JPH0918521A - Interface circuit for two-way communication - Google Patents

Abstract

PURPOSE: To make the switching of a transmission/reception state secure and stable as to a two-line type two-way communication. CONSTITUTION: When there is no input to a sent signal input terminal 7, a 2nd transistor(TR) 2 turns on and when a sent signal is inputted to an input/ output terminal 10 from outside and this signal exceeds the base voltage Vcc/4 of the 2nd TR 2, 1st and 4th TRs 1 and 4 turn off. The output of nearly 0(v) is generated at a received signal output terminal 15, and when a sent signal with a logic value 'High' is inputted to the sent signal input terminal 7, a 3rd TR turns on instead of the 2nd TR 2. Unless an external sent signal is applied to the input/output terminal 10 at the same time, the voltage of nearly Vcc is outputted at the received signal output terminal 15 and only when the external sent signal is inputted, the output of nearly 0(v) is outputted to the received signal output terminal 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bidirectional communication interface circuit used in a communication device or the like for transmitting / receiving various data to / from each other, and more particularly to an improvement of the interface circuit in so-called two-wire bidirectional communication. Regarding

[0002]

2. Description of the Related Art Conventionally, as a circuit of this type, for example,
As shown in FIG. 2, in a case where mutual communication devices are connected via a two-wire cable, the reference voltage of the receiving comparator is set in the transmitting state and the receiving state.
Some are configured to be switched by a switch.

That is, in the figure, the communication device 20
a and 20b are connected to each other by a so-called two-wire type connection cable 21, and each communication device 20a,
20b is provided with a bidirectional communication interface circuit S1 having the same circuit configuration so as to interface with a communication circuit (not shown).

This bidirectional communication interface circuit S
1 is a buffer amplifier (FIG. 2) for buffering and amplifying a transmission signal.
22) and a comparator (notated as “Comp” in FIG. 2) 23 for detecting the received signal.
And the changeover switch 24 as main constituent elements. The output terminal of the buffer amplifier 22 includes an output resistance (denoted as “R” in FIG. 1) 25 and an input / output terminal 2.
While being connected to the core wire of the connection cable 21 via 6a, the comparator 2 is connected to the connection point between the output resistor 25 and the core wire.
3 non-inverting input terminal (displayed as "+" in Fig. 2)
Is connected.

A switching contact 24a of a changeover switch 24 is connected to an inverting input terminal (indicated by "-" in FIG. 2) of the comparator 23, while the first of the changeover switch 24 is connected.
The fixed contact 24b of the
The voltage of / 4 is applied to the second fixed contact 24c and the voltage of Vcc / 4 is applied to the second fixed contact 24c from the second DC power supply 28. Then, the switching contact 24a corresponds to the first fixed contact 24b according to the level of the transmission signal as described later.
Alternatively, it is connected to the second fixed contact 24c.

For example, when a transmission signal is output, that is, when a signal having a logical value "High" is applied to the input of the buffer amplifier 22, its output terminal also becomes a logical value "High", and the changeover switch 24 The switching contact 24a will be connected to the first fixed contact 24b. Therefore,
The reference voltage at the non-inverting input terminal of the comparator 23 is 3V.
It becomes cc / 4.

In such a state, for the voltage at the input / output terminal 26a, the other communication device 20b is also provided with the same bidirectional communication interface circuit, and the output resistance having the same resistance value is provided at the input / output terminal. Two
Since it is connected to 6b, it becomes Vcc / 2. Therefore, the reference voltage of 3 Vcc / 4 at the inverting terminal becomes higher than the input voltage at the non-inverting input terminal, so that the output signal of the comparator 23 is maintained at the logical value “Low”, and No transmission signal of is output from the comparator 23.

On the other hand, in the case of reception, that is, when the input of the buffer amplifier 22 is set to the logical value "Low" and the transmission signal from the other communication device 20b is received, the changeover contact 24a of the changeover switch 24 is , The second fixed contact 24c is connected, and the reference voltage of the comparator 23 is Vcc / 4.
Becomes In this receiving state, the transmission signal input to the non-inverting input terminal of the comparator 23 from the other communication device 20b is Vcc / 2 as in the case where the own station was in the transmitting state first.
Therefore, the comparator 23 outputs a signal of logical value “High”, and the transmission signal is detected.

[0009]

However, in the above-mentioned conventional circuit, when the changeover switch 24 operates without causing a time delay according to the level of the transmission signal, the transmission state and the reception state are not hindered. The changeover switch 24 in the conventional circuit has a changeover contact 24a and first and second fixed contacts 24b and 24c.
Since it is a so-called mechanical contact point, there is a delay in the operation time between the change in the level of the transmission signal and the switching of the contact point. For this reason, when the level of the transmission signal changes too fast, the transmission signal of the local station is output to the comparator 23, which causes a problem that switching between transmission and reception cannot be performed smoothly.

That is, for example, the own station (communication device 20a
In the case where a transmission signal is output from the other communication device 20b immediately after the end of the transmission state from
When the transmission of the own station is completed, the changeover contact 24a of the changeover switch 24 changes from the first fixed contact 24b to the second fixed contact 24.
It is assumed that the transmission signal of the logical value “High” is input from the outside (from the communication device 20b) before the switching contact 24a is connected to the second fixed contact 24c because the timing of switching to c is delayed. In this case, the reference voltage at the inverting input terminal of the comparator 23 is still 3 Vcc / 4, while the input transmission signal from the outside is smaller than this reference voltage. `` Hig
While the signal of "h" should be output, the signal of the logical value "Low" is output, and until the switching contact 24a is reliably connected to the second fixed contact 24c, the correct reception signal is output. There was a problem that you could not get it.

The present invention has been made in view of the above situation, and provides a bidirectional communication interface circuit in which switching of transmission / reception states can be performed reliably and stably. Another object of the present invention is to provide a highly reliable bidirectional communication interface circuit in which a received signal is not lost and a transmitted signal does not leak to a receiving line.

[0012]

A bidirectional communication interface circuit according to the present invention includes a transmission signal circuit section connected to an input / output terminal via a resistor and a transmission signal circuit section connected to the input / output terminal via the resistor. And a received signal circuit section, wherein the transmitted signal circuit section buffers and amplifies a transmitted signal transmitted to the outside through the input / output terminal. On the other hand, the reception signal circuit section includes a buffer amplifier, and the reception signal circuit section includes a comparison circuit that outputs a predetermined output signal according to the level of the voltage signal at the input / output terminal. The circuit includes a transistor to which a sum of an output voltage of the buffer amplifier and a first DC voltage is applied as a bias voltage, and a second DC voltage which is higher than the first DC voltage as a bias voltage. It is configured such that the transistor operates dynamically difference is made with a reference voltage changing circuit which changes the reference voltage in response to the operation of these two transistors.

In particular, the comparison circuit has a current mirror circuit, and a reference voltage changing circuit is provided in one current path of the current mirror circuit, while the other current path is provided in the other current path.
It is preferable that a transistor is provided so as to operate differentially with the transistor forming the reference voltage changing circuit, and the base of the transistor is connected to the input / output terminal.

[0014]

When the output of the buffer amplifier is zero, the bias voltage of one transistor of the reference voltage changing circuit becomes the first DC voltage and the bias voltage of the other transistor becomes the second DC voltage larger than that. Depending on the type of transistor used, one of the transistors becomes conductive, and the reference voltage is determined according to the operating states of these two transistors.

Further, when the buffer amplifier outputs a predetermined voltage, the bias voltage of one transistor of the reference voltage changing circuit is the sum of the output voltage of the buffer amplifier and the first DC voltage, The bias voltage of the other transistor is the second DC voltage as before. And 2
The operating state of the two transistors is opposite to that when the output of the buffer amplifier is zero, and a new reference voltage is determined according to the operating state of these two transistors. Unlike the conventional method, no mechanical contact is used. That is, the reference voltage is electronically changed, and the delay in setting the reference voltage is extremely small as compared with the related art, and a reliable comparison operation can be obtained.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A bidirectional communication interface circuit according to the present invention will be described below with reference to FIG. The bidirectional communication interface circuit according to the present embodiment is used in a communication device (not shown). A so-called two-wire type connection cable is connected to input / output terminals, and a signal is transmitted / received to / from another communication device (not shown). Is to be switched.

The bidirectional communication interface circuit in this embodiment is mainly composed of a so-called Karen mirror circuit connected as a load to a differential amplifier circuit.
The circuit configuration will be specifically described below. First, the transmission signal input terminal 7 is applied with a transmission signal from a circuit for generating a transmission signal (not shown) of the communication device. The transmission signal input terminal 7 is a transmission signal circuit unit for buffering and amplifying the transmission signal. Buffer amplifier ("Buff" in Fig. 1)
Connected to the input terminal 8). The output terminal of the buffer amplifier 8 is connected to the input / output terminal 10 via the output resistance (resistance value R) 9.

Further, a pnp type first transistor (denoted as "Q1" in FIG. 1) 1, a pnp type second transistor (denoted as "Q2" in FIG. 1) 2, and pn
The p-type third transistor (denoted as “Q3” in FIG. 1) 3 constitutes a differential amplifier circuit, while the npn-type fourth and fifth transistors (“Q4” in FIG. 1,
"Q5") and 5 form a so-called current mirror circuit and are connected so as to be a load of a differential circuit including first, second and third transistors 1, 2 and 3. There is.

A circuit portion composed of the first to fifth transistors 1 to 5 and a sixth transistor 6 which will be described later functions as a comparison circuit as a reception signal circuit portion ( Details will be described later). Also,
The portion formed by the second and third transistors 2 and 3 functions as a reference voltage changing circuit (details will be described later).

That is, the emitters of the first, second and third transistors 1, 2 and 3 are both connected to the constant current source 11 so that a constant current is supplied. In addition, the collector of the first transistor 1 is connected to the collector of the fourth transistor 4, and npn
The sixth type transistor (denoted as “Q6” in FIG. 1) 6 is connected to the base, while the base is connected to the input / output terminal 10.

The emitter of the fourth transistor 4 has a fifth
, And the emitters of the sixth transistors 5 and 6 are grounded, while the base is connected to the base of the fifth transistor 5. The fifth transistor 5 has a base and a collector connected to each other so as to function as a diode, and the collector is the second and third transistors 2 and 2 described above.
It is connected with 3 collectors.

The second and third transistors 2 and 3 form a differential circuit between the constant current source 11 and the collector of the fifth transistor 5. A positive side of a first DC power supply (output voltage is Vcc / 4) 12 is connected to the base, and a negative side of the first DC power supply 12 is connected to the output terminal of the buffer amplifier 8. Therefore, the sum of Vcc / 4 (v) as the first DC voltage and the output voltage of the buffer amplifier 8 is applied to the base of the second transistor 2 as the bias voltage. The base of the third transistor 3 is the second DC power source (output voltage is 3 Vcc /
4) By 13, the second DC voltage is 3 Vcc / 4
(V) is always biased.

On the other hand, the power supply voltage Vcc is applied to the collector of the sixth transistor 6 through the collector resistor 14, and the received signal output terminal 15 is connected to this collector.

Next, the circuit operation in the above configuration will be described. First, as a premise, a so-called two-wire type connection cable is connected to the input / output terminal 10 in the same manner as the conventional one (see FIG. 2), though not shown, and the other communication device having the same bidirectional communication interface circuit ( (Not shown),
It is assumed that the communication device (not shown) provided with the interface circuit for bidirectional communication shown in FIG. 1, that is, the local station is in a state capable of mutual communication.

When there is no transmission signal, that is, when the transmission signal input terminal 7 is set to the logic value "Low", the output terminal of the buffer amplifier 8 is held at the so-called ground level, and the first The negative electrode side of the DC power supply 12 is grounded. Therefore, the base of the second transistor 2 is biased to Vcc / 4, and the base potential of the second transistor 2 is lower than the base potential of the third transistor 3, so that the second transistor 2 has a lower base potential. 2 becomes conductive, while the third transistor 3 becomes non-conductive.

A predetermined current flows through the second and fifth transistors 2 and 5 due to the conduction of the second transistor 2, but the same current mirror circuit is formed by the fourth and fifth transistors 4 and 5. Currents are the first and the fourth
The current also flows to the transistors 1 and 4 of the above, and the first and fourth transistors 1 and 4 are rendered conductive.

Therefore, the base of the sixth transistor 6 has a potential near the ground potential, so that the sixth transistor 6 becomes non-conductive and the reception signal output terminal 15
Will be held at approximately the power supply voltage Vcc. That is, in the bidirectional communication interface circuit of the present embodiment, the reception signal output terminal 15 is set to the logical value “High” state when no transmission signal is input from the outside. is there.

If a transmission signal from another communication device (hereinafter referred to as "partner station") is applied to the input / output terminal 10 via a connection cable (not shown), in this case,
The other communication device is also provided with the same bidirectional communication interface circuit, and the output resistance is similarly connected to the input / output terminal thereof. The level is Vcc / 2.

Therefore, the base voltage of the first transistor 1 is equal to the base voltage of the second transistor 2 (Vcc /
The voltage becomes Vcc / 2, which is higher than that in 4), the first transistor 1 becomes non-conductive, and no current flows in the first and fourth transistors 1 and 4. For this reason, the base voltage of the sixth transistor 6 becomes substantially near the power supply voltage Vcc, and the sixth transistor 6 becomes conductive. Therefore, the reception signal output terminal 15 has a logic value " A voltage of substantially zero (v) as "Low" appears.

In other words, in other words, in the above-mentioned state, the base voltage Vcc / 4 (v) of the second transistor 2
Is a reference voltage as a comparison circuit, and when a transmission signal is externally applied to the input / output terminal 10, the base voltage of the first transistor 1 exceeds the reference voltage, and as a result, the reception signal is received. The signal as described above is obtained at the output terminal 15.

Next, when a transmission signal is output from the own station to the other station, that is, when a signal having a logical value "High" is applied to the transmission signal input terminal 7, the output terminal of the buffer amplifier 8 is It is approximately the power supply voltage Vcc. As a result, the negative side of the first DC power supply 12 becomes approximately the power supply voltage Vcc, and as a result, the base voltage of the second transistor 2 becomes approximately (Vcc +
It becomes Vcc / 4). Further, the base voltage of the first transistor 1 becomes Vcc / 2 in consideration of the output resistance of the partner station connected through the connection cable (not shown) connected to the input / output terminal 10.

Therefore, when the base voltages of the second and third transistors 2 and 3 are compared with each other, the base voltage (3Vcc / 4) of the third transistor 3 is lower than the base voltage of the second transistor 2. Therefore, the second transistor 2 becomes non-conductive while the third transistor 3
Becomes conductive, and the third and fifth transistors 3, 5
A predetermined current will flow through.

The currents flowing through the third and fifth transistors 3 and 5 also flow into the first and fourth transistors 1 and 4 by the action of the current mirror circuit formed by the fourth and fifth transistors 4 and 5. It will be. Therefore,
Since the fourth transistor 4 is in a conductive state, the sixth transistor 6 is in a non-conductive state, and the reception signal output terminal 15 has a logical value “High” indicating that there is no reception signal from the outside. That is, the power supply voltage Vcc is output.

Therefore, in this case, the base voltage 3 of the third transistor 3 is used as a new reference voltage for the comparison circuit.
It can be said that Vcc / 4 (v) is set. Then, in such a state, if a transmission signal from the partner station is applied to the input / output terminal 10 via a connection cable (not shown), the base voltage of the first transistor 1 becomes equal to the transmission signal of the own station. Since it is the sum of the signal transmitted from the partner station, it is approximately Vcc. Therefore, the first
The base voltage of the first transistor 1 exceeds the base voltage (3Vcc / 4) of the third transistor 3, so that the first transistor 1 becomes non-conductive.

Therefore, the fourth transistor 4 also becomes non-conductive, and the sixth transistor 6 becomes conductive because the base voltage becomes approximately Vcc. Therefore, the reception signal output terminal 15 is in a substantially zero (v) state as the logical value "Low" according to the transmission signal from the partner station.

That is, even if the own station and the partner station simultaneously output the transmission signal of the logical value "High", only the signal corresponding to the transmission signal of the partner station is output from the reception signal output terminal 15 of the own station. Is output and the transmission signal of the local station does not appear.

In the above embodiment, the second and third
The differential circuit portion formed by the transistors 2 and 3 serves as a reference voltage changing circuit, and serves as one current path of the current mirror circuit formed by the fourth and fifth transistors 4 and 5, that is, the fifth
The configuration is provided on the collector side of the transistor 5. Further, the first transistor 1 operates so as to operate differentially with the second and third transistors 2 and 3 that function as a reference voltage changing circuit, that is, the first current path of the current mirror circuit, that is, the first current path. 4 is provided on the collector side of the transistor 4.

It should be noted that the transistor types and types in the above-described embodiment are merely examples and are not limited to the types and types in this embodiment, and other types can be used as long as the same operation can be obtained. Of course, the circuit may be configured by using transistors of different types.

[0039]

As described above, according to the present invention,
By changing the reference voltage of the comparison circuit, in the interface circuit for bidirectional communication, which switches the operation between the receiving state and the transmitting state, the reference voltage can be switched without using a mechanical switch. With the configuration, since there is almost no delay in switching as in the conventional case, it is possible to provide a bidirectional communication interface circuit in which switching of the transmission / reception state is performed reliably and stably. Further, since there is almost no delay in switching the reference voltage in the comparison circuit, it is possible to prevent the reception signal from being lost or the transmission signal from leaking to the reception line due to the delay in switching, and thus the highly reliable bidirectional communication interface. A circuit can be provided.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an example of a bidirectional communication interface circuit according to the present invention.

FIG. 2 is a circuit diagram showing an example of a conventional bidirectional communication interface circuit.

[Explanation of symbols]

 7 ... Transmission signal input terminal 8 ... Buffer amplifier 9 ... Output resistance 10 ... Input / output terminal 12 ... First DC power supply 13 ... Second DC power supply 15 ... Reception signal output terminal

Claims (2)

[Claims] 1. A bidirectional circuit comprising: a transmission signal circuit section connected to an input / output terminal via a resistance; and a reception signal circuit section connected to the input / output terminal via the resistance. In the communication interface circuit, the transmission signal circuit unit includes a buffer amplifier that buffers and amplifies a transmission signal transmitted to the outside through the input / output terminal, while the reception signal circuit unit is A comparison circuit for outputting a predetermined output signal in accordance with the level of the voltage signal at the input / output terminal, wherein the comparison circuit includes an output voltage of the buffer amplifier and a first DC voltage. And a transistor to which a second DC voltage higher than the first DC voltage is applied as a bias voltage are configured to operate differentially. One of the two-way communication interface circuit characterized by comprising a reference voltage changing circuit which changes the reference voltage in response to the operation of the transistor. 2. The comparison circuit has a current mirror circuit, and a reference voltage changing circuit is provided in one current path of the current mirror circuit, while the reference voltage changing circuit is provided in the other current path. 2. The bidirectional communication interface circuit according to claim 1, wherein a transistor is provided so as to operate differentially with the constituting transistor, and the base of the transistor is connected to the input / output terminal.