U.S. patent number 3,818,192 [Application Number 05/305,109] was granted by the patent office on 1974-06-18 for remote control and display for a liquid dispensing system.
This patent grant is currently assigned to Lockheed Electronics Company, Inc.. Invention is credited to Elmer C. Anderson, John Dow, Jr., William B. Gutman, William A. Oetting.
United States Patent |
3,818,192 |
Anderson , et al. |
June 18, 1974 |
REMOTE CONTROL AND DISPLAY FOR A LIQUID DISPENSING SYSTEM
Abstract
An electronic calculator and control system for a gasoline
dispenser or the like for remotely controlling the dispenser and
displaying the accumulated sale and/or volume. A pulse generator is
actuated as the fuel is dispensed and the count is stored in a
decade counter. A plurality of flip-flops (latches) and control
logic provide control functions such as applying power to the
dispenser, lighting proper indicators on a control panel,
transferring the accumulated count to readout devices, preserving a
count for at least one readout, resetting the counters, turning off
the dispenser in the event the sale exceeds a certain amount, and
an override if it is desirable to exceed this amount. Provision is
made for operating a plurality of dispensers utilizing the same
readout devices. The pulse generator is preferably an A.C. to D.C.
converter utilizing an optical coupler.
Inventors: |
Anderson; Elmer C. (Colonia,
NJ), Dow, Jr.; John (Atlantic, NJ), Gutman; William
B. (Scotch Plains, NJ), Oetting; William A. (Green
Brook, NJ) |
Assignee: |
Lockheed Electronics Company,
Inc. (Plainfield, NJ)
|
Family
ID: |
23179370 |
Appl.
No.: |
05/305,109 |
Filed: |
November 9, 1972 |
Current U.S.
Class: |
377/21;
377/30 |
Current CPC
Class: |
B67D
7/228 (20130101) |
Current International
Class: |
B67D
5/22 (20060101); G06m 003/08 () |
Field of
Search: |
;235/92FL |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Henon; Paul J.
Assistant Examiner: Gnuse; Robert F.
Attorney, Agent or Firm: Corber; Billy G. Geer; Albert
K.
Claims
What is claimed is:
1. A calculator and control system for fuel dispensing or the like
comprising a plurality of dispensers, control logic and a decade
counter for each dispenser, pulse generator means at each dispenser
producing pulses as a function of fuel dispensed, means connecting
the pulse generator output to the decade counters for each
dispenser, the control logic comprising
a first latch means for resetting the counters and energizing a
relay for applying AC power to the dispenser,
a dispenser switch when actuated resets the dispenser counters and
actuates the pulser,
a second latch means responsive to resetting of the second latch
for inhibiting further operation of said first latch until at least
one readout from a counter decade has been made,
a third latch means for transferring the accumulated count from the
decade counters to a readout display, and actuating said second
latch to remove the inhibit from said first latch,
a fourth latch means responsive to a predetermined count for
de-energizing said relay applying power to the dispenser, and
a fifth latch means for overriding said fourth latch when said
predetermined count is to be exceeded.
2. A system as defined in claim 1 wherein the second latch resets
the fifth latch.
3. A system as defined in claim 1, wherein the first latch resets
the second and fourth latches.
4. A system as defined in claim 1, and further including transfer
gates for each decade and a decoder for converting the binary
output of the counters to a decimal readout and wherein the third
latch actuates the transfer gates and a blanking driver responsive
to the third latch actuates the decoder.
5. A system as defined in claim 1, wherein the pulse generator
comprises a pulse responsive to fuel dispensed for opening and
closing the AC potential at the dispenser for providing a pulsed
AC, a light emitting device responsive to the pulsed AC, a light
sensitive element responsive to the light emitting device for
generating a ramp-type voltage, and a Schmitt-type pulse shaper for
generating a DC pulse for application to the decade counter.
Description
This invention relates to a fluid dispensing system for calculating
the monetary amount of the sale, and more particularly to a control
and display unit for such a system. As described herein, the
invention is applied to a gasoline dispenser of the type presently
in use at gasoline service stations. A typical dispenser includes a
fluid pump, control switch, hose with a nozzle which has a flow
control and means to start and stop the pump and reset the
calculators. Generally, there are three such dispensers at each
service island; therefore, it is desirable to provide control for
each island.
In the past, calculators for gasoline dispensers and the like have
been largely mechanical or electromechanical. One calculator of
this type is shown in U.S. Pat. No. 3,400,255. However, in more
recent years, a number of electronic calculators have been
proposed. For example, see U.S. Pat. No. 3,666,928 and the patents
cited therein. These patents are primarily concerned with various
arrangements for counting pulses, which are generated as a function
of fluid flow.
Present trends indicate that self-service gasoline dispensing is
becoming more popular, not only from the public viewpoint but from
the station operators or owners as well. In such cases, the
customer pumps his own gasoline and then pays an attendant.
Generally, however, except in small service stations a number of
attendants are required. It is apparent that a reduction in the
number of attendants will result in a lowering of operating costs
of the price of gasoline. Furthermore, many of the known
calculating systems using mechanical or moving parts were subject
to frequent repair and service due to wear and lack of
reliability.
Briefly, the calculator includes a pulse generator which produces
pulses as a function of the monetary sale and an accumulator for
counting and storing the pulses. The control/display unit includes
control logic, which provides for operation of a plurality of
dispensers by applying power to the dispenser at the beginning of
fuel delivery and to remove power at the end of the delivery. The
control logic also provides signals to transfer an accumulated
count to a readout display. Provision is made to turn off the
dispenser in the event the sale exceeds a predetermined amount or
an override if the actual sale is expected to exceed such amount.
Interlocks are provided to prevent a turn on of the dispenser until
a previous transaction has been completed, to prevent erroneous
operation by the attendant and to prevent operation of the
dispenser until turned on by the attendant.
Accordingly, it is a primary object of this invention to provide a
control system for self-service gasoline stations which is not only
highly reliable but requires a minimum number of attendants.
Another object of the invention is to provide a control system for
a plurality of dispensers, and utilizing a single set of display
devices.
Another object of the invention is to provide a calculator for a
dispensing arrangement which is under the control of the station
operator.
A further object of the invention is to provide a control system
for a plurality of dispensers, which includes a system of
interlocks.
Another object of the invention is to provide a new and improved
pulse generator.
These and other objects of the invention will become apparent from
the following description when taken with the accompanying
drawings, in which:
FIG. 1 is a partial block diagram illustrating a general
arrangement of a dispenser and a control and display unit in
accordance with the invention;
FIGS. 2A and 2B are more detailed block diagrams of the control and
display unit;
FIG. 3 is a schematic diagram of a preferred embodiment of an AC to
DC converter or pulse generator; and
FIG. 4 illustrates the waveforms at the designated points of FIG.
3.
With reference to FIG. 1, the preferred embodiment of the invention
is illustrated and comprises a pump (or dispenser) unit 1 and a
control and display unit 2. While one pump is shown, additional
pumps may be controlled by the control and display unit as will be
explained later. A pulser 3, which is shown as a reed switch, is
actuated by small magnets located under the digits on the penny
wheel of the money wheels of the pump. The output of the pulser is
a series of bursts of the AC voltage and is coupled to converter 4
in the control unit. The converter output is a series of pulses,
each representing one cent ($0.01) of the amount of fuel or other
material dispensed. The pulses are then counted in the accumulator
5, which may be conventional binary coded digital counters, with
the output of each decade driving one element of a four element
readout display 6.
At the start of a transaction, the operator (attendant) turns on
the dispenser by depressing the PUMP ON button 7 in the control
unit. If the previous transaction has been completed, the control
logic 8 will turn on the Triac (Thyristor) switch 9 which applies
the AC voltage to the pump over the AC switched line 10. The
customer then lifts the hose nozzle and puts the handle switch 11
to the ON position. The handle switch contacts 12 close applying
the AC through the normally closed (NC) contacts 13 of the reset
switch assembly to the reset motor 14. After the reset cycle first
sets all the money wheels to the zero position, the contacts of the
reset switch assembly are activated. This serves to close all the
normally open (NO) contacts and open the normally closed (NC)
contacts. Thus, contacts 13 are now open, removing the AC from the
reset motor, and the motor stops. Contacts 15, 16 and 17 are now
closed, applying AC to the flow valve solenoid, penny pulser and
pump motor. Closed contacts 16 also apply AC to the AC reset line
18 which energizes the pump relay 19 in the control unit. The now
closed contacts 20 of the pump relay signals the control logic 8
that the dispenser 1 is in use, and the pump in use indicator 21 is
energized.
Note, however, that the AC switched by contacts 15 is under control
of the Triac switch 9. This allows the control logic to stop the
flow of gasoline when a stop decision is initiated. This would
originate by setting the EMERGENCY switch (not shown) to the STOP
position or when the sale reached $10.00 (see line 22 from the
accumulator to the control logic) and the OVER $10.00 button 23 has
not been depressed. (The details of the stop decision will be fully
explained hereinafter.) If a stop decision is not initiated, the
customer can proceed to use the dispenser. As the gasoline is
dispensed, the penny wheel is turning, thus keeping track of the
amount of the sale. As the penny wheel turns, it opens and closes
the penny pulser, applying pulsed AC to the AC pulser line. This
pulsed AC activates the AC to DC converter 4 and the DC pulses are
passed on to the accumulator 5.
When the customer is finished using the dispenser, the handle
switch 11 is placed in the OFF position. This opens the handle
switch contacts 12 and sets the switch contacts 13, 15, 16 and 17
of the reset switch assembly back to the original position. The AC
is removed from the pump motor, the solenoid flow valve and the
penny pulser. This also removes the AC from the AC reset line 18
(via contacts 16) and the pump relay 19 is de-energized, which in
turn signals the control logic that the customer is finished with
the dispenser and that the transaction should be readied for
display on the readout tubes 6.
At this point, the Triac switch is turned off and cannot be turned
on by depressing the pump on button 7 until the operator completes
the readout cycle of the present transaction by depressing button
24. Also, when the Triac switch is held off by the control logic,
the handle switch 11 cannot initiate another reset cycle until the
control logic reactivates the Triac switch.
Referring now to FIG. 2, the control and interlock system is shown
in FIG. 2A and the accumulator and readout indicators are shown in
FIG. 2B. Before describing FIG. 2, consideration of the symbology
used will be helpful toward a better understanding of the
invention. A small circle (o) at the input of a device indicates
that a low voltage will activate the device, and the absence
thereof indicates that a high voltage will activate the device.
Similarly, at the output of a device the small circle indicates
that the device is inverting and the absence thereof indicates a
noninverting device. For purposes of illustration only, a low may
be zero volts (0V), or substantially so, and a high may be five
volts (+5V). Also in keeping with industry standards Q is NOT Q (or
the inverse of Q).
The control logic previously referred to (FIG. 1) consists of five
(5) flip-flops, designated as latches, and the associated
circuitry. The five latches 32 through 36 are designated RTL (Read
Transfer Latch), ROL (Read Once Latch), PUL (Pump Unlock Latch), SL
(Stop Latch) and IL (Inhibit Latch), respectively. Considering
FIGS. 2A and 2B, actuation of PUL 34 resets the decade counters and
energizes relay 45, which results in applying power to the
dispenser. RTL 32 functions to transfer the accumulated count to
the readout tubes. ROL 33 prevents actuation of PUL 34 until at
least one readout has been made. SL 35 and IL 36 determine whether
or not a stop decision has been made. Keeping in mind the overall
system as previously described, a typical operation is now
considered.
Prior to turning on the equipment, the operator should ensure that
the EMERGENCY switch 30 is in the closed (N) position, as shown,
and the manual switch 31 is open, i.e., the equipment as shown, is
in automatic operation when the proper voltage is applied and the
switches are as indicated. At turn on, RTL 32 and PUL 34 are reset
while ROL 33 is set, thereby resetting IL 36. It will be noted that
at this time the contacts 20B of pump relay 19 are closed and
contacts 20A and 20C are open. However, since the Q output of ROL
33 is low, the "readout" lamp 37 is off.
Assuming any previous transaction has been completed, and a
customer desires to use a particular dispenser, the operator will
depress the corresponding pump unlock button 7 (in this case No.
1). RTL 32 is reset through the OR gate 32A, where set by a
previous transaction and the PUL 34 is set, the Q output is high
and the Q output is low. The Q output activates the reset driver 39
which resets the decade counter (FIG. 2B) over line 40. The Q
output is gated through OR gates 41 and 42 to the lamp/relay driver
43, which in turn energizes the pump unlock lamp 44 and the DC
relay 45. The relay contacts 46 are closed, turning on the Triac
switch 9 and AC is applied to the dispenser over line 10, as
previously described. The low Q (PUL 34) also resets the SL 35, and
resets ROL 33. The now low Q (ROL 33) is applied to latch 34 as an
inhibit (I) to prevent further operation of the set state until at
least one readout has been accomplished. The customer removes the
hose nozzle and sets the handle switch 11 (FIG. 1) to ON. The
dispenser money wheels are reset and the reset switch assembly is
energized, which applies AC to the pump relay 19 (FIG. 1), and
contacts 20A and 20C close and contacts 20B open. The "in use" lamp
47 is now on and the "readout" lamp 37 stays off. The now closed
contacts 20C connect the output of inverter 48 to one input of the
OR gate 41 and to latch 34 reset. The output of the inverter 48 is
normally low and when the contact 20C closes, a low signal resets
PUL 34 and gates through OR 41 to maintain the DC relay 45 in the
energized state. This is because SL 35 is reset, Q is low and the
NAND gate 49 output is high. Resetting of PUL 34 also removes the
reset pulse from line 40 and the counters.
Gasoline is delivered and the pulse output from converter 4 is
counted in conventional decade counter, as shown in FIG. 2.
Counting continues until the desired amount of sale is reached,
unless a stop decision is made. A stop decision may be initiated by
opening the emergency switch 30, or when the sale reaches $10.00 to
avoid excess spillage. When the delivery of less than $10.00 is
completed, the customer returns the switch handle to the off
position, which results in the opening of contacts 16 and thereby
de-energizing AC relay 19, which opens contacts 20A and 20C and
closes contact 20B. The "in use" lamp 47 goes out, and the low
input to OR gate 41 is removed and rises toward +5 volts. Since PUL
34 is reset, its Q output is also high and as a result the DC relay
is de-energized, the Triac switch is turned off and AC is removed
from the dispenser. Pump unlock lamp 44 goes out.
It will be recalled that ROL 33 was previously reset (Q high).
Also, PUL 34 is reset (Q high). Since both inputs to AND gate 50
are high, an output is provided to lamp driver 51 to turn on the
readout lamp 37, which indicates to the operator that gasoline
delivery is complete and readout is now ready.
The readout cycle begins when the operator depresses the readout
button 24. The read transfer latch (RTL) 32 is now set. ROL 33 is
also set, the Q output is now low, which in turn extinguishes the
readout lamp 37. The Q output of RTL 32 is now high and is applied
over line 53 to open the transfer gates 54 (FIG. 2B). The Q output
of RTL 32 is low and is gated through OR gate 55 to start an 8
second one shot 56, the output being applied to the blanking driver
58 which generates a blanking pulse on line 59 to decoders 60 to
turn on the readout tubes 6. Thus, the contents of the decade
counters are transferred to the readout display. At the same time,
the Q output of ROL 33 is high and removes the inhibit (I) from PUL
34. The operator may now press the pump unlock button, when
desired. The inhibit function is to ensure against accidentally
depressing the pump unlock before depressing the readout
button.
The readout will remain displayed for about 8 seconds. At the end
of the 8 seconds the RTL 32 is reset over line 61 (output of one
shot 56) to OR gate 32A, and the transfer and blanking signals are
removed, turning off the readout tubes. Since the decade counters
still hold the present count, the amount of sale can be
redisplayed, simply by depressing the readout button 52 which
repeats the readout cycle. Otherwise, the pump can be assigned to a
new customer by depressing the pump unlock button 38.
Now, let it be assumed that the pump is in operation, fuel is being
delivered and the count is being registered as before. At a
predetermined amount of sale, say $10.00, provision is made to turn
off the pump. Also, the operator may override the turn off when so
desired. Returning now to FIGS. 2A and 2B, the foregoing functions
are provided by latches 35 and 36, the stopover $10.00 latch (SL)
and the Inhibit over $10.00 latch (IL), respectively.
When the counters reach the predetermined amount ($10.00), an
output from the fourth decade (1.times.10.sup.3 cents) (FIG. 2B)
will be applied over line 62 to the set input of latch 35, the Q
output thereof goes high. Latch 36 (IL) was previously reset either
at turn on or by a previous readout, and the Q output is high.
Since both inputs to NAND gate 49 are high, the output goes low and
the output of the inverter is high. Since the Q output of latch 34
is high (previously reset when contacts 20C closed), both inputs to
OR gate 41 are high, the output is low which results in the DC
Relay 45 being de-energized. AC power is removed from the pump,
whereupon the flow valve closes and the pump motor stops.
Now suppose the operator recognizes that the particular customer
desires more fuel than the predetermined amount, such as for
example in the case of a truck. In such case, the operator presses
the "over $10.00" button 23, which sets latch 36 (IL), the Q output
goes high and the "over $10.00" lamp 64 illuminates to act as a
remainder that this transaction is being allowed to exceed the
predetermined limit. At the same time, the Q output of IL 36 is
low, and thus when the predetermined amount is reached and latch 35
is set, one input to NAND gate remains low and the DC relay remains
energized.
Thus, the "over $10.00" button should be depressed prior to
reaching the predetermined amount to avoid removing AC from the
pump. However, once the "over $10.00" button has been depressed, no
further action is necessary by the operator and the $10.00 limit
will be in operation again once the transaction is completed, i.e.,
the readout cycle.
Earlier it was stated that additional pumps could be controlled by
the present invention. It is common practice at most service
stations to have three pumps at each service island. Thus, for
convenience and other reasons which will become apparent, the
following description will illustrate how the invention will
accommodate three pumps with one set of readout tubes.
Referring again to FIGS. 2A and 2B, it will be noted that there are
five inputs to OR gate 32A. Two of these inputs are designated
"From Readout Transfer Latches 2 and 3", lines 81 and 82. These
lines are also connected to the OR gate 55, and also to the input
of the "One of Three to BCD converter". The signals on these lines
are generated by the Q output of the corresponding latch 32 (RTL)
for pumps No. 2 and No. 3. Likewise, the Q output for latch 32 is
connected to the OR gates 32A of pumps No. 2 and No. 3 over line
83. The output of the 8 second one shot 56, which is connected to
the OR gate 32A of pump No. 1, is also connected to the OR gates
32A of the other pumps. Thus, the OR gate 32A of each pump receives
a signal from the 8 second one shot 56 and a signal from ROL 32 of
the other two pumps. Thus, it will be apparent that, except as
noted later, each pump is assigned the equipment below the broken
line A--A, viz. the three push buttons, the five latches, etc. The
elements above the broken line are shared by all pumps. In FIG. 2B,
a set of decade counters and transfer gates are assigned to each
pump, whereas all pumps share the decoders 60 and readout tubes
6.
As will be seen, the control logic and counters for each pump
operate independently of the control logic and counters of the
other pumps, and the interconnections to the OR gates 32A of the
three pumps provide an interlocking feature so that readout is
accomplished for one pump at a time, and at the same time protect a
count (sale) being registered for one or both of the other pumps.
Consider now the operation of pump No. 1, as before described, and
with pumps No. 2 and No. 3 on the line.
When the operator depresses pump unlock switch 7 for pump No. 1,
the latch 32 is reset, and Q goes high. This output is applied to
OR gate 55, and OR gates 32A of the control logic for the other
pumps. Since the OR gates respond only to a low signal, there is no
reaction in the control logic for pumps No. 2 and No. 3. Therefore,
all three pumps can be in operation, gasoline being dispensed and
the count accumulating in each set of counters. The pump unlock
lamp(s) 44 and the pump in use lamp(s) 47 are on, indicating
current status to the operator.
Recalling that during fuel delivery the latches 32 (RTL) are reset,
let it be assumed that the readout lamp 37 for No. 1 pump comes on.
The operator presses the readout button 24, which sets latch 32 for
pump No. 1, and Q is high, Q is low. As previously described the Q
output is used to energize the transfer gates 54. The low Q output
is now applied to OR gate 55, starting the one shot 56, and to OR
gates 32A for pumps No. 2 and No. 3, resetting the latches 32 or
holding them in the reset state. The blanking driver 58 energizes
the decoders 60 and the decade counter contents for No. 1 pump are
displayed on the readout tubes. The readout display will remain for
approximately 6 to 8 seconds. If the operator completes the readout
of No. 1 pump within the period of the one shot 56, he can
immediately depress the readout button of another pump which is
ready for readout. The latch 32 for the other pump (say No. 2) is
set which resets latch 32 for pump No. 1, removing the transfer
pulse from the transfer gates for the No. 1 counters, and
simultaneously the transfer pulse for pump No. 2 is applied to the
transfer gates 54 of pump No. 2. The contents of No. 2 decade
counters are then displayed (via lines 84, FIG. 2B) on the readout
tubes for the remainder of the blanking period. In the event of
insufficient time to complete the readout for No. 2 pump, the
operator depresses the readout button again and the one shot
recycles.
The Q outputs of the three latches 32 (RTL) are connected to a "one
of three to BCD converter" 57 (FIG. 2A), the output thereof being
connected to the adder 85 (FIG. 2B) over line 86, where it is
combined with a programmed binary coded digital signal such that
the output of the adder when applied to the decoder will provide an
indication of the pump number on the readout tube, i.e., pump No.
1, 2, 3 or 4, 5, 6 or 7, 8, 9. For example, during readout, one of
the inputs to converter 57 will be low and the other inputs will be
high, and the output will be the binary for No. 1, No. 2 or No. 3,
which is added to binary for 0, 3, 6, etc. to obtain the pump
number. Thus, any number of three dispenser control units can be
stacked to accommodate any number of pumps.
When it is desired to operate the pumps manually, the MANUAL switch
31 is closed which connects the +5 volts to the input of OR gate 42
and the DC relay 45 remains energized. The pumps may now be used
without regard to the control functions of the control unit.
It is understood that the various blocks of FIG. 2, such as
converter 57 and decoder 60, are conventional state of the art
devices.
With reference to FIG. 3, a preferred embodiment of an AC-DC
converter 4 (FIG. 1) and a pulse shaper is shown. The penny pulser
3 is connected in series with a neon lamp 70 across the AC lines
(switch 16, FIG. 1, not shown, is assumed to be closed). The neon
lamp and photosensitive material represented by the variable
resistance 71 comprise an optical coupler, which couples the pulsed
AC from the penny pulser 3 to the pulse shaper consisting of the
NPN transistors Q.sub.1 and Q.sub.2 and associated components.
Considering also the waveforms of FIG. 4, as the penny pulser opens
and closes, the AC (V.sub.1) turns the neon lamp 70 on and off.
When the lamp is on, the resistance of the photosensitive material
decreases and the voltage V.sub.2 falls. With the lamp off, the
resistance of the photosensitive material increases and V.sub.2
rises. Note that the rise of V.sub.2 has a long time constant,
which is due to the response of the photosensitive material. The
transistor circuit takes advantage of this fact to integrate the 60
Hz AC, i.e., the optical coupler not only serves as a groundless
connection but also as a filter to the AC. The transistor circuit
itself acts as a Schmitt trigger circuit where the lag provided by
capacitor C.sub.2 introduces a hysteresis that overlaps any random
ripple that occurs on the fall of V.sub.2. Capacitor C.sub.3
provides positive feedback to reenforce the trigger action and to
improve the rise and fall times of V.sub.3. The rise and fall times
of V.sub.3 are typically less than 2 microseconds. These rapid rise
and fall times are necessary when interfacing with diode transistor
logic as is the case here. Not only does the circuit provide a
groundless connection between the logic circuitry and the AC, it is
also immune to contact bounce in the penny pulser and acts as an
integrating filter without the need for large value capacitors. In
addition, the need for two power supplies, both AC and DC at the
dispenser, is eliminated.
It is believed readily apparent that the invention has many
advantages over known calculating systems. The operator need only
follow the information on the display panel. The indicator lamps
show the status throughout a transaction. Thus, a single operator
can readily supervise or attend several control units, each
controlling three dispensers. The invention also permits the use of
the latest integrated circuit technology with the acknowledged high
reliability. It is also apparent that the outputs of the
accumulators can be used as inputs to totalizers, computers or
printers for credit card transactions.
While the control-display system has been described in connection
with a penny pulser and the monetary value of the sale is
displayed, it will be recognized that where the dispenser pulser
provides pulse proportional to volume or gallons, counting and
control is the same. Since as described, the decimal would be on
the readout tube displaying the count of the second decade
(1.times.10.sup.1), conversion to count one-tenths gallons may be
made by shifting the readout tube with decimal to the least
significant digit (LSD).
While a preferred embodiment has been described, it will be
apparent to those skilled in the art that various modifications may
be made without departing from the scope of the invention as
defined by the appended claims. For instance, where a high output
is shown as connected as a low input to another device, those
skilled in the art will recognize the use of an inverter.
* * * * *