Terminating resistance circuit

A terminating resistance circuit is used for a bus line in which a power source line, signal lines and ground lines are connected between computer equipment via terminators, respectively. An input terminal of a regulator is connected to the power source line, and the terminator is connected between an output terminal of the regulator and the signal line. A zener diode is connected between the output terminal and the ground line in such a manner that a cathode terminal and an anode terminal of the zener diode are connected to the output terminal of the regulator and the ground line, respectively.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a terminating resistance circuit in an 
SCSI (Small Computer System Interface) system, and more particularly, to a 
terminating resistance circuit for a bus line between computer equipment 
in an improved version of an SCSI system. 
2. Description of the Background Art 
A connection of a bus line such as an SCSI-2, etc. for computer equipment 
is described in Japanese Patent Application No. 3-61174, which constitutes 
a background of the invention. 
More specifically, a connection example of such an SCSI-2 is shown in FIG. 
4. FIG. 4 shows an example of a system for the SCSI-2, which is called as 
a single-ended type. In FIG. 4, the reference numeral 21 denotes an 
initiator such as a host computer, the reference numeral 22 denotes a 
target such as a CD-ROM device, and the reference numeral 23 denotes an 
SCSI bus line for connecting the initiator 21 and the target 22. As the 
bus line 23, a cable a with connector, the number of terminals of which is 
50 or 68 is used. The bus line 23 includes a power source line 23a, a 
signal line 23b and a ground line 23c. The power source line 23a is a 
single line, to which 5V is applied normally. The signal line 23b is 
illustrated as a single line, but in fact, 18 signal lines are included 
therein. The ground line 23c also includes a plurality of ground lines. 
The reference numerals 24 and 25 denote a driver and a receiver, 
respectively for sending a signal to the signal line 23b and for receiving 
a signal from the signal line 23b. 
The reference numeral 26 denotes a terminator connected to the signal line 
23b. The terminator 26 is used for matching an impedance of a bus cable, 
and therefore, a signal of "0" or "1" is transmitted between the SCSI 
devices such as a initiator 21, the target 22 etc. reliably as a 
rectangular wave. Accordingly, it is necessary to provide the terminator 
26 for each of the SCSI devices such as the initiator 21, the target 22 
etc. Since the number of the signal lines 23b is 18, eighteen terminators 
26 are to be provided in a case of a circuit of the single-ended type. In 
addition, a resistance value of the terminator 26 is 110 ohms. The 
reference numeral 27 denotes a regulator having an input terminal 27a and 
an output terminal 27b which are connected to a side of the power source 
line 23a and a side of the terminator 26, respectively. The regulator 27 
is for adjusting a voltage at the power source line 23a, that is, normally 
5V, to 2.85V to supply a constant voltage to the terminator 26. The 
reference numeral 28 denotes a capacitor provided between the vicinity of 
the regulator 27 and the ground line 23c, which is for removing noise and 
ripple on the power source line 23a and an output side of the regulator 
27. 
In order to attach or detach the terminator 26 to or from the bus line 23 
in the terminating resistance circuit, a plurality of switching elements 
(not shown) each of which is an active element are provided between the 
terminator 26 and the output terminal 27b of the regulator, that is, at B 
point. Since the number of the terminators 26 is to be equal to the number 
of the signal lines 23b, the number of switching elements is also 
corresponding to the number of the terminators 26. 
In general, the terminator 26, the regulator 27, the capacitor 28, etc. 
constituting the above described terminating resistance circuit are 
incorporated within an integrated circuit together with the switching 
elements, and one example of the integrated circuit is shown in FIG. 5. C 
point, D point and E point of FIG. 5 are respectively corresponding to C 
point, D point and E point of FIG. 4. In FIG. 5, the reference numeral 29 
denotes a switching element provided between the terminator 26 and the 
regulator 27, which is constituted by two PNP transistors. The reference 
numeral 30 denotes a bias resistor for biasing the switching element 29, 
and the reference numeral 31 (a portion surrounded by a dotted line) is a 
switching element which is a transistor incorporating resistor. The 
switching element 31 functions as a switch for turning the switching 
element 29 on or off in response to a signal from an exterior source. 
Although only 9 of the terminators 26 are illustrated for simplifying the 
drawing, in fact, the number of terminators 26 is 18, that is, the number 
of the signal lines. In addition, in a case where only 9 terminators 26 
are provided as shown in FIG. 5, two integrated terminating resistance 
circuits may be utilized. 
Next, transmission of signals will be described. FIG. 6 is an illustrative 
view showing a major portion of a terminating resistance circuit in a case 
where an opened collector type driver 24a is utilized for the driver 24 
shown in FIG. 4. Vref of FIG. 6 shows the output voltage 2.85V from the 
regulator shown in FIG. 4. Furthermore, the driver 24a is mainly 
constituted by a single NPN transistor, and a signal responsive to a 
signal at the signal line 23b is inputted to a base thereof. The signal at 
the signal line 23b is a rectangular wave having levels of 0 to 2.85V as 
shown in FIG. 7. Therefore, the above described transistor is turned-on or 
off at a level of a threshold value shown by a dotted line in FIG. 7. 
Therefore, in FIG. 6, if the transistor is turned-on, the signal line 23b 
is connected to the ground, and thus, a signal of the low level is 
transmitted. In contrast, if the transistor is turned-off, the signal line 
23b is not connected to the ground, and therefore, a signal of the high 
level is transmitted. 
However, in a case where the above described driver 24a is utilized, when a 
cable of the bus line 23 is long or when a high frequency signal is to be 
transmitted, there is an occasion that the approximately 2.85V signal 
level of the high level at the signal line 23b is lowered by about 1V, and 
therefore, it becomes impossible for the transistor of the receiver within 
the other target to detect the high level. Resultingly, there was a 
problem that the signal cannot be transmitted reliably. 
Therefore, the above described problem can be solved if an active negation 
circuit 24b is utilized for the driver as shown in FIG. 8. The driver 24b 
is mainly constituted by two NPN transistors 31a and 31b and a power 
source VCC for supplying a power voltage of 5V. To the bases of the 
transistors 31a and 31b, signals are applied which are alternately 
inverted to each other in response to a signal responsive to a signal at 
the signal line 23b. Therefore, if the transistor 31a is turned-off and 
the transistor 31b is turned-on, the signal line 23b is connected to the 
ground, and therefore, the signal line 23b becomes the low level. In 
contrast, if the transistor 31a is turned-on and the transistor 31b is 
turned-off, since the voltage of 5V is supplied to the signal line 23b, 
the signal line 23b becomes the high level. Thus, when the driver 24b is 
utilized, since the high level of 5V is higher than that of the driver 
24a, the high level is not erroneously detected even if the level thereof 
is lowered by about 1V. 
However, as shown in FIG. 9, it is necessary to provide the above described 
driver 24 for each signal line 23b. If the high level of the signal line 
23b, 5V, is applied to the regulator 27, a current flows to a signal line 
which is at the low level. For example, in the terminating resistance 
circuit which is connected to 9 signal lines as shown in FIG. 5, when the 
signals at 8 signal lines are the high levels, an overcurrent will flow to 
a driver which is connected to the remaining signal line which is at the 
low level. Resultingly, there was an occasion that the driver is 
destroyed. 
SUMMARY OF THE INVENTION 
Therefore, a principal object of the present invention is to provide a 
novel terminating resistance circuit. 
Another object of the present invention is to provide a terminating 
resistance circuit in which a signal can be transmitted reliably and a 
controller such as a driver etc. cannot be destroyed. 
In order to implement above described object, a terminating resistance 
circuit according to the present invention is a terminating resistance 
circuit in a bus line in which a power source line, a signal line, and a 
ground line are connected between computer equipment through terminators, 
respectively, and characterized in that an input terminal of a regulator 
is connected to the power source line, a terminator is connected between 
an output terminal of the regulator and the signal line, and a zener diode 
is connected between the output terminal of the regulator and the ground 
line in a manner that a cathode terminal of the zener diode is connected 
to the output terminal of the regulator and an anode terminal of the zener 
diode is connected to the ground line. 
As described above, in a terminating resistance circuit according to the 
present invention, since the zener diode is connected between the output 
terminal of the regulator and the ground line, if a zener diode having a 
zener voltage of approximately 3V is selected for example, even if a 
voltage of 5V is applied to the terminator when a signal at the signal 
line is the high level, a voltage of the terminator at a side of regulator 
becomes approximately 3V, and therefore, no overcurrent flows in a 
controller such as a driver, receiver etc. connected to a signal line of 
the low level. Therefore, the controller is not destroyed. 
The above described objects and other objects, features, aspects and 
advantages of the present invention will become more apparent from the 
following detailed description of the present invention when taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 1 which shows a connection example of SCSI-2, the 
reference numeral 1 denotes an initiator such as a host computer, the 
reference numeral 2 denotes a target such as a CD-ROM device, and the 
reference numeral 3 denotes an SCSI bus line for connecting the initiator 
1 and the target 2. As the bus line 3, a cable with connector the number 
of terminals of which is 50 or 68 is used. The bus line 3 includes a power 
source line 3a, a signal line 3b and a ground line 3c. The power source 
line 3a is a single line, to which 5V is applied normally. The signal line 
3b is illustrated as a single line, but in fact, 18 signal lines are 
included therein. The ground line 3c also includes a plurality of ground 
lines. The reference numerals 4 and 5 denote a driver and a receiver, 
respectively for sending the signal line 3b and for receiving a signal 
from the signal line 3b. 
The reference numeral 6 denotes a terminator connected to the signal line 
3a. The terminator 6 is used for matching an impedance of a bus cable such 
that a signal of "0" or "1" can be transmitted between the SCSI devices 
such as an initiator 1, the target 2 etc. as a precise rectangular wave. 
Accordingly, it is necessary to provide the terminator 6 for each of the 
SCSI devices such as an initiator 1, the target 2, etc. Since the number 
of the signal lines 3b is 18, eighteen terminators 6 are to be provided in 
a case of a circuit of the single-ended type. In addition, a resistance 
value of the terminator 6 is 110 ohms. The reference numeral 7 denotes a 
regulator having an input terminal 7a and an output terminal 7b which are 
connected to a side of the power source line 3a and a side of the 
terminator 6, respectively. The regulator 7 is for adjusting a voltage at 
the power source line 3a, that is, normally 5V, to 2.85V to supply a 
constant voltage to the terminator 6. The reference numeral 8 denotes a 
capacitor provided between the vicinity of the regulator 7 and the ground 
line 3c, which is for removing a noise and ripple on the power source line 
3a and an output side of the regulator 7. 
The reference numeral 9 denotes a zener diode having a cathode terminal 9a 
connected to the output terminal 7b of the regulator 7 and an anode 
terminal 9b connected the the ground line 3c. The zener diode 9 is one 
having a zener voltage of approximately 3V in this embodiment, and it is 
preferable to use a zener diode having a zener voltage which is 
approximately equal to the output voltage of the regulator 7, that is, 
Vref, but, the zener voltage is needed to be higher than the Vref. A 
reason why is that if the zener voltage is lower than Vref, the current 
always flows from the regulator 7 to the zener diode 9, and therefore, the 
reliability of the regulator 7 and the zener diode 9 is lowered. 
In order to attach or detach the terminator 6 to or from the bus line 3 in 
the terminating resistance circuit, a plurality of switching elements (not 
shown) each of which is an active element are provided between the 
terminator 6 and the output terminal 7b of the regulator, that is, at B 
point. Since the number of the terminators 6 is to be equal to the number 
of the signal lines 3b, the number of switching elements is also 
corresponding to the number of the terminators 6. 
In general, the terminator 6, the regulator 7, the capacitor 8, the zener 
diode 9, etc. constituting the above described terminating resistance 
circuit are incorporated within an integrated circuit together with the 
switching elements, and one example of the integrated circuit is shown in 
FIG. 2. C point, D point and E point of FIG. 2 are respectively 
corresponding to C point, D point and E point of FIG. 1. In FIG. 2, the 
reference numeral 10 denotes a switching element provided between the 
terminator 6 and the regulator 7, which is constituted by two PNP 
transistors. The reference numeral 11 denotes a bias resistor for applying 
a bias voltage to the switching element 10, and the reference numeral 12 
(a portion surrounded by a dotted line) is a further switching element 
which is a transistor incorporating resistor. The switching element 12 
functions as a switch for turning the switching element 10 on or off in 
response to a signal from an exterior source. Although only 9 terminators 
6 are illustrated for simplifying the drawing, in fact, the number of 
terminators 6 is 18, that is, the number of the signal lines. In addition, 
in a case where only 9 terminators 6 are provided as shown in FIG. 2, two 
integrated terminating resistance circuits may be utilized. 
Meanwhile, the terminating resistance circuit shown in FIG. 2 is only an 
example, and therefore, an arbitrary circuit can be utilized while an 
object of the present invention can be performed. For example, in a case 
where the terminating resistance circuit is to be integrated on the 
substrate, the switching element 10, the switching element 12, the 
capacitor 8, etc. may be provided on a separate substrate. In a case where 
the terminator 6 is not needed to be electrically attached or detached, it 
is unnecessary to provide the switching elements 10 and 12. 
In addition, the present invention can be applied to the single-ended type 
as well as a differential type shown in FIG. 3. In FIG. 3, elements and 
parts corresponding to those in FIG. 2 are labeled with the same reference 
numerals. 
Furthermore, it is preferable that the terminating resistance circuit 
according to any one of the embodiments is manufactured by utilizing a 
hybrid IC technique, but the same may be manufactured by utilizing another 
method such as a monolithic IC technique. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.