Patent Abstract:
A transmitting apparatus of simple circuit configuration is provided as having turn off time (cut off delay time) of an output transistor shortened. The potential of a low-level signal is inputted to an input terminal of a first transistor through a resistor. The potential of a high-level signal is inputted to an input terminal of a second transistor through a resistor. A control circuit is connected to the input terminals of the first and second transistors and when a high-level signal is to be transmitted to the communication line, the control circuit enters a high-impedance state with respect to the input terminal of the first transistor and outputs a low-level signal to the input terminal of the second transistor. When a low-level signal is to be transmitted over the communication line, the control circuit outputs a high-level signal to the input terminal of the first transistor and develops a high-impedance state with respect to the input terminal of the second transistor.

Full Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transmitting apparatus for outputting a signal of binary levels over a communication line or the like. 
     2. Description of the Background Art 
     In Japanese Patent Laid-open No. Hei 5-292101, there is proposed a communication apparatus in which signals are exchanged among a plurality of communication units through a communication line. Each communication unit includes a transmitting circuit for alternately generating a high level signal or a low-level signal and for outputting the generated signal to the communication line. Each communication unit further includes a transmission control circuit for inputting a control signal to the transmitting circuit so that the transmitting circuit outputs the high-level signal or the low-level signal. 
     In Japanese Patent Laid-open No. Hei 5-292101, the communication apparatus is adapted to reduce the effect of stray capacitance between the communication line and ground which slows down communication, and hence to improve communication speed. The transmitting circuit includes a timing circuit for outputting a timing signal for a predetermined period of time after inversion of the level of the control signal from a first level to a second level, a first transistor responsive to the timing signal for outputting either of the high level signal and the low level signal, and a second transistor responsive to the inversion of the level of the control signal from the second level to the first level for outputting a signal at the other level to the communication line. 
     A further conventional type of transmitting apparatus is the so-called totem pole type in which a first transistor for outputting a high-level signal to a communication line and a second transistor for outputting a low-level signal to the communication line are connected in series and interposed between power source terminals. FIG. 6 is a circuit configuration diagram showing an example of a conventional totem pole type transmitting apparatus. 
     FIG. 6 illustrates transmitting apparatus  101  including a non-inverting signal output terminal  103  for outputting a signal having a same logical level as the logical level of transmitted data supplied to the data input terminal  102  and an inverting signal output terminal  104  for outputting a signal having a logical level obtained by reversing the logical level of the transmitted data supplied to the data input terminal  102 . Transmitting apparatus  101  further includes an invertor (logically inverting circuit)  105  for inverting the logical level of the transmitted data supplied to the data input terminal  102  and two output circuits  106  and  107 . 
     Output circuit  106  includes a PNP transistor Q 1 , an NPN transistor Q 2 , a P-channel enhancement field effect transistor Q 3 , an N-channel enhancement field effect transistor Q 4 , and the corresponding peripheral circuits for each of the respective transistors. The emitter of PNP transistor Q 1  is connected to a positive power source V+ and the collector of PNP transistor Q 1  is connected to the non-inverting output terminal  103 . The base of PNP transistor Q 1  is connected to the output terminal of invertor  105  through base resistor R 1 . The emitter of NPN transistor Q 2  is connected to ground (or a negative power source) and the collector of NPN transistor Q 2  is connected to non-inverting output terminal  103 . The base of NPN transistor Q 2  is connected to the output terminal of invertor  105  through base resistor R 3 . Non-inverting output terminal  103  is connected to positive power source V+ through pull-up resistor R 5 . When PNP transistor Q 1  and NPN transistor Q 2  are both in an OFF state (idle state), the logical level of non-inverting output terminal  103  is held at a HIGH level by means of pull-up resistor R 5 . 
     Output circuit  107  is substantially the same as output circuit  106  described above except that output terminal  104  is connected to ground through resistor R 10 . Also, there is no invertor inserted between the base inputs of transistors Q 5  and Q 6  and data input terminal  102  in output circuit  107 . Operation of transmitting apparatus  101  will be described as follows. 
     Output circuit  106  is adapted such that the base currents of PNP transistor Q 1  and NPN transistor Q 2  are controlled in accordance with the output of invertor  105  such that either of transistors Q 1  and Q 2  are turned on based on the output of invertor  105 . When the logical level of transmitted data supplied to data input terminal  102  is HIGH, the output of invertor  105  is brought to a LOW level. When the output of invertor  105  is LOW, no base current is supplied to NPN transistor Q 2  and NPN transistor Q 2  is thus brought to an OFF state. Meanwhile, since a base current is not supplied to PNP transistor Q 1  through base resistor R 1  in this case, PNP transistor Q 1  is maintained in an ON state. Thereby, the output of the non-inverting output terminal  103  is brought to a HIGH level. 
     When the logical level of the transmitted data supplied to data input terminal  102  is LOW, the output of the invertor  105  is brought to a HIGH level. When the output of invertor  105  is HIGH, a base current is supplied to NPN transistor Q 2  through base resistor R 3  and NPN transistor Q 2  is brought to an ON state. At the same time, PNP transistor Q 1  is brought to an OFF state. Thereby, the output of non-inverting output terminal  103  is brought to a LOW level. 
     In bipolar transistors such as PNP transistors and NPN transistors, even when the supply of base current is cut off, a time delay is produced until the collector current is cut off by the effect of electric charge stored in the base region and the like. In order to shorten the cut-off delay time (turnoff time), conventional transmitting apparatus  101  includes a field effect transistor between the base and the emitter of each transistor. By turning the field effect transistor on to short-circuit the base with the emitter through a low impedance, charge on the base is forcibly discharged. By forcibly discharging the charge stored on the base, cut-off delay time (turn off time) can be shortened. This operation will be described as follows. 
     When the logical level of the transmitted data supplied to data input terminal  102  is HIGH, then P-channel enhancement field effect transistor Q 3  is in an OFF state and PNP transistor Q 1  is brought to an ON state by the LOW level output of invertor  105 . When the logical level of the transmitted data supplied to data input terminal  102  is changed from HIGH level to LOW level, P-channel enhancement field effect transistor Q 3  is brought to an ON state. By turning on P-channel enhancement field effect transistor Q 3 , the charge stored on the base of PNP transistor Q 1  is forcibly discharged. Thereby, the cut-off delay time (turn off time) of PNP transistor Q 1  is shortened. 
     On the other hand, when the logical level of the transmitted data supplied to data input terminal  102  is LOW, N-channel enhancement field effect transistor Q 4  is in an OFF state and NPN transistor Q 2  is brought to an ON state by the HIGH level output of invertor  105 . When the logical level of the transmitted data supplied to data input terminal  102  is changed from LOW level to HIGH level, N-channel enhancement field effect transistor Q 4  is brought to an ON state. By turning on N-channel enhancement field effect transistor Q 4 , the charge stored on the base of NPN transistor Q 2  is forcibly discharged. Thereby, the cut-off delay time (turn off time) of NPN transistor Q 2  is shortened. 
     Since conventional transmitting apparatus  101  of FIG. 6 employs field effect transistors for shortening the cut-off delay time (turn off time) of the bipolar transistors, the number of discrete components constituting each of output circuits  106  and  107  is increased. It is therefore considered advantageous to provide circuits for shortening cut-off delay time (turn off time) of bipolar transistors as in the form of an integrated circuit (IC) by employing three-status buffers as will be described as follows. 
     FIG. 7 is a circuit configuration diagram of a transmitting apparatus adapted to shorten cut-off delay time (turn off time) of bipolar transistors by the use of three-status buffers. The transmitting apparatus  111  shown in FIG. 7 includes a logical circuit portion  112  and output circuits  113  and  114 . The logical circuit portion  112  can be provided in the form of an IC with the logical circuit portions put together. Each of output circuits  113  and  114  are realized by eliminating the field effect transistors from each of output circuits  106  and  107  of FIG.  6 . Otherwise, the circuit configuration of output circuits  113  and  114  are the same as the circuit configurations of output circuits  106  and  107  of FIG.  6 . 
     Logical circuit portion  112  includes an invertor  105  and four three-status buffers G 1 -G 4 . The input terminal G 1   a  of first three-status buffer G 1  is connected to the output terminal of invertor  105 . The output terminal G 1   b  of first three-status buffer G 1  is connected to the base of PNP transistor Q 1 . The output enable terminal G 1   c  of first three-status buffer G 1  is connected to the output terminal of invertor  105 . 
     First three-status buffer G 1 , when the logical level of the output enable signal supplied to output enable terminal G 1   c  is LOW, brings output terminal G 1   b  to a high-impedance state. When the logical level of the output enable signal supplied to output enable terminal G 1   c  is HIGH, first three-status buffer G 1  outputs a signal at the same logical level as the logical level of the input signal supplied to input terminal G 1   a . The output impedance of first three-status buffer G 1  is sufficiently smaller than the resistance value of base-emitter resistor R 2  of PNP transistor Q 1 . Operation of the first three-status buffer G 1  will be briefly described as follows. 
     When the logical level of the transmitted data supplied to data input terminal  102  is HIGH, the output of invertor  105  is brought to a LOW level and the output enable terminal G 1   c  of first three-status buffer G 1  is brought to a LOW level. Hence, the output of first three-status buffer G 1  is brought to a high-impedance state. In view of the low-level output of the invertor  105 , no base current is supplied to PNP transistor Q 1  through base resistor R 1  and, PNP transistor Q 1  is therefore maintained in an ON state. The output of non-inverting output terminal  103  is brought to a HIGH level. 
     When the logical level of the transmitted data supplied to data input terminal  102  is changed from a HIGH level to a LOW level, the output of invertor  105  is changed from a LOW level to a HIGH level and both output enable terminal G 1   c  and input terminal G 1   a  of first three-status buffer G 1  are brought to a HIGH level. The output of first three-status buffer G 1  is thus brought to a HIGH level. The base of PNP transistor Q 1  is therefore brought to such a state that it is connected to the side of positive power source V+ by a low impedance through a high-level output transistor within first three-status buffer G 1 . Thus, it becomes possible to forcibly discharge the charge stored on the base of PNP transistor Q 1  and shorten the cut-off delay time (turn off time). 
     With further regard to FIG.  6  and FIG. 7, when such a circuit configuration capable of short-circuiting the base and the emitter of a bipolar transistor by a low impedance is employed for shortening the turn off time of the bipolar transistor driving the output terminal, four signal lines are required to be placed between each output circuit and the logical circuit portion. Thus, the interface between the logical circuit portion and each output circuit becomes complicated. Furthermore, when it is attempted to fabricate the logical circuit portion as an IC, the number of output pins is undesirably increased. 
     When such a circuit configuration is employed in which a field effect transistor is used in place of the bipolar transistor driving the output terminal, the turn off time can be shortened by supplying through a low impedance a voltage to the gate of the field effect transistor for controlling the field effect transistor to be turned to an OFF state. However, there arises a problem similar to the above that the interface between the logical circuit portion and each output circuit becomes complicated. 
     SUMMARY OF THE INVENTION 
     The present invention was made to solve the above described problems. An object of the present invention is to therefore provide a transmitting apparatus of simple circuit configuration which is capable of shortening the turn off time of an output transistor. 
     According to the invention described above, there is provided a transmitting apparatus including a transmitting circuit for alternately generating a high-level signal and a low-level signal and transmitting the generated signal over a communication line; a control circuit for providing a control signal for controlling the transmitting circuit; a first transistor for activating the communication line to a HIGH level when the control signal is at a first level; and a second transistor for activating the communication line to a LOW level when the control signal is at a second level. The potential of the low-level signal is inputted to the input terminal of the first transistor through a resistor and the potential of the high-level signal is inputted to the input terminal of the second transistor through a resistor. The control circuit is connected to the input terminals of the transistors. The control circuit develops a high-impedance state with respect to the input terminal of the first transistor and outputs a low-level signal to the input terminal of the second transistor when a high-level signal is to be transmitted to the communication line. The control circuit further outputs a high-level signal to the input terminal of the first transistor and develops a high-impedance state with respect to the input terminal of the second transistor when a low-level signal is to be transmitted to the communication line. 
     In a preferred embodiment, the control circuit may be made up of three-status buffers. Each of the three-status buffers can output the low-level signal, output the high-level signal, and develop the high-impedance state. 
     In a further preferred embodiment, the first and second transistors may be bipolar transistors and the bases may be used as the inputs. The bipolar transistors can be controlled with their bases used as the inputs or input terminals. 
     In a still further preferred embodiment, the first and second transistors may be field effect transistors and the gates may be used as the inputs. The field effect transistors can be controlled with their gates used as the inputs or input terminals. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention and wherein: 
     FIG. 1 is a circuit configuration diagram of a transmitting apparatus according to a first embodiment of the invention; 
     FIG. 2 is a timing chart showing operation of the transmitting apparatus of FIG. 1; 
     FIG. 3 is a circuit configuration diagram of a transmitting apparatus of a second embodiment as provided with a transmission stopping function; 
     FIG. 4 is a circuit configuration diagram of a transmitting apparatus of a third embodiment as provided with a transmission stopping function and a base drive inhibiting function; 
     FIG. 5 is a circuit configuration diagram of a variation of the transmitting apparatus shown in FIG. 1; 
     FIG. 6 is a circuit configuration diagram of a conventional transmitting apparatus; and 
     FIG. 7 is a circuit configuration diagram of a transmitting apparatus in which a design for shortening the turn off time of transistors is made by employing three-status buffers. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the invention will be described with reference to the accompanying drawings as follows. FIG. 1 is a circuit configuration diagram of a transmitting apparatus according to a preferred embodiment of the present invention. Transmitting apparatus  1  shown in FIG. 1 includes a non-inverting output terminal  3  for outputting the same logical level as the logical level of transmitted data supplied to data input terminal  2  and an inverting output terminal  4  for outputting the logical level obtained by reversing the logical level of the transmitted data supplied to data input terminal  2 . Transmitting apparatus  1  further includes a logical circuit portion  5  and two output circuits  6  and  7 . The data provided to data input terminal  2  may alternate between a high level signal (or data) and a low level signal (or data). 
     Output circuit  6  includes a PNP transistor Q 1 , an NPN transistor Q 2 , and corresponding peripheral circuits of transistors Q 1  and Q 2 . The emitter of the PNP transistor Q 1  is connected to positive power source V+ and the collector of PNP transistor Q 1  is connected to non-inverting output terminal  3 . The base of PNP transistor Q 1  is connected to ground (or a negative power source) through base resistor R 1 . The resistance value of base resistor R 1  is set so that it can supply a base current sufficient to control turning on of PNP transistor Q 1 . Base-emitter resistor R 2  is connected in is parallel with the base-emitter circuit of PNP transistor Q 1 , although base-emitter resistor R 2  is not necessary and may be dispensed with. Diode D 1  is provided between the collector and the emitter of PNP transistor Q 1  for protecting PNP transistor Q 1  against backward voltage and backward current. As for diode D 1 , the cathode of diode D 1  is connected to the emitter side of PNP transistor Q 1  and anode of the diode D 1  is connected to the collector side of PNP transistor Q 1 . 
     The emitter of NPN transistor Q 2  is connected to ground (or a negative power source) and the collector of NPN transistor Q 2  is connected to non-inverting output terminal  3 . The base of NPN transistor Q 2  is connected to positive power source V+ through a base resistor R 3 . The resistance value of base resistor R 3  is set so that it can supply a base current sufficient to control turning on of NPN transistor Q 2 . Base-emitter resistor R 4  is connected in parallel with the base-emitter circuit of NPN transistor Q 2 , although base-emitter resistor R 4  is not necessary and may be dispensed with. Diode D 2  is provided between the collector and the emitter of NPN transistor Q 2  for protecting NPN transistor Q 2  against backward voltage and backward current. As for diode D 2 , the cathode of diode D 2  is connected to the collector side of NPN transistor Q 2  and the anode of diode D 2  is connected to the emitter side of NPN transistor Q 2 . Non-inverting output terminal  3  is connected to positive power source V+ through a pull-up resistor R 5 . 
     The other output circuit  7  is made up of PNP transistor Q 5 , NPN transistor Q 6 , and corresponding peripheral circuits of transistors Q 5  and Q 6 . The emitter of the PNP transistor Q 5  is connected to positive power source V+ and the collector of PNP transistor Q 5  is connected to inverting output terminal  4 . The base of PNP transistor Q 5  is connected to ground (or a negative power source) through base resistor R 6 . The resistance value of base resistor R 6  is set so that it can supply a base current sufficient to control turning on of PNP transistor Q 5 . Base-emitter resistor R 7  is connected in parallel with the base-emitter circuit of PNP transistor Q 5 , although base-emitter resistor R 7  is not necessary and may be dispensed with. Diode D 3  is provided between the collector and the emitter of PNP transistor Q 5  for protecting PNP transistor Q 5  against backward voltage and backward current. As for diode D 3 , the cathode of diode D 3  is connected to the emitter side of PNP transistor Q 5  and the anode of diode D 3  is connected to the collector side of PNP transistor Q 5 . 
     The emitter of NPN transistor Q 6  is connected to ground (or a negative power source) and the collector of NPN transistor Q 6  is connected to inverting output terminal  4 . The base of NPN transistor Q 6  is connected to positive power source V+ through base resistor R 8 . The resistance value of base resistor R 8  is set so that it can supply a base current sufficient to control turning on of NPN transistor Q 6 . Base-emitter resistor R 9  is connected in parallel with the base-emitter circuit of NPN transistor Q 6 , although base-emitter resistor R 9  is not necessary and may be dispensed with. Diode D 4  is provided between the collector and the emitter of NPN transistor Q 6  for protecting NPN transistor Q 6  against backward voltage and backward current. As for diode D 4 , the cathode of diode D 4  is connected to the collector side of NPN transistor Q 6  and the anode of diode D 4  is connected to the emitter side of NPN transistor Q 6 . 
     Inverting output terminal  4  is connected to ground (or a negative power source) through a pull-down resistor R 10 . When PNP transistor Q 5  and NPN transistor Q 6  are both in an OFF state (idle state), the logical level of inverting output terminal  4  is held at a LOW level by means of pull-down resistor R 10 . 
     The logical circuit portion  5  is made up of four three-status buffers  51 ,  52 ,  53 , and  54 . Input terminal  51   a  of first three-status buffer  51  is connected to data input terminal  2 . Output terminal  51   b  of first three-status buffer  51  is connected to base of the PNP transistor Q 1 . Output enable terminal  51   c  of first three-status buffer  51  is connected to data input terminal  2 . 
     When the logical level of the output enable signal supplied to output enable terminal  51   c  is HIGH, first three-status buffer  51  brings its output terminal  51   b  to a high-impedance state. When the logical level of the output enable signal supplied to output enable terminal  51   c  is LOW, first three-status buffer  51  outputs a signal via output terminal  516  at the logical level obtained by reversing the logical level of the input signal supplied to input terminal  51   a . First three-status buffer  51  outputs, as the high level output, the voltage of the positive power source V+ through a low impedance. 
     Input terminal  52   a  of second three-status buffer  52  is connected to data input terminal  2 . Output terminal  52   b  of second three-status buffer  52  is connected to the base of NPN transistor Q 2 . Output enable terminal  52   c  of second three-status buffer  52  is connected to data input terminal  2 . 
     When the logical level of the output enable signal supplied to output enable terminal  52   c  is LOW, second three-status buffer  52  brings its output terminal  52   b  to a high-impedance state. When the logical level of the output enable signal supplied to output enable terminal  52   c  is HIGH, second three-status buffer  52  outputs a signal via output terminal  52   b  at the logical level obtained by reversing the logical level of the input signal supplied to input terminal  52   a . Second three-status buffer  52  outputs, as the low level output, the voltage of ground (or a negative power source) through a low impedance. 
     Input terminal  53   a  of a third three-status buffer  53  is connected to data input terminal  2 . Output terminal  53   b  of third three-status buffer  53  is connected to the base of PNP transistor Q 5 . Output enable terminal  53   c  of third three-status buffer  53  is connected to data input terminal  2 . 
     When the logical level of the output enable signal supplied to output enable terminal  53   c  is LOW, third three-status buffer  53  brings its output terminal  53   b  to a high-impedance state. When the logical level of the output enable signal supplied to output enable terminal  53   c  is HIGH, third three-status buffer  53  outputs a signal via output terminal  53   b  at a logical level which is the same as the logical level of the input signal supplied to input terminal  53   a . Third three-status buffer  53  outputs, as the high level output, the voltage of positive power source V+ at a low impedance. 
     Input terminal  54   a  of fourth three-status buffer  54  is connected to data input terminal  2 . Output terminal  54   b  of fourth three-status buffer  54  is connected to the base of NPN transistor Q 6 . Output enable terminal  54   c  of fourth three-status buffer  54  is connected to data input terminal  2 . 
     When the logical level of the output enable signal supplied to output enable terminal  54   c  is HIGH, fourth three-status buffer  54  brings its output terminal  54   b  to a high-impedance state. When the logical level of the output enable signal supplied to output enable terminal  54   c  is LOW, fourth three-status buffer  54  outputs a signal via output terminal  54   b  at a logical level which is the same as the logical level of the input signal supplied to input terminal  54   a . Fourth three-status buffer  54  outputs, as the low level output, the voltage of ground (or a negative power source) through a low impedance. 
     Operation of transmitting apparatus  1  shown in FIG. 1 will be described as follows. FIG. 2 is a timing chart showing the operation of transmitting apparatus shown in FIG.  1 . When the transmitted data supplied to data input terminal  2  (the input shown in FIG. 2A) is at a HIGH level, then the output of first three-status buffer  51  is brought to a high-impedance state as shown in FIG.  2 B and the output of the second three-status buffer  52  is brought to a LOW level as shown in FIG.  2 C. Accordingly, a base current is supplied to PNP transistor Q 1  through base resistor R 1 , whereby PNP transistor Q 1  is brought to an ON state and NPN transistor Q 2  is brought to an OFF state. Hence, the output of non-inverting output terminal  3  is brought to a HIGH level as shown in FIG.  2 D. 
     When the transmitted data supplied to data input terminal  2  is brought to a LOW level, the output of first three-status buffer  51  is brought to a HIGH level as shown in FIG.  2 B and the output of second three-status buffer  52  is brought to a high-impedance state as shown in FIG.  2 C. Accordingly, PNP transistor Q 1  is brought to an OFF state and a base current is supplied to NPN transistor Q 2  through base resistor R 3 , whereby NPN transistor Q 2  is brought to an ON state. Hence, the output of non-inverting output terminal  3  is brought to a LOW level as shown in FIG.  2 D. 
     When the transmitted data supplied to the data input terminal  2  (the input shown in FIG. 2A) is at a HIGH level, then the output of third three-status buffer  53  is brought to a HIGH level as shown in FIG.  2 E and the output of fourth three-status buffer  54  is brought to a high-impedance state as shown in FIG.  2 F. Accordingly, PNP transistor Q 5  is brought to an OFF state and a base current is supplied to NPN transistor Q 6  through base resistor R 8 , whereby NPN transistor Q 6  is brought to an ON state. Hence, the output of inverting output terminal  4  is brought to a LOW level as shown in FIG.  2 G. 
     When the transmitted data supplied to data input terminal  2  is brought to a LOW level, the output of third three-status buffer  53  is brought to a high-impedance state as shown in FIG.  2 E and the output of fourth three-status buffer  54  is brought to a LOW level as shown in FIG.  2 F. Accordingly, a base current is supplied to PNP transistor Q 5  through base resistor R 6 , whereby PNP transistor Q 5  is brought to an ON state and NPN transistor Q 6  is brought to an OFF state. Hence, the output of inverting output terminal  4  is brought to a HIGH level as shown in FIG.  2 G. 
     Transmitting apparatus  1  as shown in FIG. 1 is adapted such that a base bias circuit is provided for holding PNP transistors Q 1  and Q 5  in an ON state. When PNP transistors Q 1  and Q 5  are to be turned off, the voltage of positive power source V+ is supplied from first and third three-status buffers  51  and  53  to the bases of PNP transistors Q 1  and Q 5  at a low impedance. Accordingly, it is possible to control turning off of each PNP transistors Q 1  and Q 5  via respective single signal lines. Further, in view of the supply of voltage of positive power source V+ to the bases of PNP transistors Q 1  and Q 5  at low impedance, the charge on the bases of PNP transistors Q 1  and Q 5  can be quickly discharged and, hence, the turn off time of PNP transistors Q 1  and Q 5  can be shortened. 
     Likewise, a base bias circuit is provided as part of transmitting apparatus  1  for holding NPN transistors Q 2  and Q 6  in an ON state. When NPN transistors Q 2  and Q 6  are to be turned off, the voltage of ground (or a negative power source) is supplied from second and fourth three-status buffers  52  and  54  to the bases of NPN transistors Q 2  and Q 6  through a low impedance. Accordingly, it is possible to control turning off of each of NPN transistors Q 2  and Q 6  via respective single signal lines. Further, in view of the supply of voltage of ground (or a negative power source) to the bases of NPN transistors Q 2  and Q 6  at a low impedance, the charge on the bases of NPN transistors Q 2  and Q 6  can be quickly discharged and, hence, the turn off time of NPN transistors Q 2  and Q 6  can be shortened. 
     FIG. 3 is a circuit configuration diagram of a transmitting apparatus provided with a transmission stopping function. Transmitting apparatus  11  shown in FIG. 3 is provided by adding, to transmitting apparatus  1  shown in FIG. 1, transmission stopping signal input terminal  8 , AND gate  55 , OR gate  56 , and invertor  57 , by which all of transistors Q 1 , Q 2 , Q 5  and Q 6  are controlled to turn off in accordance with a transmission stopping signal (idle signal) supplied to transmission stopping signal input terminal  8 . 
     In transmitting apparatus  11 , when an input signal at a High level is inputted to transmission stopping signal input terminal  8 , all of the transistors Q 1 , Q 2 , Q 5 , and Q 6  are controlled to turn off. When an input signal at a LOW level is inputted to transmission stopping signal input terminal  8 , an output corresponding to the logical level of the transmitted data supplied to data input terminal  2  is generated. 
     In the transmission stopped state, the logical level of non-inverting output terminal  3  is held at a HIGH level through pull-up resistor R 5  and the logical level of inverting output terminal  4  is held at a LOW level through pull-down resistor R 10 . Since all of transistors Q 1 , Q 2 , Q 5 , and Q 6  are turned off in the transmission stopped state, current consumption at the time of stand by and the like can be suppressed. 
     When an input signal at a High level is inputted to transmission stopping signal input terminal  8 , one input signal to AND gate  55  is brought to a LOW level through invertor  57  and the output signal from AND gate  55  is brought to a LOW level. The output signal from AND gate  55  is supplied to first three-status buffer  51  and fourth three-status buffer  54 . Since the output signal from AND gate  55  is brought to a LOW level, the output signal from terminal  51   b  of first three-status buffer  51  is brought to a HIGH level. In view of this High level output from first three-status buffer  51 , PNP transistor Q 1  is brought to an OFF state. Further, since the output signal from AND gate  55  is brought to a LOW level, the output signal from output terminal  54   b  of fourth three-status buffer  54  is brought to a LOW level. In view of this LOW level output from output terminal  54   b , NPN transistor Q 6  is brought to an OFF state. 
     When an input signal at a High level is inputted to the transmission stopping signal input terminal  8 , this HIGH level signal is supplied to second three-status buffer  52  and third three-status buffer  53  through OR gate  56 . Since the output signal from OR gate  56  is brought to a HIGH level, output terminal  52   b  of second three-status buffer  52  is brought to a LOW level. In view of this LOW level output from output terminal  52   b , NPN transistor Q 2  is brought to an OFF state. Since the output signal from OR gate  56  is brought to a HIGH level, the output signal from output terminal  53   b  of third three-status buffer  53  is brought to a HIGH level. In view of this HIGH level output from output terminal  53   b , PNP transistor Q 5  is brought to an OFF state. 
     FIG. 4 is a circuit configuration diagram of a transmitting apparatus provided with a base drive inhibiting function in addition to the transmission stopping function. Transmitting apparatus  21  shown in FIG. 4 is provided by adding, to transmitting apparatus  11  shown in FIG. 3, PNP transistor Q 7  for controlling the supply of base currents to NPN transistors Q 2  and Q 6 , NPN transistor Q 8  for controlling the supply of base currents to PNP transistors Q 1  and Q 5 , and corresponding peripheral circuits of transistors Q 7  and Q 8 . 
     In the transmitting apparatus shown in FIG.  1  and FIG. 3, if the operation of logical circuit portion  5  becomes unstable upon rising of the power source and all the outputs of three-status buffers  51 - 54  are brought to high-impedance states, there is the possibility of all transistors Q 1 , Q 2 , Q 5 , and Q 6  turning on and short-circuiting the power source. In the transmitting apparatus  21  shown in FIG. 4, upon detection that the source voltage has reached a predetermined voltage by a voltage detection circuit or the like (not shown), first bias supply control terminal  22  is brought to a LOW level in accordance with the detected output of the source voltage. A base current is supplied through base resistor R 21  to PNP transistor Q 7 , which has an emitter connected to positive power source V+, to turn PNP transistor Q 7  ON, whereby bias voltages are supplied to outputting NPN transistors Q 2  and Q 6 . 
     Further, upon detection that the source voltage has reached a predetermined voltage by a voltage detecting circuit or the like (not shown), a signal at a HIGH level is inputted to second bias supply control terminal  23  in accordance with the detected output of the source voltage. A base current is supplied through base resistor R 22  to NPN transistor Q 8 , which has an emitter connected to ground (or a negative power source), to turn NPN transistor Q 8  ON, whereby bias voltages are supplied to outputting PNP transistors Q 1  and Q 5 . 
     When both PNP transistor Q 7  and NPN transistor Q 8  are in OFF states, no bias voltage is supplied to each of transistors Q 1 , Q 2 , Q 5 , and Q 6 . Therefore, all the outputting transistors Q 1 , Q 2 , Q 5 , and Q 6  can be brought to OFF states regardless of the outputting state of each of three-status buffers  51 - 54 . 
     Reference numeral R 23  denotes a base-emitter resistor for PNP transistor Q 7  and R 24  denotes a pull-up resistor for first bias supply control terminal  22 . The configuration may be modified such that only resistor R 23  or resistor R 24  are provided therein. Reference numeral R 25  denotes a base-emitter resistor for NPN transistor Q 8  and R 26  denotes a pull-down resistor for second bias supply control terminal  23 . The configuration may be modified such that only resistor R 25  or resistor R 26  are provided therein. 
     Further, it is also possible to provide such a configuration that a constant-voltage circuit constituted of a voltage regulating diode or the like, not shown, may be interposed between first bias supply control terminal  22  and second bias supply control terminal  23  and that base currents are supplied to PNP transistor Q 7  and NPN transistor QB when power source V+ exceeds the regulated voltage determined by the constant-voltage circuit, transistors Q 7  and Q 8  are brought to ON states, and bias voltages are supplied to outputting transistors Q 1 , Q 2 , Q 5 , and Q 6 . In this case, pull-up resistor R 24  and pull-down resistor R 26  may be eliminated. 
     FIG. 5 is a circuit configuration diagram of an example of a variation of the transmitting apparatus shown in FIG.  1 . Transmitting apparatus  61  of FIG. 5 is of such a configuration that P-channel enhancement field effect transistors Q 21  and Q 23  are used in place of output PNP transistors Q 1  and Q 5  shown in FIG.  1  and N-channel enhancement field effect transistors Q 22  and Q 24  are used in place of output NPN transistors Q 2  and Q 6  shown in FIG.  1 . 
     The gate voltage necessary for controlling turning on of output P-channel enhancement field effect transistor Q 21  is generated by voltage-dividing the source voltage with resistor  61  and resistor  62 . The voltage generated by voltage division using resistor  61  and resistor  62  is supplied to the gate of output P-channel enhancement field effect transistor Q 21 . 
     The gate voltage necessary for controlling turning on of output N-channel enhancement field effect transistor Q 22  is generated by voltage-dividing the source voltage with resistor  63  and resistor  64 . The voltage generated by voltage division using resistor  63  and resistor  64  is supplied to the gate of output N-channel enhancement field effect transistor Q 22 . 
     The gate voltage necessary for controlling turning on of output P-channel enhancement field effect transistor Q 23  is generated by voltage-dividing the source voltage with resistor  65  and resistor  66 . The voltage generated by voltage division using resistor  65  and resistor  66  is supplied to the gate of output P-channel enhancement field effect transistor Q 23 . 
     The gate voltage necessary for controlling turning on of output N-channel enhancement field effect transistor Q 24  is generated by voltage-dividing the source voltage with resistor  67  and resistor  68 . The voltage generated by voltage division using resistor  67  and resistor  68  is supplied to the gate of output N-channel enhancement field effect transistor Q 24 . 
     When controlling each of field effect transistors Q 21 -Q 24  from an ON state to an OFF state, the turn off time of each field effect transistor Q 21 -Q 24  can be shortened by supplying, through a low impedance, the voltage for turning off each field effect transistor Q 21 -Q 24  to the gate of each field effect transistor Q 21 -Q 24 . Further, such a configuration may be provided as shown in FIG. 5 wherein the gate voltage for bringing each field effect transistor Q 21 -Q 24  to an OFF state is supplied via respective three-status buffers  51 - 54  through a low impedance. 
     The transmitting apparatus according to the invention as thus described is configured such that the base or gate of a transistor driven in an ON state is supplied with a voltage to control turning off of the transistor through a three-status buffer. Hence, switching operation of transistors can be controlled by having each transistor interfaced with a single signal line. 
     Further, since the transmitting apparatus is configured such that the voltage for controlling turning off of the transistor is output from a three-status buffer through a low impedance, the turn off time of the transistor can be shortened and a signal changing between high level and low level can be transmitted more quickly. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Technology Classification (CPC): 7