive side), and a transmitting-side board connecting terminal7b(negative side). The transmitting circuit1and the receiving circuit2are each constituted by, for example, a large scale integrated (LSI) circuit.

While the transmitting-side board10remains connected to the receiving-side board11via the transmitting-side board connecting terminals7aand7b, the positive signal included in the differential signal transmitted from the transmitting circuit1is input to the positive side of the receiving circuit2via the transmission line4a, transmitting-side board connecting terminal7a, transmission line5aand the positive side of the matching-termination circuit6. Similarly, the negative signal included in the differential signal transmitted from the transmitting circuit1is input to the negative side of the receiving circuit2via the transmission line4b, transmitting-side board connecting terminal7b, transmission line5band the negative side of the matching-termination circuit6.

As shown in this figure, in the differential signal transmitting circuit, the transmitting circuit1and the receiving circuit2are mounted on different boards, and transmission lines5aand5band the matching-termination circuit6are mounted on the receiving-side board11. In the differential signal transmitting circuit, the positive and negative sides of the receiving circuit2are set to the same potential, rendering the differential potential of the circuit2undefined, if the transmitting circuit1is disconnected from the receiving circuit2because of the expansion slots. In this case, a through current may flow in the receiving circuit2or an undefined logic state may propagate into the receiving circuit2. Consequently, a logic problem may arise.

The undefined logic can be avoided if the receiving circuit is a single-end receiving circuit merely by connecting a pull-up resistor of high resistance or a pull-down resistor of high resistance to the input. However, the undefined logic cannot be avoided in the differential signal transmitting circuit if the matching-termination circuit6is provided on the differential signal receiving side.

As a conventional technique related to the present invention, a circuit is available, in which a selector connects a pull-up resistor and a pull-down resistor to the positive and negative sides of the differential input of the receiving circuit when the transmitting circuit is found not connected, and disconnects these resistors from the differential input when the transmitting circuit is found not connected. (See, for example, Patent Document 1.)

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

With the technique disclosed in Patent Document 1, however, it is necessary to use a dedicated external circuit having the function of a selector or the like, or to use a receiving circuit having the function of a selector or the like. This may increase the cost.

The present invention has been made to solve the problem described above. An object of the invention is to provide a differential signal transmitting apparatus and a differential signal receiving apparatus, in which the receiving circuit to and from which the transmitting circuit can be connected and disconnected is prevented from malfunctioning.

Means for Solving the Problems

To solve the above-mentioned problem, a differential signal transmitting apparatus according to the present invention transmits a differential signal through two transmission lines. The apparatus includes: transmitting circuit connecting terminals that are provided for each of the transmission lines, and can connect a transmitting circuit for transmitting the differential signal; receiving circuit connecting terminals that are provided at a far end of a daisy-chain connection extending from the transmitting circuit connecting terminals provided for each of the transmission lines, and connect a receiving circuit for receiving the differential signal; and a potential-difference output circuit that is connected to a near end of the daisy-chain connection extending from the transmitting circuit connecting terminals provided for each of the transmission lines, and output a prescribed potential difference to the receiving circuit connecting terminals when the transmitting circuit is not connected to the transmitting circuit connecting terminals.

The differential signal transmitting apparatus according to the present invention is characterized in that the potential-difference output circuit increases an output impedance at the receiving circuit connecting terminals when the transmitting circuit is connected to the transmitting circuit connecting terminals.

The differential signal transmitting apparatus according to this invention further includes a transmitting circuit detecting unit, which is connected to the transmitting circuit, together with the transmitting circuit connecting terminals, and outputs a signal indicating whether the transmitting circuit is connected to the transmitting circuit connecting terminals by acquiring a signal from the transmitting circuit and, the potential-difference output circuit switches the output to the receiving circuit connecting terminals in accordance with a signal supplied from the transmitting circuit detecting unit.

The differential signal transmitting apparatus according to the present invention is characterized in that the potential output circuit includes enable buffers provided for each of the transmission lines, and receives fixed logics as input signals of the buffers and receives a signal from the transmitting circuit detecting unit as an enable signal, and the enable buffers become enabled when the transmitting circuit is not connected to the transmitting circuit connecting terminals.

The differential signal transmitting apparatus according to the present invention is characterized in that the enable buffers are tri-state buffers.

The differential signal transmitting apparatus according to the present invention is characterized in that the enable buffers are bi-directional buffers.

A differential signal receiving apparatus according to the present invention receives a differential signal through two transmission lines. The apparatus includes: transmitting circuit connecting terminals that are provided for each of the transmission lines, and can connect a transmitting circuit for transmitting the differential signal; a receiving circuit that is provided at a far end of a daisy-chain connection extending from the transmitting circuit connecting terminals provided for each of the transmission lines, and receives the differential signal; and a potential-difference output circuit that is connected to a near end of the daisy-chain connection extending from the transmitting circuit connecting terminals provided for each of the transmission lines, and outputs a prescribed potential difference to the receiving circuit connecting terminals when the transmitting circuit is not connected to the transmitting circuit connecting terminals.

The differential signal receiving apparatus according to the present invention is characterized in that the potential-difference output circuit increases an output impedance at the receiving circuit connecting terminals when the transmitting circuit is connected to the transmitting circuit connecting terminals.

The differential signal receiving apparatus according to the present invention further includes a transmitting circuit detecting unit, which is connected to the transmitting circuit, together with the transmitting circuit connecting terminals, and outputs a signal indicating whether the transmitting circuit is connected to the transmitting circuit connecting terminals by acquiring a signal from the transmitting circuit and, the potential-difference output circuit switches the output to the receiving circuit connecting terminals in accordance with a signal supplied from the transmitting circuit detecting unit.

The differential signal receiving apparatus according to the present invention is characterized in that the potential output circuit includes enable buffers provided for each of the transmission lines, and receives fixed logics as input signals of the buffers and receives a signal from the transmitting circuit detecting unit as an enable signal, and the enable buffers become enabled when the transmitting circuit is not connected to the transmitting circuit connecting terminals.

The differential signal receiving apparatus according to the present invention is characterized in that the enable buffers are tri-state buffers.

The differential signal receiving apparatus according to the present invention is characterized in that the enable buffers are bi-directional buffers.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, preferred embodiments of the present invention will be described by referring to the accompanying drawings.

First, the configuration of a differential signal transmitting apparatus according to this embodiment will be described.

FIG. 1is a circuit diagram showing an example of the configuration of the differential signal transmitting apparatus according to this embodiment. InFIG. 1, the components identical or equivalent to those shown inFIG. 7are designated by the same reference numerals. The component identical or equivalent will not be described here. Unlike the apparatus ofFIG. 7, the apparatus ofFIG. 1has a receiving-side board20, in place of the receiving-side board11. Unlike the receiving-side board11, the receiving-side board20has some additional components, i.e., an undefined-logic preventing circuit31, a transmission line32a(positive side), a transmission line32b(negative side), and a transmitter-side detecting terminal33.

An undefined-logic preventing circuit31is connected to the near end of the daisy-chain connection extending from transmitting-side board connecting terminals7aand7b, and a matching-termination circuit6and a receiving circuit2are connected to the far end thereof via the transmission lines32aand32b.

The transmitter-side detecting terminal33is used as a connection notifying signal such as connect. In this instance, the terminal33is held at a prescribed potential as long as the transmitting-side board10remains not connected to it. When the transmitting-side board10is connected to it, the transmitter-side detecting terminal33is set to the ground potential.

The undefined-logic preventing circuit31is implemented by a tri-state buffer or a bi-directional buffer. These buffers can be disabled or enabled by an output enable (OE) signal, which is a control signal.FIG. 2is a circuit diagram showing an example of the undefined-logic preventing circuit31that has a tri-state buffer.FIG. 3is a circuit diagram showing an example of the undefined-logic preventing circuit that has a bi-directional buffer.

If the OE signal is enabled, the undefined-logic preventing circuit31outputs a prescribed potential from the left, in accordance with a predetermined fixed logic input from the right. When two different fixed logics are input for the positive side and the negative side, respectively, the circuit31inputs a prescribed potential difference to the receiving circuit2. On the other hand, if the OE signal is disabled, the impedance from the left (output) becomes high, and the undefined-logic preventing circuit31is electrically disconnected from the left-side transmission line.

The operation of the differential signal transmitting apparatus will be explained below.

When power is supplied to the receiving-side board20and the transmitting-side board10is not connected to the receiving-side board20, the transmitter-side detecting terminal33is not connected and therefore has a prescribed potential. The undefined-logic preventing circuit31therefore becomes enabled. As a result, the undefined-logic preventing circuit31outputs a prescribed potential difference in accordance with the fixed logic input. The potential difference is therefore imparted to the input of the receiving circuit2so that the undefined logic is avoided.

When the transmitting-side board10is connected to the receiving-side board20, the transmitter-side detecting terminal33is connected to the ground of the transmitting-side board10. The OE signal is thereby set to the ground potential. The undefined-logic preventing circuit31is therefore inverted to be disabled. As a result, the output impedance of the undefined-logic preventing circuit31becomes high impedance. The undefined-logic preventing circuit31is therefore electrically disconnected.

The waveform of the input to the receiving circuit2, which has been determined by simulation, will be explained next.

The waveform of the input to the receiving circuit2provided in the differential signal transmitting apparatus shown inFIG. 1was simulated, on the assumption that the transmission lines4aand4bare 50 cm long and that the transmission lines32aand32bare 25 cm long.FIG. 4shows the result of this simulation, i.e. the waveform of an input to the receiving circuit2according to the embodiment. The solid line indicates the waveform of the input to the positive side, and the broken line indicates the waveform of the input to the negative side. As seen from this diagram, the input to the receiving circuit2acquires a good waveform in the differential signal transmitting apparatus according to the present embodiment.

The result of simulation, in which the undefined-logic preventing circuit31was connected in a different manner, will be explained.

FIG. 5is a circuit diagram showing an example of a differential signal transmitting apparatus in which the undefined-logic preventing circuit31is connected in a different manner. InFIG. 5, the components identical or equivalent to those shown inFIG. 1are designated by the same reference numerals. The component identical or equivalent will not be described here. Unlike the apparatus ofFIG. 1, the apparatus ofFIG. 5has a receiving-side board21in place of the receiving-side board20. The receiving-side board21is different from the receiving-side board20, in that the matching-termination circuit6and receiving circuit2are connected at the position of the undefined-logic preventing circuit31and that the undefined-logic preventing circuit31is connected at the position of the matching-termination circuit6and receiving circuit2.

That is, the matching-termination circuit6and receiving circuit2are connected to the near end of the daisy-chain connection extending from the transmitting-side board connecting terminals7aand7b, and the undefined-logic preventing circuit31is connected to the far end thereof via the transmission lines32aand32b.

The waveform of the input to the receiving circuit2provided in the differential signal transmitting apparatus shown inFIG. 5was simulated, on the assumption that the transmission lines4aand4bare 50 cm long and that the transmission lines32aand32bare 25 cm long as in the apparatus ofFIG. 1.FIG. 6is a waveform acquired by simulation, which an input to the receiving circuit2has if the undefined-logic preventing circuit is connected in a different manner. InFIG. 6, the solid line indicates the waveform of the input to the positive side, and the broken line indicates the waveform of the input to the negative side, as inFIG. 4. In this differential signal transmitting apparatus in which the undefined-logic preventing circuit31is connected to the far end of the daisy-chain connection, the input to the receiving circuit2acquires a waveform that is deformed as shown inFIG. 6, because the wiring extending from the receiving circuit2to the undefined-logic preventing circuit31works as an antenna, inevitably generating a reflection noise. In order to reduce this reflection noise, a matching-termination circuit may be connected to the undefined-logic preventing circuit3, too. In this case, two matching-termination circuits are connected, and levels of the signal waveform cannot be ensured.

A problem, such as transmission of reflection noise, does not arise in the present embodiment. This is because the undefined-logic preventing circuit31is connected to the near end of the daisy-chain connection extending from the transmitting-side board connecting terminals7aand7b, and the receiving circuit2is connected to the far end of the daisy-chain connection extending from the transmitting-side board connecting terminals7aand7b.

The differential signal transmitting apparatus according to the present embodiment can be easily adapted to the transmission of high-frequency signals, e.g., clock signals, in information processing apparatuses, and can enhance the performance of the information processing apparatuses. The information processing apparatuses include servers, work stations and personal computers.

The transmitting circuit connecting terminal corresponds to the transmitting-side board connecting terminals used in the embodiments. The receiving circuit corresponds to the receiving circuit and matching-termination circuit that are used in the embodiments. Further; the potential-difference output circuit corresponds to the undefined-logic preventing circuit used in the embodiments. The transmitting circuit detecting unit corresponds to the transmitter-side detecting terminal used in the embodiments.

INDUSTRIAL APPLICABILITY

As has been described above, the present invention can prevent a receiving circuit from malfunctioning, to and from which a transmitting circuit can be connected and disconnected.