Driving circuit for transmission lines

Abstract

A driving circuit for transmission lines. The driving circuit transforms outgoing signals of an internal circuit to signals large enough to be sent to an external circuit by the transmission lines, especially for RS-232 transmission lines. It comprises three transistors, six resistors, three capacitors and one diode. The expensive signal driver chip needed in the conventional circuit is not necessary in the present invention. The expensive chip is replaced by cheap discrete components. This reduces the cost of the driving circuit.

Description

Pursuant to 35 U.S.C. §119( a )-( d ), this application claims priority from Taiwanese application no. 91100577, filed on Jan. 16, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a driving circuit for transmission lines, particularly to a driving circuit for transmission lines which is composed of discrete components so that the cost of the driving circuit is reduced.

2. Description of the Prior Art

FIG. 1 is a diagram showing a conventional driving circuit for an RS-232 transmission line. An internal circuit 11 sends an outgoing signal T× to a signal driver chip 12 . The chip 12 transforms the outgoing signal T× to a signal T×d large enough to be sent to an external circuit 13 by the RS-232 transmission line 14 . The external circuit 13 sends an incoming signal R×d through the RS-232 transmission line 14 to the chip 12 . The chip 12 transforms the incoming signal R×d to a signal R× sent to the internal circuit 11 .

However, the signal driver chip 12 is very expensisve so that the conventional driving circuit of RS-232 transmission line suffers a high cost.

SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide a driving circuit for transmission lines which is composed of cheap discrete components. This effectively reduces the cost of the driving circuit.

The present invention provides a driving circuit for transmission lines, receiving a first outgoing signal from an internal circuit and an incoming signal from an external circuit, and sending a second outgoing signal to the external circuit. The driving circuit comprises a first transistor having an emitter connected to receive a first voltage, a collector and a base, a second transistor having a collector connected to the collector of the first transistor, an emitter and a base, a third transistor having a collector connected to receive a second voltage, an emitter through which the second outgoing signal is sent and a base, a first, second, third, fourth, fifth and sixth impedance connected between a node through which the first outgoing signal is received and the base of the first transistor, the emitter and the base of the first transistor, the collector and the base of the third transistor, the base of the third transistor and the emitter of the second transistor, the emitters of the second and third transistor, and the base of the second transistor and the collector of the third transistor, a first, second and third capacitor connected between the node and the base of the second transistor, the collector of the first transistor and the emitter of the third transistor, and the emitter of the second transistor and the collector of the third transistor, and a diode having an anode connected to the emitter of the second transistor and a cathode connected to receive the incoming signal.

Thus, the expensive signal driver chip in the conventional driving circuit is replaced by cheap discrete components in the invention, which reduces the cost.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and not intended to limit the invention solely to the embodiments described herein, will best be understood in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram showing a conventional driving circuit of an RS-232 transmission line.

FIG. 2 is a diagram showing a driving circuit of an RS-232 transmission line according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a diagram showing a driving circuit of an RS-232 transmission line according to one embodiment of the invention. The driving circuit 2 includes an outgoing circuit 21 receiving an outgoing signal T× from an internal circuit (not shown) and sending an outgoing signal T×d to an external circuit (not shown), and an incoming circuit 22 receiving an incoming signal R×d from the external circuit and sending an incoming signal R× to the internal circuit.

The outgoing circuit 21 comprises transistors M 1 , M 2 and M 3 , capacitors C 1 , C 2 and C 3 , resistors R 1 , R 2 , R 3 , R 4 , R 5 and R 6 , and diode D 1 . The transistor M 1 is a P type transistor, and the transistors M 2 and M 3 are N type transistors. The transistor M 1 has an emitter connected to receive a voltage Vcc(3V). The transistor M 2 has a collector connected to the collector of the transistor M 1 . The transistor M 3 has a collector connected to a ground (0V), an emitter through which the outgoing signal T×d is sent. The resistors R 1 , R 2 , R 3 , R 4 , R 5 and R 6 are connected between an input node through which the outgoing signal T× is received and the base of the transistor M 1 , the emitter and the base of the transistor M 1 , the collector and the base of the transistor M 3 , the base of the transistor M 3 and the emitter of the transistor M 2 , the emitters of the transistors M 1 and M 3 , and the base of the transistor M 2 and the collector of the transistor M 3 (the ground). The capacitor C 1 , C 2 and C 3 are connected between the input node and the base of the transistor M 2 , the collector of the transistor M 1 and the emitter of the transistor M 3 , and the emitter of the transistor M 2 and the collector of the transistor M 3 (the ground). The diode D 1 has an anode connected to the emitter of the transistor M 2 and a cathode connected to receive the incoming signal R×d.

The incoming circuit 22 comprises a transistor M 4 , resistors R 7 , R 8 and R 9 , and a diode D 2 . The transistor M 4 has an emitter connected to the ground and a collector through which an incoming signal R× is sent to the internal circuit. The resistors R 7 , R 8 and R 9 are connected between the emitter of the transistor M 1 (Vcc) and the collector of the transistor M 4 , the cathode of the diode D 1 and the base of the transistor M 4 , and the base of the transistor M 4 and the collector of the transistor M 3 . The diode D 2 has an anode connected to the ground and a cathode connected to the base of the transistor M 4 .

The operation of the driving circuit 2 will be explained in the following.

At the beginning, the incoming signal R×d has a voltage level of −12V when the incoming signal R×d is not asserted. This turns the diodes D 1 and D 2 on, and further turns the transistor M 2 on and the transistor M 4 off. Thus, the incoming circuit 22 is not active. There is a voltage of 5V on the base of the transistor M 3 by selecting a proper ratio of resistors R 3 to R 4 . The voltage on the base and emitter of the transistor M 3 is approximately the same since the transistor M 3 acts as a follower. Therefore, there is also a voltage of 5V on the emitter of the transistor M 3 . Additionally, there is a voltage of 10V on the transistor M 2 which has been turned on. This results in a voltage difference of 5V across the capacitor C 2 . The outgoing signal T×d has a voltage level of −5V.

Next, the voltage level of the outgoing signal T× is suddenly switched from a high level to a low level when the internal circuit starts to send out the outgoing signal T×. This results in a sudden voltage drop where the base of the transistor M 2 connects with the capacitor C 1 , and where the resistor R 1 connects with R 2 . Thus, the transistor M 2 is turned off and the transistor M 1 is turned on. There is a voltage of approximately Vcc (5V) on the collector of the transistor M 2 , whereby the voltage on the emitter of the transistor M 3 increases to 5V. Therefore, the voltage level of the outgoing signal T×d may swing in a range between 5V and −5V, which is large enough for signal transmission on the RS-232 transmission line 14 .

When the external circuit starts to send the incoming signal R×d to the internal circuit, the signal R×d is asserted by being switched from −12V to 12V. This turns off the diode D 1 and D 2 , and inactivates the outgoing circuit 21 . The transistor M 4 acts as an inverter and generates on the collector the inverted signal R×d of the incoming signal R×. The signal R×d has an amplitude smaller than the signal R×.

In this embodiment, the driving circuit 2 is composed of discrete transistors, capacitors and resistors. There is no serious impact on the RS-232 signal transmission even if the operating voltage (Vcc) is somehow decreased to 2V.

In conclusion, the present invention provides a driving circuit for transmission lines which are composed of cheap discrete components replacing the expensive signal driver chip in the conventional driving circuit. This effectively reduces the cost of the driving circuit.

While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

Claims

1. A driving circuit for transmission lines, receiving a first outgoing signal from an internal circuit and an incoming signal from an external circuit, and sending a second outgoing signal to the external circuit, the driving circuit comprising:a first transistor having an emitter connected to receive a first voltage, a collector and a base; a second transistor having a collector connected to the collector of the first transistor, an emitter and a base; a third transistor having a collector connected to receive a second voltage, an emitter through which the second outgoing signal is sent and a base; a first, second, third, fourth, fifth and sixth impedance connected between a node through which the first outgoing signal is received and the base of the first transistor, the emitter and the base of the first transistor, the collector and the base of the third transistor, the base of the third transistor and the emitter of the second transistor, the emitters of the second and third transistor, and the base of the second transistor and the collector of the third transistor, respectively; a first, second and third capacitor connected between the node and the base of the second transistor, the collector of the first transistor and the emitter of the third transistor, and the emitter of the second transistor and the collector of the third transistor; and a diode having an anode connected to the emitter of the second transistor and a cathode connected to receive the incoming signal.

2. The driving signal as claimed in claimed 1 further comprising:a fourth transistor having an emitter connected to receive the second voltage and a collector through which a second incoming signal is sent to the internal circuit and a base; a seventh, eighth and ninth impedance connected between the emitter of the first transistor and the collector of the fourth transistor, the cathode of the diode and the base of the fourth transistor, and the base of the fourth transistor and the collector of the third transistor; and a second diode having an anode connected to receive the second voltage and a cathode connected to the base of the fourth transistor.

3. The driving circuit as claimed in claim 1, wherein the first transistor is a P type transistor, and the second and third transistor are N type transistors.

4. The driving circuit as claimed in claim 1, wherein the first voltage is 3V and the second voltage is 0V.

5. The driving circuit as claimed in claim 1, wherein the fourth transistor is a N type transistor.