JPS6033643Y2 - half duplex digital communication equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in half-duplex digital communication equipment.

A conventional example of a half-duplex digital communication device is as shown in FIG.

In other words, a pull-up voltage EEVCC is applied to one line of a pair of two-wire communication lines at both stations via a resistor R□, and a driver is applied to the communication line to which this pull-up voltage EVCC is applied at both stations. There is a system in which the output end of the DR and the input end of the receiver RV are connected, and the common point G of the pull-up voltage Vcc in both stations is connected by the other communication line.

In such a device, when the driver DR of one station transmits a low-level active signal, the transmission end of the communication line appears to be short-circuited, and other circuits, such as those seen from the reception side, appear to be short-circuited. It will look like Figure 2.

Therefore, the input voltage of the receiver RV is 2rVCC/R1+ga, where r is the one-way resistance of the communication line.

By selecting r to be sufficiently smaller than R1, the input voltage of receiver RV is made lower than its threshold level by some margin, so that a low-level active signal is reliably received.

However, since the resistance r increases as the communication distance increases, as the communication distance increases, the margin for the threshold level decreases, making it more susceptible to noise.

An object of the present invention is to provide a half-duplex digital communication device in which the level of a low-level active signal is not affected by the resistance of a communication line or the like.

The present invention consists of a pair of two-wire communication lines, at both ends of which a pull-up voltage is applied to the first of the two lines through a resistor, and the second line is connected to a common point. means for connecting, at both ends of the communication line, respective output circuits are connected between the first line and the second line, each substantially shorting the lines when transmitting active level signals; two drivers, each consisting of a non-linear element such as a diode and a resistor connected between each line at both ends of the communication line;
two series circuits, the voltage drop of each non-linear element in these series circuits being greater than the sum of the voltage drops that occur on each of the two communication lines when the driver transmits an active level signal; The above object is achieved by a half-duplex digital communication device comprising: two series circuits connected to each other, and two receivers at both ends of the communication line, each having a high input impedance and whose respective input signals are the voltages across the resistors in the series circuit. is a distant relative.

The present invention will be explained below with reference to the drawings.

FIG. 3 is a conceptual diagram of the embodiment of the present invention.

In Figure 3, DR is a driver, RV is a receiver, and L
is a communication line.

A pull-up voltage Vcc is applied to one line of the communication line at both stations A and B through a resistor w1, and the other line of the communication line is connected to the respective common point G at both stations.

A diode D and a resistor R are installed between the communication lines at both stations.
Two series circuits are connected.

The output circuits of the drivers DR are also connected between the communication lines at both stations.

The driver DR is, for example, an open collector type TTL circuit that generates a low-level active signal.

At both stations, the voltage drop across resistor R2 becomes an input signal to each receiver RV.

The operation of the device configured in this way is as follows.

When the driver of one station generates a low-level active output signal, the transmitting end of the communication line appears to be short-circuited, and the equivalent circuit seen from the receiving end is
It will look like the figure.

That is, the series circuit of the diode D and the low-loss B2 and the reciprocating resistance of the communication line are connected in parallel between the resistance R□ and the common point G.

If we consider the current 12 flowing through the series circuit of diode D and resistor R2, and the current 12 flowing through communication line resistance Δ, the voltage-current characteristics of diode D are non-linear, so the voltage drop across communication line resistance Δ is Until the forward drop voltage of the diode D exceeds 1, the diode D can be considered to be virtually off, and virtually all of the current due to the pull-up voltage Vcc flows through the diode D.

Also, by using a receiver RV with high input impedance, it is possible to connect the resistor R from the input end of the receiver RV.
The current flowing through 2 is made extremely small.

Therefore, the voltage drop across resistor R2 becomes virtually zero, and a low-level active input signal with sufficient margin for the threshold level of receiver RV is obtained.

This state is maintained as long as diode D does not conduct.
The low level active input signal of the receiver RV is not affected by the communication line resistance and the resistance R2.

Therefore, by appropriately selecting the diode D,
The value of the communication line resistance can be increased to a large value that was previously unacceptable due to the threshold level of the receiver RV.

In other words, the distance of signal transmission can be extended.

Note that a Zener diode may be used in place of the diode D, if necessary.

When the driver DR on the transmitting side stops generating low-level active signals, the equivalent circuit seen from the receiving side becomes as shown in Figure 5. If the circuit conditions of both stations are balanced, there is no pull to the receiver RV. - The difference between the up voltage Vcc and the forward drop voltage (■) of the diode D is determined by the resistor ju1. A divided voltage is given by R2.

In addition to appropriately determining the values of the resistors R□ and R2, this voltage is set to a value sufficiently larger than the threshold level of the receiver RV.

As described above, according to the present invention, it is possible to obtain a communication device in which the level of a low-level active signal is not affected by the resistance of a communication line or the like.

[Brief explanation of the drawing]

Fig. 1 is a conceptual block diagram of the conventional example, Fig. 2 is an equivalent circuit diagram of the device shown in Fig. 1 during operation, Fig. 3 is a conceptual block diagram of the embodiment of the present invention, and Figs. 4 and 5. is an equivalent circuit diagram of the device shown in FIG. 3 when it is in operation; DR...Driver, Rv...Receiver, L...Communication line, R1, R2-...Resistance, D...Diode.

Claims (1)

[Claims for Utility Model Registration] A pair of two-wire communication lines, at both ends of which a pull-up voltage is applied to the first of the two lines through a resistor, and the second line is means for connecting each to a common point; at both ends of the communication line, respective output circuits are connected between the first line and the second line, and when transmitting active level signals, the lines are substantially connected to each other; two drivers that are short-circuited, each consisting of a non-linear element such as a diode and a resistor connected between each line at both ends of the communication line;
two series circuits, the voltage drop of each non-linear element in these series circuits being greater than the sum of the voltage drops that occur on each of the two communication lines when the driver transmits an active level signal; a half-duplex digital communication device, comprising: two series circuits arranged in series, and two receivers at both ends of the communication line, each having a high input impedance and whose input signal is a voltage across a resistor in the series circuit.