Combination of terminator apparatus enhancements

An SCSI bus line apparatus including in combination at least two of any of an active deassertion (ADR) circuit, a signal line impedance matching (SLIM) circuit and a signal line increased circuit kicker (SLICK) circuit. In the ADR circuit, if a current sinking circuit senses an increased voltage on the common node of the signal lines of a bus due to active deassertion of a signal line, the current sinking circuit sinks enough current from the signal lines to prevent an over-current condition on asserted signal lines or soon-to-be asserted signal lines. In the SLIM circuit, transient voltages are removed from a signal line by limiting to within a range the voltages that can appear on the signal line. In the SLICK circuit, a current switching device is controlled to provide current to raise the voltage level of a notch occurring in a signal on a signal line when the line is deasserted, but is responsive to a monitoring circuit for disconnecting the current when a programmed length of time has been exceeded after assertion.

BACKGROUND OF THE INVENTION 
The present invention generally relates to terminator apparatus. More 
specifically, the present invention relates to terminator apparatus used 
with a SCSI (acronym for small computer system interface) bus line. 
One application where reliable data transfer becomes important is in the 
field of data transmission on a fully loaded SCSI bus line for 
communication between a plurality of data transceivers and a central 
processing unit (CPU) in a small computer. A SCSI system has a 
transmission line over which a plurality of units coupled to the 
transmission line may communicate. A regulated terminator, in accordance 
with the standard set forth in American National Standard for Information 
Systems X3T9.2/82-2 (the "ANSI standard"), is coupled to the two physical 
ends of the transmission line. 
The SCSI system has drivers which drive the individual signal lines of the 
transmission line. A signal line may be in one of three states. First, a 
signal line is said to be "asserted" if a driver drives the signal line to 
the ground. Second, signal lines that are released from the asserted state 
are said to be "deasserted." Third, signal lines that are driven from the 
asserted state by a driver are said to be "actively deasserted." 
The ANSI standard requires that asserted signal lines draw no more than 24 
milliamperes (mA) of current. However, it has previously been difficult, 
if not impossible, to comply with this standard when active deassertion is 
used. 
Deasserted signal lines have 2.85 volts on them because the regulated 
terminator made in accordance with the ANSI standard has a 2.85 volt 
voltage regulator that pulls deasserted lines up to 2.85 volts. If other 
signal lines are asserted (i.e., grounded), they will draw no more than 
the 24 mA allowed by the ANSI standard due to the 110 ohm resist or in 
each signal line [(2.85-V.sub.ol of driver)/110 ohms]. 
However, when a signal line is actively deasserted, it may have anywhere 
from 3.0 to 5 volts on it. If the voltage regulator of the regulated 
terminator cannot sink current, this higher voltage on the actively 
deasserted signal line is not regulated to 2.85 volts. Thus, if a signal 
line is asserted, it will draw more than 24 mA (e.g., 4.0 volts/110 
ohms=36 mA). This may damage the driver or cause it to malfunction. 
Although others have attempted to address driver damage and/or malfunction, 
attempts have fallen short of an adequate solution. For instance, Texas 
Instruments claims that its part number TL1431 can be used, along with 
other components, to sink current such that it is possible for an asserted 
signal line to draw an amount of current within the ANSI standard even if 
another signal line is actively deasserted. However, the TL1431, by Texas 
Instruments' admission, is limited to applications wherein only a limited 
number of signal lines are actively deasserted. 
Typically, control signals REQ and ACK on respective request and 
acknowledge signal lines are used to perform a "handshake" so as to 
transfer the data back and forth between a target (i.e., a disk drive) and 
an initiator (i.e., a host computer). Each of the control signals REQ and 
ACK is either a high logic value or a low logic value. When the control 
signal REQ or ACK is asserted, it is at the low logic value. When the 
control signal REQ or ACK is deasserted, it is at the high logic value. 
In operation, when the control signal REQ on the request line makes the 
transition from the asserted condition to the deasserted condition (i.e., 
low-to-high transition), there exists a condition of the SCSI bus line 
which can cause the corruption of data. At the rising edge of the control 
signal REQ, there will appear what is referred to as a "notch." This notch 
is typically accompanied by a reflection caused by stub drive cabling. The 
combined effect of the notch and the stub reflection will cause the rising 
edge of the control signal REQ to reverse its direction and "double back" 
before reaching the deassertion level (i.e., +2.0 volts). If this 
reversing control signal REQ falls below the +1.5 volt level, it may 
result in a "double trigger" and be interpreted as a valid request signal 
(i.e., another assertion), thereby causing erroneous data to be 
transferred. 
As is generally known in the art of computer equipment manufacturing, a 
termination device is typically connected to each end of one OR-WIRED SCSI 
bus line for supplying a fixed supply voltage with a predetermined 
impedance. A bus line is one of a plurality of signal lines of a bus. 
In FIG. 1, there are shown prior art termination networks 10 and 12 
sometimes referred to as "220/330 terminators." Each of the termination 
networks 10 and 12 includes a voltage divider formed of two resistors T1 
and T2 connected in series. One end of the resistor T1 is connected to an 
input power supply voltage TERMPWR, which is typically at +4.75 volts. One 
end of the resistor T2 is connected to a ground potential GND, which is 
typically at zero volts. The junction J1 of the resistors T1 and T2 for 
the termination network 10 is connected to one end of a bus line 14, and 
the junction J2 of the resistors T1 and T2 for the termination network 12 
is connected to the other end of the bus line 14. For the SCSI bus, the 
value of the resistors T1 and T2 are shown to be 220.OMEGA. and 330.OMEGA. 
respectively so as to provide approximately +2.85 volts at the junction 
points J1 and J2 when the bus line 14 is not active. 
The OR-WIRED SCSI bus 14 has a plurality of transceivers 16a, 16b and 16c 
which are connected thereto by respective signal lines 18a, 18b and 18c. 
Each of the transceivers 16a-16c includes a controller driver 20 having an 
open collector output (i.e., NAND logic gate type 7438) and a receiving 
device 22 (i.e., Schmitt trigger input type 7414). When the controller 
driver 20 is turned off, the signal line (i.e., line 18a) is at a high 
logic level which has a voltage value of approximately +2.85 volts. When 
the controller driver 20 is turned on, the signal line is at a low logic 
level since the open collector output device will pull the voltage value 
on the signal line down to approximately zero volts (i.e.,0.2 volts 
through the driver transistor Q1). 
In FIG. 2, there is shown another prior art termination network 10a. This 
alternative arrangement of FIG. 2 is sometimes referred to as a "110 
regulated terminator." The termination network 10a includes a voltage 
regulator 24 for receiving the voltage TERMPWR on line Vin and for 
generating a regulated voltage on line Vout of +2.85 volts with 110 ohm 
resistors to the respective 18 signal lines DB(0)-DB(7), DB(P), ATN, BSY, 
ACK, RST, MSG, SEL, C/D, REQ and I/O. 
The principal unsolved problem of the prior art termination networks 10 or 
10a was that neither one had the capability of raising the notch above the 
critical "double trigger" area (between +1.5 volts and +2.0 volts). As can 
be seen from the curve A of FIG. 5, the signal line in the 220/330 
terminator has a notch N1 occurring at approximately +1.0 volts. 
Similarly, there is shown in the curve B of FIG. 5 that the signal line in 
the 110 regulated terminator has a notch N2 occurring at approximately 
+1.4 volts. Thus, these prior art termination networks did not teach how 
the notch could be raised above the critical area. 
Further, with reference again to the prior art terminator of FIG. 1, it is 
also generally known that the voltage level of the notch is determined by 
the cable impedance and the amount of current present in bus line 14a when 
it is released or deasserted by its respective controller driver 20. While 
the SCSI specification defines cable impedance to be "no less than 90 
ohms," twisted pair or "round cable" impedance is seldom found to be 
greater than 90 ohms and may be as low as 45 ohms. 
Even though these controller drivers 20 have the capability of sinking more 
current, a problem occurs, referred to as "metal migration" when sinking 
high current over prolonged periods of time. This situation arises when 
the request control signal (REQ) is asserted, (low logic level) and the 
system fails to respond with the associated acknowledge control signal 
(ACK). In other words, the control signal REQ is being constantly asserted 
and does not deassert itself. Thus, this will cause the system to "hang" 
until the error is cleared up by deasserting the control signal REQ. If 
the unattended system hangs over an extended holiday weekend, this 
situation could last for many days without being corrected. 
There is also known in the prior art of a terminator design which utilizes 
a diode clamping technique for raising the level of the notch by providing 
current in excess of the 24 mA. However, this prior art technique suffers 
from the disadvantage that this extra current is uncontrolled and may vary 
between 24 mA to 45 mA. Further, this extra current will be provided to 
the controller driver indefinitely when the system "hangs" and thus may 
result in the destruction of the costly controller driver. 
Accordingly, a need has arisen in the computer equipment industry for an 
improved terminator apparatus for raising the level of the notch occurring 
in the data and/or control signals on the SCSI bus line so as to provide 
reliable and accurate data transmission. Further, it would be expedient 
that the terminator apparatus be capable of being incorporated internally 
into conventional termination networks or can be connected externally to 
existing systems having conventional termination networks. 
Still further, a need has arisen in the computer equipment industry for an 
improved terminator apparatus for removing noise spikes or transients 
occurring in the control signals REQ and ACK on the SCSI bus line so as to 
provide reliable and accurate data transmission. It would also be 
expedient that the terminator apparatus be capable of being readily 
modified so as to accommodate a range of SCSI cable impedances. 
Yet another need exists for an apparatus and a method for using the 
apparatus which complies with the ANSI standard in that it will not allow 
an asserted signal line to draw more current than is specified by the ANSI 
standard, in cases where the maximum allowable number of signal lines are 
actively deasserted. 
SUMMARY OF THE INVENTION 
The present invention provides an SCSI bus line apparatus including in 
combination at least two of any of an active deassertion (ADR) circuit, a 
signal line impedance matching (SLIM) circuit and a signal line increased 
circuit kicker (SLICK) circuit. In the ADR circuit, if a current sinking 
circuit senses an increased voltage on the common node of the lines of a 
bus due to active deassertion of a signal line, the current sinking 
circuit sinks enough current from the signal lines to prevent an 
over-current condition on asserted signal lines or soon-to-be asserted 
signal lines. In the SLIM circuit, transient voltages are removed from a 
signal line by limiting to within a range the voltages that can appear on 
the signal line. In the SLICK circuit, a current switching device is 
controlled to provide current to raise the voltage level of a notch 
occurring in a signal on a signal line when the line is deasserted, but is 
responsive to a monitoring circuit for disconnecting the current when a 
programmed length of time has been exceeded after assertion. 
In an embodiment, the present invention provides a terminator apparatus 
used with a SCSI bus line for controlling the voltage level of a notch 
occurring on the bus line. Such terminator apparatus includes a 
termination network, a current switching device, and a programmed 
monitoring circuit. The termination network is interconnected between an 
input termination power supply voltage and a plurality of data and/or 
control signal lines coupled to the bus line for generating a first 
current. A current switching device is interconnected between the power 
supply voltage and each of the plurality of data and/or signal lines to be 
so terminated for generating a second current. 
The programmed monitoring circuit is connected to a control input of the 
current switching device and the at least one of the plurality of data 
and/or control signal line for controlling a programmed length of time 
that the second current is generated. The current switching device is 
responsive to the monitoring circuit for disconnecting the second current 
when the programmed length of time has been exceeded. The second current 
serves to raise the voltage level of the notch occurring in the at least 
one of the plurality of data and/or control signal lines when it is 
deasserted so as to prevent erroneous data from being transferred. 
In an embodiment, the present invention further provides an active 
deassertion circuit and a method of using the same. The active deassertion 
circuit is comprised of a means for providing a voltage reference that has 
an input and an output. Further, the active deassertion circuit also has a 
means for sinking current that has a first input, a second input, and 
output. The first input of the means for sinking current is coupled to the 
output of the means for providing a voltage reference. The means for 
sinking current compares the first input to the second input. If the 
voltage at the second input is higher than the voltage at the first input, 
due to active deassertion of at least one signal line of the transmission 
line, the means for sinking current sinks enough current to prevent an 
overcurrent condition on signal lines that are asserted or are going to be 
asserted. 
In an embodiment, the present invention further provides a terminator 
method and apparatus used with a SCSI bus line for removing noise spikes 
or transients from data and/or control signals received on the bus line. 
The terminator apparatus includes a voltage regulator which is responsive 
to an input termination power supply voltage for generating a regulated 
output voltage of a predetermined value. A low voltage regulating circuit 
is responsive to the regulated output voltage for generating a reference 
voltage. Resistors are interconnected between the regulated output voltage 
and a plurality of data and/or control signal lines coupled to the bus 
line for generating termination impedances. A suppression network is 
interconnected between at least one of the plurality of data and/or 
control signal lines and the reference voltage for removing transients 
from the at least one signal line by limiting the range of the upper 
voltage level and the lower voltage level appearing thereon. 
It is, therefore, an advantage of the present invention to provide an 
improved terminator apparatus which includes a termination network, a 
current switching device and a programmed monitoring circuit. 
It is yet another advantage of the present invention to provide a 
terminator method used with an SCSI bus line for controlling the voltage 
level of a notch occurring in data and/or control signals transferred on 
the bus line. 
Yet another advantage of the present invention is to provide an improved 
terminator method and apparatus for removing transients occurring in the 
data and/or control signals which is relatively simple and economical to 
manufacture and assemble. 
A still further advantage of the present invention is to provide an 
improved terminator apparatus for removing noise spikes or transients 
occurring in the control and/or data signals on the SCSI bus line so as to 
provide reliable, accurate data transmission. 
Moreover, another advantage of the present invention is to provide an 
improved terminator apparatus which includes a voltage regulator, a low 
voltage regulating circuit, a plurality of pull-up termination resistors, 
and a transient suppression network. 
Another advantage of the present invention is to provide a terminator 
method used with a SCSI bus line for removing noise spikes from data 
and/or control signals transferred on the bus line. 
Yet another advantage of the invention is the provision of a terminator 
apparatus including a circuit for sinking current appearing on a bus due 
to active deassertions of a signal line, thereby preventing an overcurrent 
condition on signal lines that are asserted or are going to be asserted. 
Additional features and advantages of the present invention are described 
in, and will be apparent from, the detailed description of the presently 
preferred embodiments and from the drawings.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
It is to be distinctly understood at the outset that the present invention 
shown in association with an 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 its application. 
The present invention has numerous applications in other fields and 
apparatus since the invention pertains to a terminator apparatus for 
controlling the voltage level of a notch occurring in data and/or control 
signals. 
Referring now in detail to the drawings, there is shown in FIG. 3 a block 
diagram of an improved signal line increased current kicker (SLICK) 
terminator apparatus 110 which is constructed in accordance with the 
principles of the present invention. The terminator apparatus 110 is 
adapted to be used with or is connected to ends of a SCSI bus line 14a 
(similar to the bus line 14 in FIG. 1) for raising the voltage level of a 
notch occurring in data and/or control signals transferred on the bus 
line. For example, the transceiver 16a of FIG. 1 can be connected to the 
bus line 14a in FIG. 3 via the signal line 18a designated as a request 
signal line for a control signal REQ. The terminator apparatus 110 is 
comprised of a conventional termination network 112 (such as termination 
network 10 or 12 of FIG. 1), a current switching device 114, and a 
programmed monitoring circuit 116. 
In FIG. 4, there is shown a detailed schematic circuit diagram of the SLICK 
terminator apparatus 110 of FIG. 3. For ease of illustration and 
discussion, the conventional termination network 112 is comprised of a 
220/330 termination network similar to the termination network 10 or 12 of 
FIG. 1. Thus, the 220/330 terminator 112 includes a voltage divider formed 
of two resistors T1 and T2 connected in series. One end of the resistor T1 
is connected to an input termination power supply voltage TERMPWR, which 
is typically at +4.75 volts. One end of the resistor T2 is connected to a 
ground potential GND, which is typically at zero volts. The junction of 
the resistors T1 and T2 for the termination network 112 is connected to 
one end of the bus line 14a. 
The current switching device 114 includes a bipolar transistor Q1 of the 
PNP-type conductivity and a current-limiting resistor R1. The transistor 
Q1 has its emitter connected also to the power supply voltage TERMPWR and 
its collector connected to one end of the resistor R1. The other end of 
the resistor R1 is connected to the bus line 14a. 
The programmed monitoring circuit 116 is comprised of resistors R2 and R3, 
a bipolar transistor Q2 of the NPN-type conductivity, and a capacitor C1. 
The transistor Q2 has its base connected to one end of the resistor R2, 
its collector connected to one end of the capacitor C1 and to one end of 
the resistor R3, and its emitter connected to the ground potential. The 
other end of the resistor R2 is connected to the bus line 14a, and the 
other end of the capacitor C1 is connected to the ground potential. The 
other end of the resistor R3 is connected to the base of the transistor 
Q1. 
It should be clearly understood to those skilled in the art that additional 
current switching devices and programmed monitoring circuits similar to 
114 and 116 could be connected to each of the other remaining signal lines 
so as to raise the voltage level of the notch appearing therein. However, 
in order to reduce costs, the current switching device and the programmed 
monitoring circuit are generally implemented only with the signal line 
which is highly sensitive to the notch. In other words, at least the 
control signal line (REQ) being the most critical to reliable and accurate 
data transfer has been implemented with such circuits. 
The operation of the present SLICK terminator apparatus 110 of FIG. 4 will 
now be explained with reference to the graphs of FIGS. 5, 6 and 7. In 
particular, the curve A of FIG. 5 illustrates the voltage level of the 
control signal REQ on the request signal line in the prior art 220/330 
terminator of FIG. 1. As can be seen, when the control signal REQ is 
switched from the asserted condition (low logic level) to the deasserted 
condition (high logic level), a notch N1 occurs at approximately +1.0 
volts. Similarly, the curve B of FIG. 5 depicts the control signal REQ on 
the request signal line in the prior art 110 regulated terminator of FIG. 
2. As can be seen, when the control signal REQ is switched from the 
asserted condition to the deasserted condition, the notch N2 occurs at 
approximately +1.4 volts. Therefore, the notches N1 and N2 could be 
interpreted by a receiving device coupled to the SCSI bus line 14a as a 
valid request signal. As a result, such notches may cause incorrect data 
to be transferred. 
The improved terminator apparatus 110 of FIG. 4 of the present invention 
will control or raise the voltage level of the notch appearing in the 
control signal REQ so as to produce the curve C of FIG. 5. When the 
control signal REQ makes the transition from the asserted condition to the 
deasserted condition, the notch N3 in the curve C will be raised above the 
"double trigger" area and now appears at approximately +1.8 volts. 
Referring now again to FIG. 4, when the control signal REQ is deasserted 
(high logic level), the transistor Q2 of the monitoring circuit 116 will 
be turned on due to the base drive current via the resistor R2. Any 
residual voltage which may be present on the capacitor C1 will be 
discharged through the transistor Q2 to ground. Additionally, the base of 
the transistor Q1 will also be placed at the ground potential via the 
resistor R3. As a result, the transistor Q1 will be rendered conductive. 
Consequently, the resistor R1 will then be connected to the power supply 
voltage TERMPWR via the transistor Q1. This stable condition causes the 
power supply voltage to be connected to the request signal line 14a via 
the resistor R1. 
When the control signal REQ on the line 18a is asserted (low logic level), 
the resistor R1 will provide instantaneously a second current path from 
the power supply voltage TERMPWR through the signal line 14a. This current 
flowing through the resistor R1 is in addition to that which is provided 
by the first current path via the resistor T1 in the termination network 
112. Since the transistor Q1 is already turned on prior to the time of 
assertion, there will be no transients created on the request signal line 
18a upon assertion. The amount of this additional current can be selected 
by controlling the value of the resistor R1 and may be set to a 
predetermined value consistent with cable impedance requirements. 
The monitoring circuit 116 will now measure the length of time that the 
request signal line is being asserted. Further, if the period of assertion 
is longer than the programmed length of time, the additional current 
furnished by the second current path will be canceled. Therefore, the 
current supplied to the controller driver in the transceiver 16a via the 
signal line 18a will be limited to the current supplied by the termination 
network 112. 
In particular, during the period of assertion, the transistor Q2 will be 
turned off. As a consequence, the capacitor C1 will begin to charge up 
towards the power supply voltage TERMPWR through the resistor R3 and the 
emitter-base junction of the transistor Q1, If the request signal line 18a 
remains asserted, such as a in the case of a "hung" system, the capacitor 
C1 will be charged to a point where the transistor Q1 will no longer be 
conductive. This condition occurs when the emitter-base voltage is less 
than approximately +0.7 volts. 
For typical SCSI voltage, this time period is approximately two time 
constants (i.e., Tc=R3.times.C1). At the end of the "timeout" period, the 
transistor Q1 will be turned off. Therefore, the current added to the 
request signal line 18a by the second current path via the resistor R1 
will be canceled, and only the current from the termination network 112 
will be provided. 
FIG. 8 illustrates another embodiment 200 of the terminator apparatus of 
FIG. 3 wherein the terminator network comprises a 110 ohm voltage 
regulated terminator. As illustrated, a data or control line 202, e.g. a 
REQ or ACK signal line, is coupled to a bus 206 via a 110 ohm resistor 
204. A suitable voltage regulator arrangement 208 is coupled between 
terminal power (TERMPWR) 210 and the bus 206. 
As also illustrated, a PNP transistor Q1 of a current switch 212 is coupled 
to line 202 via resistor 214. The base of the transistor Q1 is coupled to 
ground via serially connected resistor 216 and capacitor 218. The base of 
an NPN transistor Q2 is coupled to line 202 via a resistor 222 while the 
collector of transistor Q2 is connected between the resistor 216 and the 
capacitor 218. 
In a manner similar to the arrangement set forth in FIG. 4, when the signal 
line 202 is deasserted (high logic level), the transistor Q2 of the 
monitoring circuit 220 will be turned on due to the base drive current via 
the resistor 222. Any residual voltage on the capacitor 218 will then be 
discharged to ground via the transistor Q2. Additionally, the base of the 
transistor Q1 will also be placed at the ground potential via the resistor 
216. AS a result, the transistor Q1 will be rendered conductive. 
Consequently, the resistor 214 will then be connected to the regulated 
power supply out Vout via the transistor Q1. This stable condition causes 
the power supply voltage Vout to be connected to the signal line 202 via 
the resistor 214. Transistor Q1 and Q2 and the monitor circuit 220 then 
function as set forth above in connection with FIG. 4. 
In FIG. 6, curve 118 shows the amount of current flowing in the signal line 
of the prior art termination network 10 when it is pulled down below the 
minimum assertion level of +0.5 volts and all the way down to zero volts. 
Similarly, curve 120 shows the amount of current flowing in the signal 
line of the prior art termination network 10a when it is pulled down 
between +0.5 volts and zero volts. However, it will be noted that there is 
no appreciable amount of increased current, and thus the respective 
notches N1 and N2 in FIG. 5 will be created. On the other hand, curve 122 
shows the amount of current flowing in the signal line of the present 
terminator apparatus 110 when it is pulled down between +0.5 volts and 
zero volts. It can be seen that the current is increased in a highly 
controlled manner so as to raise the notch N3 (FIG. 5) and its associated 
stub reflection well above the critical "double trigger" area. 
In FIG. 7, there is shown the amount of current flowing in the signal line 
of the present terminator apparatus 110 as a function of time. As noted, 
when the programmed length of time is exceeded (i.e., 40 ms), the current 
will be slowly changed from approximately 41 mA to 24 mA. This gradual 
change is controlled by the capacitor C1 and the transistor Q1 and thus 
again avoids the generation of transients. 
In the typical situation, the control signal REQ will be deasserted (high 
logic level) before the expiration of this programmed length of time. When 
this deassertion occurs, the transistor Q2 will be turned on again so as 
to discharge the capacitor C1. As a result, the terminator apparatus 110 
is reset to its original condition, and a new cycle is repeated. It should 
be understood that the terminator apparatus is always reset when the 
request signal line is deasserted. In other words, whenever the error is 
cleared after the system is "hung," the terminator apparatus is returned 
automatically to its original condition. 
From the foregoing detailed description it can thus be seen that the 
present invention provides an improved terminator apparatus used with a 
SCSI bus line for controlling the voltage level of a notch occurring in 
data and/or control signals transferred on the bus line. The terminator 
apparatus is comprised of a termination network, a current switching 
device, and a programmed monitoring circuit. The terminator apparatus 
serves to prevent erroneous data from being transferred by raising the 
voltage level of the notch above the critical "double trigger" area. 
Referring now to FIG. 9, an improved signal line impedance matching (SLIM) 
terminator apparatus 310 is illustrated which is constructed in accordance 
with the principles of the present invention. 
Reference is also made to U.S. Pat. No. 5,239,559, the disclosure of which 
is incorporated herein by reference, wherein such SLIM terminator 
apparatus also is disclosed. 
The terminator apparatus 310 is adapted to be used with or is connected to 
ends of the SCSI bus line (such as bus line 214) for removing noise spikes 
or transients from data and/or control signals received on the bus line. 
The terminator apparatus 310 includes a voltage regulator 311 formed of an 
adjustable 3-terminal I.C. low voltage dropout regulator 312 such as 
LT1086CT, a low voltage regulating circuit 314, a plurality of pull-up 
termination resistors R1-R18, and transient suppression networks TS1 and 
TS2 for removing or attenuating transients or noise spikes. 
The low voltage dropout regulator 312 has its input pin 3 connected to an 
input terminal 316 for receiving an input termination power supply voltage 
TERMPWR (Vin), which is typically at +4.75 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. The voltage regulator 312 has an output pin 2 for 
generating a regulated output voltage (Vout), which is typically at 2.85 
volts. However, this output voltage can be adjusted to any value lower 
than +4.25 volts by selecting the values of series-connected resistors R19 
and R20 which are coupled between the output pin 2 and a ground potential 
GND. The junction of the resistors R19 and R20 is connected to an 
adjustment pin 1 of the voltage regulator. 
A capacitor C1 is connected between the input terminal 316 and the ground 
potential GND for shunting any low frequency noise appearing on the input 
termination power supply voltage TERMPWR to the ground potential. A 
capacitor C3 is connected between the output pin 2 of the voltage 
regulator and the ground potential GND for shunting any low frequency 
noise appearing on the output voltage Vout to the ground potential. A 
capacitor C2 is also connected between the output pin 2 and the ground 
potential GND for shunting any high frequency noise appearing on the 
output voltage Vout to the ground potential. 
The low voltage regulating circuit 314 is comprised of a current-limiting 
resistor R21, diodes D9 and D10, and a capacitor C4. One end of the 
resistor R21 is connected to the regulated output voltage on the pin 2 of 
the voltage regulator, and the other end of the resistor R21 is connected 
to one end of the capacitor C4, to the anode of the diode D10, and to a 
reference line 318. The cathode of the diode D10 is connected to the anode 
of the diode D9. The cathode of the diode D9 is connected to the other end 
of the capacitor C4 and to the ground potential GND. The reference line 
318 provides a reference voltage VREF for the suppression networks TS1 and 
TS2. Assuming that the forward voltage drop across the diodes D9 and D10 
is each 0.75 volts, the reference voltage VREF is approximately +1.5 
volts. Thus, the value of the resistor R21 is selected so that the current 
flowing through these diodes will produce the reference voltage of +1.5 
volts at V reference. 
One end of each of the plurality of pull-up termination resistors R1 
through R18 is commonly connected together and to the regulated output 
voltage on line 320. The other ends of the resistors R1 through R18 are 
connected to one end of the respective data and/or control signal lines 
ACK, REQ, DB(0) through DB(7), DB(P), ATN, BSY, RST, MSG, SEL, C/D, and 
I/O. The pull-up resistors R1 through R18 function in the same manner as 
the resistors P1-P18 in FIG. 2 of the prior art and serve to supply the 
current to a respective open collector transistor device formed in a 
corresponding transmitting device that is coupled to the SCSI bus line. In 
particular, when the open collector transistor device associated with the 
respective data and/or control signal lines is turned off, such signal 
line will be at a high logic level and no current will flow through the 
corresponding pull-up resistor. When the open collector transistor device 
associated with the respective date and/or control signal lines is turned 
on (signal line is active), such signal line will be at a low logic level 
and the maximum current through the open collector transistor device is 
limited by the corresponding pull-up resistor. 
The transient suppression network TS1 is interconnected between the control 
signal line (ACK) and the reference line 318. The transient suppression 
network TS1 is a diode array formed of diodes D1 through D4. The anode of 
the diode D1 is connected to the cathode of the diode D3 and to the 
control signal line (ACK). The cathode of the diode D1 is connected to the 
anode of the diode D2. The anode of the diode D3 is connected to the 
cathode of the diode D4. The cathode of the diode D2 is connected to the 
anode of the diode D4 and to the reference voltage VREF on the line 318. 
The transient suppression network TS1 functions as a variable resistor 
which dynamically changes its impedance dependent upon the amount of 
current flowing therethrough. 
Similarly, the transient suppression network TS2 is interconnected between 
the control signal (REQ) and the reference line 318. The transient 
suppression network TS2 is a diode array formed of diodes D5 through D8. 
The anode of the diode D5 is connected to the cathode of the diode D7 and 
to the control signal line (REQ). The cathode of the diode D5 is connected 
to the anode of the D6. The anode of the diode D7 is connected to the 
cathode of the diode D8. The cathode of the diode D6 is connected to the 
anode of the diode D8 and to the reference voltage VREF on the line 318. 
The transient suppression network TS2 functions likewise as a variable 
resistor which dynamically changes its impedance dependent upon the amount 
of current flowing therethrough. 
It should be clearly understood to those skilled in the art that additional 
transient suppression networks similar to TS1 could be connected between 
each of the other remaining signal lines and the reference line 318 so as 
to suppress transients. However, in order to reduce costs, the transient 
suppression networks are generally implemented only with the signal lines 
which are highly susceptible to transients. In other words, at least the 
control signal lines (ACK) and (REQ) being the most critical to reliable 
and accurate data transfer have been implemented with such transient 
suppression networks. 
The operation of the present terminator apparatus 310 of FIG. 9 will now be 
explained with reference to FIGS. 10A and 10B. As can be seen, when the 
signal line is at the high logic level (i.e., between .+-.2.5 to .+-.3.0 
volts), there is a positive spike at point A extending above the high 
logic level. Also, when the signal line is at the low logic level (i.e., 
between 0.0 to 0.5 volts) there is a negative spike at point B extending 
below the low logic level. If these transients appearing at the points A 
and B are sufficiently long in duration and have large enough amplitudes, 
these transients could be interpreted by a receiving device coupled to the 
SCSI bus line as a valid signal. As a result, such transients may cause 
incorrect data to be transferred. 
The improved terminator apparatus 310 in FIG. 9 of the present invention 
will remove or attenuate the transients appearing on the control signal 
lines REQ and ACK so as to produce the waveform 122 of FIG. 10B. When the 
signal line (i.e. control signal line REQ) is at the high logic level and 
a transient appears like the one at the point A (FIG. 5) which exceeds the 
high logic level, the diodes D5 and D6 will be rendered conductive so as 
to clamp the control signal REQ to a voltage equal to two diode drops 
above the reference voltage VREF or approximately +3.0 volts, thereby 
effectively eliminating the transient. In other words, the diodes D5 and 
D6 limit the upper voltage level on the control signal line REQ. 
When the control signal line REQ is at the low logic level and the 
transient appears like the one at the point B (FIG. 10A) which falls below 
the low logic level, diodes D7 and D8 will be rendered conductive so as to 
clamp the control signal REQ to the low logic level or approximately zero 
volts. In other words, the diodes D7 and D8 limit the lower voltage level 
on the control signal REQ. 
For completeness in the disclosure of the above-described terminator 
apparatus but not for purposes of limitation, the following representative 
values and component identifications are submitted. These values and 
components were employed in a terminator apparatus that was constructed 
and tested and which provides a high quality performance. Those skilled in 
the art will recognize that many alternative elements and values may be 
employed in constructing the circuits in accordance with the present 
invention. 
______________________________________ 
T TYPE OR VALUE 
______________________________________ 
C1 4.7 .mu.F 
C2 .1 .mu.f 
C3 22 .mu.f 
C4 2.2 .mu.f 
R19 121 Ohms 
R1-R18 110 Ohms 
R20 154 Ohms 
D1-D10 BAV 99 
______________________________________ 
Therefore, with the above values used, the terminator apparatus 310 of the 
present invention will generally provide a regulated output voltage of 
+2.85 volts with a termination impedance of 110 Ohms which meets the 
maximum current specification. Further, the suppression networks TS1 and 
TS2 will remove any transients by limiting the range of the upper voltage 
level and the lower voltage level of the control signals REQ and ACK. 
However, since it has been encountered in the computer equipment industry 
that many of the SCSI cables have a lower impedance than 110 Ohms and is 
more on the order of 80 Ohms, it would be desirable to easily modify the 
terminator apparatus 110 so as to accommodate the different impedances. It 
is merely needed to lower the regulated output voltage Vout so that the 
maximum current requirement is met when the termination impedances 
(R1-R18) are lowered to 80 Ohms. Accordingly, it has been discovered that 
the output voltage should be reduced to approximately +2.15 volts when 
driving an 80 Ohm termination impedance. Therefore, it will be apparent to 
those skilled in the art that the output voltage further can simply be 
adjusted to a desired value so as to accommodate the variety of different 
impedances of the SCSI cables. 
From the foregoing detailed description, it can thus be seen that the 
present invention provides an improved terminator apparatus used with an 
SCSI bus line for removing noise spikes or transients from data and/or 
control signals received on the bus line. The terminator apparatus is 
comprised of a voltage regulator, a low voltage regulating circuit, a 
plurality of pull-up resistors, and suppression networks. The suppression 
networks serve to remove transients from the signal lines by limiting the 
range of the upper voltage level and the lower voltage level of the 
control signals appearing thereon. 
FIG. 11 shows an active deassertion circuit 422 in block diagram form. The 
active deassertion circuit 422 is comprised of a means for providing a 
voltage reference 424 and a means for sinking current 426. The means for 
providing a voltage reference 424 has an input 428 and an output 430. The 
means for sinking current 426 has a first input 432 that is coupled to the 
output 430 of the means for providing a voltage reference 424. The means 
for sinking current 426 also has a second input 434 and an output 436. 
FIG. 12 shows the active deassertion circuit 422 and its placement within 
a SCSI system denoted by reference numeral 420. The regulated terminator 
418 is comprised of a 2.85 volt voltage regulator 438, the output 440 of 
which is coupled to each signal line 416a through 416r of the transmission 
line 414. The voltage regulator 438 is connected to a common voltage 444. 
Other features of the regulated terminator 418, such as its input 445 and 
its capacitors, 446, 448 and 450, are connected as shown. The means for 
sinking current 426 of the active deassertion circuit 422 is coupled to 
each signal line 416a through 416r, and the output 440 of the voltage 
regulator 438 via its second input 434. These are the only connections 
that need be made between the SCSI system 420 and the active deassertion 
circuit 422. However, preferably, the input 428 of the means for providing 
a voltage reference 424 is connected to the input 445 of the regulated 
terminator 418 which enables both the regulated terminator 418 and the 
active deassertion circuit 422 to operate off of the same power means (not 
shown). 
Again referring to FIG. 12, the means for providing a voltage reference 
424, or voltage reference circuit, is preferably comprised of a voltage 
regulator 452, a capacitor 454 and a voltage divider circuitry 456. The 
voltage regulator 452 has an input 428 that is identical to the input 428 
of the means for providing a voltage reference 424. The voltage regulator 
has an output 458 and is connected to the common voltage 444, preferably 
ground. Preferably, the voltage regulator 452 is part LT11172.85 
manufactured by Linear Technology Corporation. The capacitor 454 is 
interposed between the output 458 of the voltage regulator 452 and the 
common voltage 444 and is, preferably, a 22 microfarad capacitor. The 
voltage divider circuitry 456 is interposed between the output 458 of the 
voltage regulator 452 and the common voltage 444. The voltage divider 
circuitry 456 is comprised of diode means 462 serially connected with 
impedance means 464 at a junction 466 that serves as the output 430 of the 
means for providing a voltage reference 424. Preferably, the diode means 
462 is part number BAV99, manufactured by Phillips. Thus, it is manifest 
that the capacitor 454 and the voltage divider circuitry 456 are in 
parallel with each other. Preferably, the diode means 462 is comprised of 
a pair of diodes 468 and 470, interposed between the output 458 of the 
voltage regulator 452 and the output 430 of the means for providing a 
voltage reference 424, or junction 466. Also, the impedance means 464 is 
preferably a resistor 472 that is interposed between the junction 466 and 
the common voltage 444 having a value of 3.3 kOhms. 
Yet again referring to FIG. 12, the means for sinking current 426, or 
current sink, is comprised of transistor means 474 having a base 476, 
emitter 478, and a collector 480. The base 476 serves as a first input 432 
to the means for sinking current 426 and is coupled to the junction 466. 
Preferably, the transistor means 474 is comprised of a Darlington 
transistor 482 arranged from two PNP transistors 484 and 486 as shown. 
Preferably, the Darlington transistor 482 is part number FTZ705, 
manufactured by Zetex. In this configuration, an emitter 488 of transistor 
486, and thus the emitter 478 of the transistor means 474, has a resistor 
490 preferably with a value of 0.22 ohms connected to it. The resistor 490 
serves as the second input 434 to the means for sinking current 426. 
Further in this configuration, the collectors 492 and 494 of the 
transistors 484 and 486 respectively serve as the output 436 to the means 
for sinking current 426 and the collector 480 of the transistor means 474. 
Preferably, the output 436 is connected to the common voltage 444 as 
shown. 
Again referring to FIG. 12, by its very nature, the quiescent operating 
point of the Darlington transistor 482 will not be stable over the entire 
environmental temperature range over which the active deassertion circuit 
422 may operate. However, matching the temperature coefficient of diode 
means 462 with the temperature coefficient of the junction of the emitter 
478 and the base 476 alleviates this problem. Thus, although, for 
instance, one can practice the claimed invention by using a zener diode 
(not shown) as the means for providing a voltage reference 424, a zener 
diode is not the preferable structure. 
Still referring to FIG. 12, the second input 434 is coupled to both the 
output 440 of the voltage regulator 438 of the regulated terminator 418 
and the signal lines 416a through 416r at a node 469. If there are no 
actively deasserted signal lines 416a through 416r, the voltage regulator 
438 will provide a voltage of 2.85 volts at the second input 434. However, 
if a signal line 416a through 416r is actively deasserted, the node 469 
will be at a voltage greater than 2.85 volts due to characteristics of its 
drivers (not shown). Since each of the signal lines 416a through 416r has 
a 110 ohm resistor, 498a through 498r, an asserted line will normally draw 
more than 24 mA (which is outside of the ANSI standard) if an active 
deassertion circuit 422 is not present. In fact, the more signal lines, 
416a through 416r, that are deasserted, the farther the current through 
each asserted line exceeds the maximum current allowed by the ANSI 
standard due to the increase in the overall current through the signal 
lines, 416a through 416r. 
Having described the structure of the active deassertion circuit 422 and 
its connections to SCSI system 420, the method of use of the active 
deassertion circuit 422 will be apparent to those of skill in the art. The 
method of use prevents overcurrent on signal lines, 416a through 416r, due 
to the active deassertion of at least one other signal line, 416a through 
416r. First, one must provide a regulated terminator 418 having an output 
440. Next, one must couple a plurality of signal lines 416 through 416r, 
to the output of the regulated terminator. Next, one must couple the 
second input 434 of the active deassertion circuit 422 to the output 440 
of the regulated terminator 418. Next, one may assert at least one of the 
plurality of signal lines 416a through 416r. Next, one must actively 
deassert at least one of the plurality of signal lines, 416a through 416r. 
Next, one must compare the first input 432 of the means for sinking 
current 426 to the second input 434 of the means for sinking current 426. 
Finally, one must sink current through the means for sinking current 426 
until none of the asserted plurality of signal lines 416a through 416r 
draws current in excess of that which is permitted by its communications 
protocol. 
The invention has been described in detail with particular reference to an 
active deassertion circuit 422 comprised of a means for providing a 
reference voltage 424 and a means for sinking current 426. However, those 
skilled in the art understand that there are numerous variations and 
modifications of the present invention. For instance, as explained above, 
the present invention could use a zener diode as the means for providing a 
voltage reference 424 and still fall within the ambit of the claims. Also 
any temperature compensating voltage reference can be used in place of the 
voltage reference. 
In FIG. 13 there is illustrated a particular embodiment of the invention 
wherein a terminator apparatus is provided with a SLICK circuit, an ADR 
circuit and a SLIM circuit in addition to a typical termination network. 
As can be appreciated, the SLICK circuit actually consists of two SLICK 
circuits, one each for the REQ and ACK signal lines of the SCSI bus. 
Similarly, the SLIM circuit includes two suppression networks, one each 
for the REQ and ACK signal lines. Only one current sink is required in the 
ADR circuit because it is connected to the output node of the regulated 
power supply voltage rather than individual signal lines. 
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. It is, therefore, intended that such 
changes and modifications be covered by the appended claims.