Abstract:
In a bus driver for driving a bus having first and second power supply terminals, an input terminal for receiving an input signal and an output terminal connected to the bus, a switching element is provided between the output terminal and the second power supply terminal, and the switching element is controlled by a voltage at the input terminal. A pull-up resistor is connected between the first power supply terminal and the output terminal.

Description:
This is a Divisional Application of application Ser. No. 09/218,069 filed Mar. 9, 1999, now abandoned, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a bus driver for a data processing system. 
     2. Description of the Related Art 
     In a data processing system, a plurality of packages each including a semiconductor integrated circuit are inserted into a back panel. The packages share buses formed on the back panel. In this case, each of the packages has a bus driver and a bus receiver. 
     A prior art bus driver includes an open drain type metal oxide semiconductor (MOS, broadly, metal insulating semiconductor (MIS)). This will be explained later in detail. 
     However, the prior art bus driver per se does not include a noise removing circuit, so that a large ringing effect is generated in the bus. Therefore, it takes a long time to converge the ringing effect, which substantially increases the propagation delay time of signals from the bus driver to its respective bus receivers. Also, it is difficult to increase the frequency of the propagated signals. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to reduce the ringing effect in a bus driver, thus substantially reducing the signal propagation delay time as well as increasing the frequency of the propagated signals. 
     According to the present invention, in a bus driver for driving a bus having first and second power supply terminals, an input terminal for receiving an input signal and an output terminal connected to the bus, a switching element is provided between the output terminal and the second power supply terminal, and the switching element is controlled by a voltage at the input terminal. A pull-up resistor is connected between the first power supply terminal and the output terminal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description as set forth below, as compared with the prior art, with reference to the accompanying drawings wherein: 
     FIG. 1 is a block circuit diagram illustrating a prior art data processing system; 
     FIG. 2 is a circuit diagram of the bus driver of FIG. 1; 
     FIG. 3 is an equivalent circuit diagram of the data processing system of FIG. 1; 
     FIG. 4 is a timing diagram showing the received signals at the bus receivers of FIG. 1; 
     FIG. 5 is a circuit diagram illustrating a first embodiment of the bus driver according to the present invention; 
     FIG. 6 is a circuit diagram illustrating a second embodiment of the bus driver according to the present invention; 
     FIG. 7 is a timing diagram showing the effect of the first and second embodiment; 
     FIG. 8 is an equivalent circuit diagram illustrating a data processing system including a third embodiment of the bus driver according to the present invention; and 
     FIG. 9 is a circuit diagram of the bus driver of FIG.  8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the description of the preferred embodiments, a prior art bus driver will be explained with reference to FIGS. 1,  2 ,  3  and  4 . 
     In FIG. 1, which illustrates a prior art data processing system, packages  100 - 1 ,  100 - 3 ,  100 - 4 , . . . ,  100 - 7  are inserted into a back panel  200  by connectors  101 - 1  and  101 - 2 . In this case, a package  100 - 2  is not inserted into the back panel  200  intentionally. Thus, the packages  100 - 1 ,  100 - 3 ,  100 - 4 , . . . ,  100 - 7  share a bus  201  formed on the back panel  200 . 
     One bus driver  102  is mounted on each of the packages  100 - 1 ,  100 - 3 ,  100 - 4 , . . . ,  100 - 7 . The bus driver  102  is connected via a signal line  103  and the connector  101 - 1  to the bus  201 . Also, one bus receiver (not shown) is mounted on each of the packages  100 - 1 ,  100 - 3 ,  100 - 4 , . . . ,  100 - 7 . The bus receiver is connected via a signal line (not shown) and the connector  101 - 2  to the bus. 
     In FIG. 2, which is a circuit diagram of the bus driver  102  of FIG. 1, the bus driver  102  is formed by an open drain type MOS transistor having a gate for receiving an input voltage V in , a grounded source and a drain for generating an output voltage V out . That is, the bus driver of FIG. 2 does not have a noise removing circuit. 
     In FIG. 3, which is an equivalent circuit diagram of the system of FIG. 1, the spacing between the packages depends upon the height of heat sinks of semiconductor integrated devices mounted thereon. For example, the spacing between the packages  100 - 1  and  100 - 2  (in this case, package  100 - 2  is removed) is 1.4 inches; the spacing between the packages  100 - 2  and  100 - 3  is 2.0 inches; the spacing between the packages  100 - 3  and  100 - 4  is 1.8 inches; the spacing between the packages  100 - 4  and  100 - 5  is 1.8 inches; the spacing between packages  100 - 5  and  100 - 6  is 2.0 inches; and the spacing between the packages  100 - 6  and  100 - 7  is 1.0 inches. Also, the length of a stub of each of the packages, i.e., the distance between the connector  101 - 1  and each of the bus drivers is 1 inch. Further, a terminal resistor R formed on the back panel  200  is connected to the connector  101 - 1  of each of the packages  100 - 1  and  100 - 7 . 
     In FIG. 3, when the input voltage V in  of the bus driver  102  included in the package  100 - 3  is switched from high to low and vice versa, the voltages at the bus receivers at the packages  100 - 1 ,  100 - 3 ,  100 - 4 , . . . ,  100 - 7  are obtained as shown in FIG.  4 . As shown in FIG. 4, a large ringing effect is generated in each of the voltages of the bus receivers since the bus driver  102  does not have a noise removing circuit. That is, it takes longer than 40 ns to converge the voltages at the bus receivers. This substantially increases the signal propagation delay time, and also it is difficult to increase the frequency of the propagated signals. 
     Note that each bus receiver of the packages  100 - 1 ,  100 - 3 ,  100 - 4 ,  100 - 5 ,  100 - 6  and  100 - 7  had the same configuration as the bus driver of FIG.  2 . 
     In FIG. 5, which illustrates a first embodiment of the bus driver according to the present invention, the bus driver includes a MOS transistor  1  as a switching element having a gate connected to an input terminal IN for receiving an input voltage V in , a drain connected to an output terminal OUT for generating an output voltage V out , and a source connected to the ground terminal GND. Also, the bus driver includes a noise removing circuit formed by pull-up resistor  2 . 
     In FIG. 6, which illustrates a second embodiment of the bus driver according to the present invention, the bus driver includes a bipolar transistor  1 ′ instead of the MOS transistor  1  of FIG.  5 . The bipolar transistor  1 ′ has a base connected to the input terminal IN, collector connected to the output terminal OUT, and an emitter connected to the ground terminal GND. The bus driver of FIG. 6 operates in the same way as the driver of FIG.  5 . 
     Assume that the bus driver of FIG. 5 or  6  is applied to the data processing system of FIGS. 1 and 3. In this case, when the input voltage V in  of the bus driver of FIGS. 5 or  6  included in the package  100 - 3  is switched from high to low and vice versa, the voltages at the bus receivers at the packages  100 - 1 ,  100 - 3 ,  100 - 4 , . . . ,  100 - 7  are obtained as shown in FIG.  7 . As shown in FIG. 7, the ringing effect generated in each of the voltages of the bus receivers is reduced, since the bus driver of FIGS. 5 or  6  has a noise removing circuit formed by the pull-up resistor  2 . Therefore, it takes less than 20 ns to converge the voltages at the bus receivers. This substantially decreases the signal propagation delay time, and also, the frequency of the propagated signals can be increased. 
     In FIG. 8, which illustrates a third embodiment of the present invention, one pull-up resistor R′ formed on the back panel  200  is connected to the connector  101 - 1  of each of the packages  100 - 2 ,  100 - 3 ,  100 - 4 ,  100 - 5  and  100 - 6  of FIG.  3 . Note that each of the packages  100 - 1 ,  100 - 3 ,  100 - 4 ,  100 - 5 ,  100 - 6  and  100 - 7  includes and open drain type MOS transistor as a bus driver or a bus receiver as illustrated in FIG. 2; however, this bus driver or bus receiver can be an open collector type bipolar transistor as illustrated in FIG.  9 . 
     In FIG. 8, when the input voltage V in  of the bus driver included in the package  100 - 3  is switched from high to low and vice versa, the voltages at the bus receiver at the packages  100 - 1 ,  100 - 3 ,  100 - 4 , . . . ,  100 - 7  are obtained in a similar way to that of FIG.  7 . That is, the ringing effect generated in each of the voltages of the bus receivers is reduced, since the packages  100 - 2 ,  100 - 3 ,  100 - 4 ,  100 - 5  and  100 - 6  have a noise removing circuit formed by the pull-up resistor R′. Therefore, it takes less than 20 ns to converge the voltages at the bus receivers. This substantially decreases the signal propagation delay time, and also, the frequency of the propagated signals can be increased. 
     In the data processing system, of FIG. 8, each of the packages  100 - 1 ,  100 - 3 ,  100 - 4 ,  100 - 5 ,  100 - 6  and  100 - 7  includes an open drain type bus driver or an open collector type bus driver; however, the bus driver as illustrated in FIG. 5 or  6  can be used as such a bus driver. 
     As explained hereinabove, according to the present invention, the signal propagation delay time can be reduced, and also, it is easy to increase the frequency of the propagated signals.