Inactive state termination tester

An inactive state termination tester for determining the number of terminators connected to a bus when the bus is inactive. The termination tester monitors the bus for determining when it is inactive. During the periods of bus inactivity, the termination tester senses the bus to determine the number of terminators connected to the bus. The termination tester then displays how many terminators were detected.

BACKGROUND OF THE INVENTION 
This invention relates generally to terminator devices and in particular, 
it relates to a detection method and apparatus used with a bus for 
detecting the number of terminators connected to the bus transmission 
lines. 
One application where reliable data transfers are important is in the field 
of data transmissions on a bus transmission line for communication between 
a plurality of peripherals and a central processing unit in a small 
computer. For example, a Small Computer System Interface (SCSI) bus 
provides for such a communication link. 
As is generally known in the art of designing bus transmission lines, a 
terminator, comprising of a resistor network, is typically connected to 
each of the physical ends of the bus in order to eliminate line reflection 
created by the transmission of signals on the bus. Both of the terminators 
have a characteristic impedance equal to that of the bus. 
Many peripherals, such as disk drives, are adapted to connect to the bus. 
These peripherals are usually equipped with a bus terminator which is to 
be removed if the peripheral is not connected to one end of the bus. 
However, with the addition and removal of various peripherals from the 
bus, terminators may inadvertently be omitted or more than one terminator 
may be inadvertently connected to the bus. 
Visual verification of the number of terminators connected to the bus 
normally requires the system to be turned off and the system components 
disassembled. However, there are prior art designs which measure the 
number of terminators connected to the bus, but all of these designs share 
the same disadvantage of requiring the computer system to be rendered 
inactive in order for the bus to be tested. 
Therefore, there is a need for a diagnostic apparatus which has the ability 
to determine the number of terminators present on a bus without 
deactivating the computer system. The present invention is such an 
apparatus. 
In view of the above, it is an object of the invention to determine if the 
bus is active. If the bus is found to be inactive, the diagnostic phase 
commences. 
It is another object of the present invention to determine the number of 
terminators connected to the bus. 
It is still another object of the present invention to display the number 
of terminators connected to the bus. 
SUMMARY OF THE INVENTION 
In one form of the invention, an inactive state termination tester 
determines the number of terminators connected to a bus when the bus is 
established to be in an inactive state. The termination tester operates by 
monitoring the bus to determine when it is inactive. When the bus is 
determined to be inactive, the termination tester senses the bus to 
determine how many terminators are connected to the bus. The termination 
tester then displays how many terminators were detected. 
Various means for practicing the invention and other advantages and novel 
feature thereof will be apparent from the following detailed description 
of an illustrative preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
Referring to the drawing, and particularly to FIG. 1, a functional block 
diagram of an inactive state termination tester apparatus 10 is depicted 
with a bus-inactivity detector 12, a terminator detector 14, and a display 
16. 
The bus-inactivity detector 12 is adapted to be used with or connected to a 
bus and receive a bus activity signal(s) 18 indicating if the bus is 
either active or inactive. When the bus is inactive, the bus-inactivity 
detector 12 transmits a signal to the terminator detector 14 indicating 
that testing of the bus should commence. The terminator detector 14 then 
determines the number of terminators connected to the bus transmission 
line 20 and this result is received and presented by the display 16. 
Turning to FIG. 2, an inactive state termination tester apparatus 110 is 
depicted which is constructed in accordance with the principles of the 
present invention for use with a SCSI bus. It is to be understood that the 
embodiment shown in association with a SCSI bus is not intended to serve 
as a limitation upon the scope or teaching thereof, but is merely for the 
purpose of convenience of illustration of one example of the invention's 
application. 
The inactive state termination tester apparatus 110 which is used with or 
connected to the SCSI bus includes a comparator U1 which is configured as 
a logic AND gate for the purpose of determining when the SCSI bus is 
inactive. To accomplish this task, one end of a resistor R1 is connected 
to the BUSY signal 112 of the SCSI bus and the other end of the resistor 
is connected to the non-inverting input of the comparator U1. Likewise, 
one end of a resistor R2 is connected to the SELECT signal 114 of the SCSI 
bus with the other end of the resistor connected to the non-inverting 
input of the comparator U1. Also, a resistor R3 is connected to the 
non-inverting input of the comparator U1 and the other end of the resistor 
is connected to ground. The connecting of the resistors R1, R2, and R3 to 
the non-inverting input of the comparator U1 results in a voltage divider 
115 of the BUSY signal 112 and the SELECT signal. 114. 
To complete the logic AND gate configuration of the comparator U1, a +0.77 
volt reference 116 is applied to the inverting input of the comparator. 
The +0.77 volt reference 116 is provided by connecting resistors R8, R9, 
R10, and R11 in series, respectively, with the end of the resistor R11 
connected to ground and the end of the resistor R8 connected to a +2.85 
voltage source 118. Resistors R9, R10, and R11 are chosen to be of equal 
value and resistor R8 is chosen in order for the +0.77 volt reference 116 
to be provided at the junction of resistors R10 and R11. It is preferred 
that the +2.85 voltage source 118 only vary between .+-.5 millivolts to 
.+-.15 millivolts and be provided by a voltage regulator resident on a 
SCSI bus terminator such as that depicted in FIG. 2 (prior art) of U.S. 
Pat. No. 5,313,105 (1994), issued to Samela et at. Therefore, in order for 
the inactive state termination tester apparatus to be connected to the 
SCSI bus terminator voltage regulator, it is also preferred that the 
inactive state termination tester apparatus be mounted onto a SCSI bus 
terminator. 
A pull-up resistor R4 is connected to the +2.85 voltage source 118 and the 
other end of the resistor connects to the output of the comparator U1. The 
pull-up resistor R4 pulls the output of the comparator U1 to a high level 
when the comparator output is in an "open" state. 
Also connected to the output of the comparator U1 is the base of an NPN 
transistor switch Q1. The collector of the transistor switch Q1 is 
connected to a bus transmission line DP0 which is part of the SCSI bus and 
is preferably that of the parity transmission line. Correspondingly, the 
bus transmission line DP0 connects to one end of all terminators 
(terminators are not shown in FIG. 2) attached to the SCSI bus. Likewise, 
the opposite end of all terminators attached to the SCSI bus are connected 
to the +2.85 voltage source 118. The total resistance of all terminators 
connected between the bus transmission line DP0 and the +2.85 voltage 
source 118 is referred to as the terminator resistance Rt. 
The emitter of the transistor switch Q1 is connected to a sensing resistor 
R5 which has its other end connected to ground. Furthermore, the emitter 
of the transistor switch Q1 also attaches to one end of a resistor R6 and 
the opposite end of the resistor is connected to the non-inverting input 
of an amplifier U2. 
The amplifier U2 is configured as a non-inverting amplifier having a gain 
such that the voltage across sensing resistor R5 causes 3.0 volts at the 
output of the amplifier when the terminator resistance Rt equals that of 
three (3) terminators connected to the SCSI bus. To set the gain of the 
amplifier U2, a resistor R7 is connected to ground with its other end 
connected to the inverting input of the amplifier U2. Likewise, one end of 
a feedback resistor Rf is connected to the inverting input of the 
amplifier U2 and the other end of the feedback resistor is connected to 
the output of the amplifier. 
The output of the amplifier U2 is connected to the inverting inputs of 
comparators U3, U4, and U5. The non-inverting input of U5 is connected to 
the +0.77 voltage reference 116. Likewise, the non-inverting input of U4 
is connected a +1.54 voltage reference 120 provided at the junction of 
resistors R9 and R10. Furthermore, the non-inverting input of U3 is 
connected to a +2.31 voltage reference 122 provided at the junction of 
resistors R8 and R9. 
To provide the +1.54 voltage reference 120 and the +2.31 voltage reference 
122, the resistors R9, R10, and R11 are chosen to be of equal value and 
resistor R8, as described previously, was chosen so that the voltage at 
the junction of resistors R10 and R11 is approximately +0.77 volts. This 
resistor combination will cause the voltage at the junction of R9 and R10 
to essentially be two times +0.77 volts and the voltage at the junction of 
R8 and R9 to essentially be three times +0.77 volts. These voltage levels 
are preferred in order that the inactive state termination tester 
apparatus effectively detects the various terminator types defined by the 
SCSI-2 specification. 
The output of comparator U3 is connected to the cathode of a light emitting 
diode (LED) D4. The anode of LED D4 is connected to one end of a resistor 
R12 and the other end of the resistor is connected to a termination power 
supply voltage TERMPOWER (TP) 124, which typically is at +5.0 volts but 
can be varied in the range of +4.25 volts to +5.25 volts in accordance 
with ANSI specification X3T9.2/86-109REV10c. Likewise, the output of 
comparator U4 is connected to the cathode of a LED D3. The anode of LED D3 
is connected to one end of a resistor R13 and the other end of the 
resistor is connected to TP 124. Finally, the output of comparator U5 is 
connected to the cathode of a LED D2. The anode of LED D2 is connected to 
one end of a resistor R14 and the other end of the resistor is connected 
to TP 124. 
One end of a resistor R15 is also connected to TP 124 with the other end of 
the resistor connected to the anode of a LED D1. The cathode of the LED D1 
is connected to ground. 
In the embodiment shown in FIG. 2, the comparators U1, U3, U4, and U5 are 
all part of a single integrated chip which has a voltage input pin 3 which 
is connected to TP 124. Similarly, voltage input pin 4 of the amplifier U2 
is connected to TP 124. 
The operation of the inactive state termination tester for use with a SCSI 
bus will now be explained with reference to FIG. 2. In particular, it must 
be recognized that a SCSI system communicates using a well defined 
protocol. One of the elements of the protocol is a means of determining if 
the bus is active or inactive. The SCSI bus is known to be inactive when 
the BUSY signal line and the SELECT signal line are simultaneously at a 
high logic level. 
When the BUSY signal 112 is inactive (i.e., at a high logic level of 
greater than +2.5 volts), and the SELECT signal 114 is inactive (i.e., 
also at a high logic level of greater than +2.5 volts), the voltage formed 
at the non-inverting input of comparator U1 will be greater than the +0.77 
voltage reference 116 connected to the inverting input of the comparator. 
This will result in the output of the comparator U1 to be pulled to a high 
logic level, thereby indicating the logical AND condition of the BUSY 
signal 112 and the SELECT signal 114. It may be shown that other 
combinations of the BUSY and the SELECT logic voltage levels will produce 
a voltage at the non-inverting input of comparator U1 which is less than 
the inverting reference voltage of 0.77 volts. In these cases the output 
of comparator U1 will go to a low logic level, thereby indicating the 
absence of the logical AND condition of the BUSY and the SELECT signal 
lines. 
The diagnostic phase begins when the terminator resistance Rt is connected 
to sensing resistor R5 through the transistor switch Q1. 
The base of the transistor switch Q1 is controlled by the output of 
comparator U1. When the logical AND condition of the BUSY signal 112 and 
the SELECT signal 114 is met, the transistor switch Q1 is rendered 
conductive, thereby connecting the terminator resistance Rt associated 
with bus transmission line DP0, through the transistor switch Q1, to the 
sensing resistor R5. The voltage at the junction of the terminator 
resistance Rt and sensing resistor R5 will be dependent upon the number of 
terminators present in the system. Correspondingly, the output voltage of 
amplifier U2 will be an indication of the number of terminators connected 
to the bus. 
Since the terminators are connected to the bus in parallel, as the number 
of terminators present on the bus increase, the equivalent terminator 
resistance Rt will decrease. Thus, both the voltage at the sensing 
resistor R5 and the output voltage of amplifier U2 will increase as the 
number of terminators present on the bus increase. 
Correspondingly, the output voltage of the amplifier U2 will exceed the 
voltage applied to the non-inverting input of the comparator U5 when one 
terminator is connected to the bus. Likewise, the output voltage of the 
amplifier U2 will exceed the voltage applied to the non-inverting input of 
both comparator U4 and comparator U5 when two terminators are connected to 
the bus. Finally, the output voltage of the amplifier U2 will exceed the 
non-inverting input of comparators U3, U4, and U5 when three or more 
terminators are connected to the bus. 
When the voltage applied to the inverting input of comparator U3, U4, 
and/or U5 exceeds the voltage applied to the non-inverting input of a 
comparator, the output of the comparator will go to a low logic level. 
When the comparator's output is at the low logic level current will flow 
through the LED, causing it to light, thereby indicating the number of 
terminators present in the system. 
LED D2 will illuminate when one terminator is attached to the bus. 
Likewise, LED D2 and LED D3 will illuminate when two terminators are 
connected to the bus. Furthermore, LED D2, LED D3, and LED D4 will 
illuminate when three or more terminators are detected. 
As a way of ensuring that power is applied to the inactive state 
termination tester 110, LED D1 will illuminate when the TP 124 voltage is 
applied to the termination tester. 
For completeness in the disclosure of the above-described inactive state 
termination tester apparatus, when it is used in conjunction with a SCSI 
bus, but not for purpose of limitation, the following representative 
values and component identifications are submitted. These values and 
components were employed in an inactive state termination tester apparatus 
that was constructed and tested and which provides a high quality 
performance. Those skilled in the art will recognized that many 
alternative elements and values may be employed in constructing the 
circuity in accordance with the present invention. 
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Part TYPE or VALUE 
______________________________________ 
R1 3.9K Ohms 
R2 3.9K Ohms 
R3 1K Ohms 
R4 470 Ohms 
R5 10 Ohms 
R6 1K Ohms 
R7 1K Ohms 
R8 680 Ohms 
R9 1K Ohms 
R10 1K Ohms 
R11 1K Ohms 
R12-15 225 Ohms 
Rf Select 
D1-D4 HLMP1585 
Q1 2N2222 
U1 LM339D 
U2 LM324D 
U3-U5 LM339D 
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As previously stated above, the value of the feedback resistor Rf is 
selected in order for 3.0 volts to be present at the output of the 
amplifier U2 when the terminator resistance Rt equals that of three (3) 
terminators connected to the SCSI bus. The feedback resistor Rf is 
selected because it is an easy way to compensate for the error tolerances 
of the other components used in the manufacture of the inactive state 
termination tester. 
It should be understood that various changes and modifications to the 
presently preferred embodiments described herein will be apparent to those 
skilled in the art. Such changes and modifications may be made without 
departing from the spirit and scope of the present invention and without 
diminishing its attendant advantages. For example, with minor variations, 
an inactive state termination tester can be used in conjunction with a 
VMEbus or any other various data transmission line configurations. 
Therefore, it is intended that such changes and modifications be covered 
by the appended claims.