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Sharp Compet 15 Electronic Desktop Calculator
The machine exhibited here, a Sharp Compet 15 calculator, is a close relative of another machine exhibited in the museum, the Sharp Compet 20. The Compet 15 appears to be an attempt by Sharp to reduce the cost-of-entry for potential buyers of electronic calculator technology. The cost reduction was implemented by trimming down the features and capabilities available on the Compet 20. The Compet 15, with its reduced capacity and fixed decimal point operation, made the machine less complex, and therefore less expensive.Profile View of Sharp Compet 15 Both the Compet 15 and
earlier Compet 20 share virtually identical design in terms of the
structural and mechanical aspects of the machines. They also use the same
general architecture. The major difference between the two machines simply
lies in the number of digits of capacity (12 in the Compet 15 versus 14
for the Compet 20); fixed-decimal point operation for the Compet 15,
versus the floating decimal capability of the Compet 20; and lastly, the
Compet 15 doesn't have circuitry in it to deal with negative numbers,
while the Compet 20 does. Mechanically, the plastic
upper cabinet half and the metal cabinet base are interchangeable between
the two machines, as are many of the other mechanical components of the
design.
The "Sharp COMPET" Dust Cover This particular calculator has an
interesting history associated with it. Apparently this calculator was
purchased in Europe by a German engineer, sometime in 1967.
The machine, because of its European heritage, is designed to run on 200,
220, or 240 volt AC power. The line voltage is adjustable via a socket
and plug arrangement accessible by removing the bottom cover of the chassis.
European line voltage standards vary, so the machine was designed to be able
to accommodate a number of different European line voltages. Fortunately,
the machine is not picky about the line frequency, which in European countries
is 50Hz, versus the 60Hz line frequency in the US. All US homes have
220 volt service supplied to them, and most have available 220V circuits in the
home for powering electric dryers and cooking ranges, so it is possible
to power up this machine in the US, which I have done. The machine
operates beautifully. Anyway, the engineer who originally owned
the calculator ended up coming to the US (Massachusetts) sometime later,
and brought his prized calculator with him. Sometime in the late 1990's,
the engineer passed away, and his family put his cherished calculator up for
sale at an estate sale. The calculator was purchased by a person who had an
appreciation for the meticulous condition that the machine was kept in, and
later ended up being acquired by the Old Calculator Web Museum as a prized
addition to the museum.
The mysterious "ADDO" Logo There's an interesting twist to this
particular machine that could have some relation to its European origins.
All of the nomenclature on the machine makes it clear that the machine
was made by Sharp. The serial/model number tag has the Sharp logo, along
with a label on the back of the plastic case that says "SHARP", along
with an emblem on the keyboard panel that says "Sharp" and "Compet 15".
However, there's an additional emblem on the upper right of the keyboard
panel that says "ADDO". Addo sold calculators in the European market,
starting out selling mechanical and electro-mechanical calculators, and
later working themselves into the electronic calculator market by
forming relationships with established electronic calculator manufacturers.
I don't have much information on the history of Addo, nor on the
machines it sold, but it is clear from this particular example that
Addo had a re-marketing agreement with Sharp. Since this machine was
originally purchased in Europe, it was probably bought at an Addo
distributor. Addo simply purchased the Compet 15 from Sharp,
and added on the "ADDO" badge, and resold the machine through their
own sales channels.
The Sharp Compet 15's Keyboard Layout The Compet 15 is a very basic
four-function electronic calculator. It provides a 12-digit
Nixie-tube display. Each NEC LD-784 Nixie tube indicates the digits 0
through 9 and includes a right-hand decimal point. The calculator operates
with fixed decimal point, with settings for 0, 2, 4 or 6 digits behind the
decimal point. The decimal point setting is selected by a rotary switch on the
keyboard panel. Two groups of settings are available, with one selection
designated in red. At this point it isn't clear what the difference in
behavior of the machine is between the use of the red decimal point
settings versus the other (which are labelled on the selector switch
in white). Further experimentation will be required to determine the
difference between the red and white decimal point settings. Addition
and subtraction work as expected, in 'adding-machine' style, with the
keys performing a "+=" or "-=" operation, even though the key cap nomenclature
simply shows "=" on the key cap, with a red key used to indicate subtraction.
Multiplication and division key caps have small red jewels in them that
light up to indicate the pending operation. Multiplication on the Compet
15 operates conventionally, as opposed to the unusual method of displaying
multiplication problems on the Compet 20.
The Keyboard Assembly of the Sharp Compet 15 The keyboard of the Compet 15 uses an
interesting combination of mechanical and electrical, the same basic design as
used on the Compet 20. The mechanical aspect of the keyboard
assembly assures that only one key can be pressed at a time. The electrical
aspect of the keyboard uses magnetically activated micro-switches to
close the electrical circuit when a key is depressed. The keyboard
connects to the rest of the calculator electronics via a couple of ribbon
cables that connect via a fairly large edge connector to a circuit board
that provides keyboard signal conditioning and encoding functions.
Operating the Compet 15 is pretty
straightforward, with the usual zero through nine and decimal point keys.
The white [=] key is used for addition and terminating
multiplication/division problems, and the red [=] key for subtraction
(or complementing of the number in the display). The [X] and [÷]
keys' functions are obvious. The [RC] key swaps the operands of
multiplication and division functions. The [CLE] key is used for clearing
entry errors, clearing the display/entry register, and the [CL] key clears
the entire machine.
Unusual Power-On Display The calculator does not have a power
on clear circuit, resulting in an interesting display when the machine is first
powered up, with all digits showing simultaneous '7' and '9' Nixie digits
lit at the same time. Pressing the [CL] key after power-up clears the
machine and readies it for normal operation.
Inside the Sharp Compet 15 Like the Compet 20, the Compet 15
is an entirely transistorized calculator. There are no integrated circuits
used in the calculator. 468 transistors (with most being the same
pancake-packaged devices used in the Compet 20), along with countless
resistors, diodes, and capacitors make up the circuitry of the machine.
The electronics are contained on a total of eleven circuit boards that plug
into a hand-wired backplane that interconnects the boards. Like the
Compet 20, a number of the circuit boards in the machine have specific
function.
One of Six "Digit" Boards in the Compet 15 Six of the eleven
boards in the machine are digit boards, with each board having
circuitry on it to drive two of the Nixie tubes, as opposed to the
digit boards in the Compet 20, that contain the circuitry for only
one digit. The two Nixie tubes are mounted to the board, supported by a
plastic fixture that is secured to the circuit board by a couple of small
screws. Individual wire leads from the Nixie tubes have "spaghetti"
insulation placed on them for electrical isolation, and are soldered to
pads on the circuit board. The Nixies are individually driven by two sets
of ten discrete transistor drivers, one transistor for each numeral-shaped
electrode in the Nixie tube. A separate transistor provides drive for the
decimal point.
Detail of Nixie Tube Mounting The circuit boards are made of
phenolic, with traces on both sides of the boards. Feed throughs
are accomplished with uninsulated pieces of wire placed through the feed-through
hole and soldered into place, providing a solid connection between each side
of the board. Edge connector fingers are tin-plated. Ten of the eleven
boards plug into the backplane in such a way that those boards
with Nixie tubes on them can shine through the display panel of the
calculator.
The Sequencing Logic Board One larger board spans the rear of the
machine, containing the main sequencing state machine
for the calculator. The right-most board in the chassis provides keyboard
signal conditioning and encoding functions, and the other three boards seem
to contain miscellaneous control logic functions.
The Compet 15's Power Supply The Compet 15 uses a very simple linear
power supply, contained in the base of the machine, underneath the
keyboard assembly. A small transformer takes the line voltage
and steps it down to one 170V secondary which is rectified and smoothed
and used for Nixie tube drive. Two other 14V secondary windings are used
to generate the logic supplies for the machine. The voltages from
the transformer are rectified and filtered by a bank of capacitors.
It appears that the logic power supplies are not transistor regulated.
The logic voltages are tweaked using adjustments (variable resistors) at
the factory to provide the proper voltages at the current load imposed
by the circuitry of the calculator.
The Compet 15's Model/Serial Tag The Compet 15, like many
early transistorized calculators, has a couple of quirks. It generates
incorrect answers for multiplication and division problems where the
multiplicand/dividend occupy the most-significant digit in the display.
For example, with the decimal point setting set at 6, dividing 123456
by 10 results in a completely non-sensical answer, while dividing 12345
by 10 results in the expected answer. As part of the cost-saving measures,
the Compet 15 does not deal with negative numbers. For example, performing
10 - 11 results in 999999999999. (with decimal point set to 0). Pressing
the red [=] key at this point will perform a tens complement of the number
in the display, so in the above case doing so would result in 000000000001.
Arithmetic overflow is not detected by the Compet 15, with the machine simply
discarding any digits beyond its 12-digit capacity. The logic of the
calculator can only handle decimal points in the lowest six digits of
the display. Entering 1.2345678, with decimal point position set at 6
results in 000012345678 on the display -- the decimal point gets lost.
The Compet 15 also does not detect input overflow -- any digits entered in
excess of the capacity of the machine are discarded with no warning.
Division by zero results in the machine churning furiously to try to find
an answer that doesn't exist. Pressing the [CL] key halts the futility
and returns the machine to normal operation.
The Compet 15 seems a bit slower
than the Compet 20 -- perhaps the 15 was purposefully slowed down to provide
another perceived advantage to the Compet 20, but more likely, it allowed
wider design tolerances in the circuitry that made the machine
simpler to build, and potentially allowing the use of lower-cost transistors.
Addition and subtraction complete virtually instantly, however, multiplication
and division can take a little while to perform. The "all-nines" benchmark
can't be performed to the full capacity of the machine due to the 11-digit
limitation. However, performing eleven 9's divided by 1 (with decimal point
set to 0) takes about 1/2 second to perform. During calculation, the
Nixie tubes are not blanked, resulting in that wonderful 'spinning'
effect that is one of my favorite aspects of Nixie tube-based
calculators.
