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Sharp Compet 16 Electronic Desktop Calculator

Updated 12/27/2008

The Sharp Compet 16 calculator was Sharp Corporation's first electronic calculator to use MOS (Metal-Oxide Semiconductor) integrated circuit technology. With the advent of MOS integrated circuits, it was possible to further shrink the size of a capable desktop electronic calculator, as MOS circuit technology allowed more logic to be placed on a single chip, reducing the size of the calculator, the complexity of the interconnects needed, as well as the cost. The MOS chips also used less power than their earlier bipolar bretheren, allowing further size reductions because power supply circuitry didn't need to be as complex.

Profile View of Sharp Compet 16 (Note Carrying Handle)

The Compet 16 was introduced in December of 1967, as the Model CS-16A, and marked the beginning of Sharp's fourth generation of electronic calculators. The first generation was the Compet 10, Sharp's first electronic calculator, using an all-discrete component Germanium transistor design. The second generation, again all based on discrete components, consisted of discrete Silicon transistors, which dramatically improved reliability over the Germanium transistor design. These machines were the Sharp Compet 20, Compet 21, and Compet 30. The third generation consisted of calculators that bridged the technology gap between discrete transistor designs and integrated circuit technology, utilitizing a majority of discrete Silicon transistors, combined with a small number of early small-scale bipolar integrated circuits. An example of this third generation of Sharp's calculators is Sharp Compet 31 (Model CS-31A) and Compet 32 (Model CS-32A), and the Sharp-designed Burroughs C3350. The fourth generation, including the Compet 16, utilized further advances in the integration levels of MOS integrated circuits, which dramatically reduced the count of discrete components, thus resulting in smaller, less-costly, and more capable calculators.

There are a number of known revisions of the Compet 16, all with subtle changes to the implementation of the logic (mainly updated integrated circuits), but visually and functionally identical. The unit exhibited here is a Model CS-16S. Given that Sharp tended to designate revisions to a model by an alphabetic postfix on the model name, it's clear quite a few revisions were made to the Compet 16 over its product lifetime. Sharp also produced a downgraded version of the Compet 16 designated model CS-17 (or Compet 17), introduced sometime in '68, that was internally virtually identical to the Compet 16, but omitted the memory functionality. The Compet 17 retailed for $100 less than the Compet 16, providing a lower-cost alternative for those users who didn't need the memory functionality offered by the higher-priced Compet 16. Along with the low-end Compet 17, Sharp also offered a high-end machine in this generation of calculators called the Compet 22 (Model CS-22), that provided all of the functionality of the Compet 16, but added two additional digits of capacity, at a retail cost about $180 above that of the Compet 16. All of the machines in this fourth generation of Sharp's electronic calculators shared the same general logic architecture, the same Japanese-made MOS IC's, and virtually identical mechanical design and packaging.

The Swedish business machine company Facit had an OEM relationship with Sharp, whereby Sharp would provide some electronics subassemblies for Sharp calculators, and Facit would package the Sharp-made guts (sometimes with slight modifications) in their own packaging, and sell the machines under the Facit brand, both in Europe and North America. Facit's equivalent to the Sharp Compet 16 was called the Facit 1127, and while packaged differently (Facit's packaging tended to be more boxy and angular than Sharp's), operated identically.

Compet 16 with Plastic Dust Cover In Place

Like all of the early Sharp calculators, the Compet 16 was built to a very high level of quality. The machine exudes a feeling of quality in all aspects. The keyboard is smooth and very easy to operate, with a heavy metal plate that houses the keyboard and serves as the bezel for the keys. The cabinetry is made of thick-wall cast plastic. A matching hard-plastic dust cover snaps in place onto the cabinet, providing nice-looking and functional protection for the keyboard and display. A fold-out carrying handle (made of a very sturdy metal casting) stows under the machine to make it easy to move the machine around. At 13 pounds, the machine isn't a lightweight, making the carrying handle a welcome addition for a user who needs to move the machine around.

The Model/Serial Number Tag on the Compet 16

This particular Compet 16 was made in January of 1969 based on the encoding in the serial number. Given the introduction date of late 1967, the Compet 16 had a relatively long lifetime in the marketplace, partly due to the revisions made along the way that likely increased the reliability (at the time the Compet 16 was introduced, MOS IC technology was quite new) and decreased the cost, allowing the older design to continue to be competitive in the marketplace. When introduced, the Compet 16 sold for just under $1000, with a retail price of $995. This price set a new price benchmark for a four function desktop calculator with memory, and created a bit of a stir in the marketplace as other calculator manufacturers struggled to match Sharp's bang for the buck.

The CS-16 is a 12-digit, fixed decimal, four function calculator with accumulating memory register. The machine provides a constant function for multiplication and division, with a push-on/ push-off keyboard key labeled [K] to enable the function. It follows Sharp's consistent design of using arithmetic logic for addition/subtraction, and algebraic input for multiplication and division. The machine provides fixed decimal point selections, via a rotary switch on the keyboard panel, of 0, 2, 3, 4, and 6 digits behind the decimal point. Two different settings are available for each selection, one which causes truncation, and the other rounding off to the selected decimal position. Strangely, an additional (blank) selection is available on the decimal point setting switch. Placing the switch in this position results in an immediate input overflow upon trying to operate on any number entered. Why Sharp did this is unclear, as it surely ended up being the source of confusion when users inadvertently set the decimal point selection swith to this position. Numbers may be entered with up to seven digits behind the decimal point, with any digits in excess of the decimal point setting of the machine being discarded when the number is operated upon. Attempting to enter more than seven digits behind the decimal will cause an input overflow.

Compet 16 Sign and Memory Status Indicators

The machine uses a wonderful Nixie tube display, with twelve individual tubes with 1/2-inch tall digits providing the answers to the user. The Nixie tubes are made by NEC, but because of the mounting hardware that keeps the tubes aligned (a fairly complex metal housing), I can't tell the part numbers of the tubes. These particular tubes have to be the sharpest and cleanest looking Nixie's I've seen, presenting a very clean and well-defined digit, even the digits that are the deepest in the tube seem to be much clearer than other Nixies I have seen. Two neon indicators are located to the right of the bank of Nixie tubes, one that that shows the sign of the number (lighting up a '-' when the number in the display is negative), and another that lights when the memory accumulator has non-zero content. The Nixie tubes are multiplexed and use discrete transistor driver circuitry, as, at the time the machine was designed, the high voltages required by Nixie tubes simply couldn't be handled by MOS integrated circuit drivers.

The Compet 16's Keyboard Layout

The keyboard of the Compet 16 is laid out in a very traditional fashion, with four groupings of keys. The keycaps are of very high-quality moulded plastic, featuring moulded-in nomenclature. The left-most group of keys contains the constant key, and the [CE] (Clear Entry), and [C] (Clear) keys. The [CE] key clears the content of the entry register, allowing entry errors to be corrected. The [C] key clears the working registers of the calculator (it does not clear the memory register), and also unlocks the machine when an overflow occurs. The next grouping of keys is the traditional numeric keypad, with a double-width zero key. The numeric entry keys feature a mechanical interlock that prevents the depression of more than one key at a time. Such a feature is good from the point of view of minimizing entry errors, but at the same time can actually slow down a skilled user, as each key must be completely released before the next key can be pressed. Later calculators abandoned the mechanical methods and used electronic means to avoid entry errors caused by inadvertent depression of multiple keys at the same time.

The math function keys make up the next group, with lighted [X] and [÷] keys. These keys have a moulded-in red-colored jeweled nomenclature, with incandescent lamps mounted beneath the key that light when a multiply or divide operation is pending. The white [=] key serves as the addition function key, and also triggers the machine to calculate the result of multiplication and division operations. The red [=] key is used for subtraction. The [RC] key swaps the content of the entry and operand registers. The right-most group of keys controls the operation of the memory register. The [CM] key clears the memory register, extinguishing the memory annunciator on the display panel. The [MR] key pulls the content of the memory register into the display. The [M-] and [M+] keys subtract or add the current content of the display to the memory accumulator register. If the memory add or subtract operation results in an overflow, the machine goes into overflow condition, but the memory register retains its previous content.

Lighted Multiply and Divide Keys

The memory register of the Compet 16 does not have an automatic power-on-clear function, meaning that the memory register can have random garbage in it when the machine is first powered on. Pressing the [MR] key after power-up can result in unusual indications on the display, with multiple Nixie tube digits lit within the same tube. Pressing the [CM] key after power-on clears the garbage out of the memory register, making it ready for normal operation. The memory register, as well as all of the other working registers of the machine, is implemented using shift registers, and is volatile, meaning that the content of the memory register is lost when the machine is powered off.

The Compet 16 is good about detecting overflow and error conditions. When either an input or arithmetic overflow occurs, or if the machine is commanded to divide by zero, the machine reacts by lighting all of the decimal points, and inhibiting any entry on the keyboard except the [C] key, which will clear the machine and return it to normal operation.

Inside the Sharp Compet 16

The benefits of using MOS integrated circuitry are clear when looking inside the machine. The electronics of the machine reside on two circuit boards. One smaller circuit board contains the display electronics, along with the bank of Nixie tubes. The display circuit board handles the multiplexing and driving of the display. The other circuit board, which takes up the base of the machine, contains the majority of the calculating logic of the machine. Compared to earlier bipolar logic machines, such as the Sharp-designed Burroughs C3350, the Compet 16 is significantly smaller, and contains significantly fewer discrete components.

Closer view of NEC Integrated Circuits

The integrated circuits in the Compet 16 are manufactured by NEC. The devices are all members of NEC's 1st generation MOS uPD1-series integrated circuits in TO-100(10-pin) or TO-101(12-pin) can-type packages. A total of 72 IC devices, combined with a large compliment of discrete transistors and diodes make up the active devices in the machine, with a large number of resistors and capacitors rounding out the components.

The Main Logic and Display Circuit Boards

The circuit boards in the Compet 16 are made of phenolic, with traces on both sides of the boards. Feed throughs are plated-through holes, which are filled with solder as a result of the flow-soldering process used to solder the components to the circuit board. The circuit board edge connector fingers are gold-plated for durability and superior electrical connectivity. The main circuit board plugs into two edge connectors, that connected to the power supply, keyboard, and display circuit board by a point-to-point, hand-wired wiring harness. The display circuit board does not plug into any connectors, rather, it is hard-wired into the wire harness. As with some other Sharp-designed calculators, an additional connector is wired into the wiring harness that is accessable from the outside of the machine. In the case of the Compet 16, if the model/serial number tag is removed, it exposes the connector. I can only surmise at this point what the purpose of the connector is, but my assumption is that it is for factory testing and diagnostic access to the circuitry of the machine.

Compet 16 Main Board Edge Connectors and Diagnostic? Connector

The linear power supply of the Compet 16, which delivers around 90V and 190V potentials for driving the Nixie tube display, and -24V for the logic supply, is split across a portion of the rear of the machine, and also at the front edge of the chassis. At the rear is the high-voltage section, with power line filter, line fuse, and power switch wiring, and at the front part of the chassis is the power transformer and active voltage regulation circuitry. The -24V logic supply is Zener diode and transistor regulated. The higher voltages used for the display drive are partially rectified and filtered, then simply resistively divided from a secondary winding of the power transformer.

The Compet 16 has a couple of operational quirks. One mentioned earlier is the 'blank' setting on the decimal point selection rotary switch that causes an input overflow when an operation is attempted with the switch in this position. Another quirk, which is quite common on transistorized and early IC-based calculators, is that the machine can't handle division problems where the dividend occupies the most-significant digit in the display. For example, dividing 999999999999 by 1, with the decimal point selection set at zero, results in an overflow, and sometimes strange indications on the display (multiple digits within an individual Nixie tube lit up at once), even though the calculation would result in a solution which is within the range of the calculator.

The early MOS integrated circuits used in the Compet 16 sacrificed some speed over earlier bipolar IC's, but made up for it with higher density, and slightly lower power requirements. 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 division bug. However, performing eleven 9's divided by 1 (with decimal point set to 0) takes about 2/3-second to perform. The Nixie tubes are not blanked during calculation, and, especially during more complex multiply and divide operations, the least and most significant digits on the display "spin" like crazy, with the other digits flickering ghosts of digits as the calculation progresses.


Thanks to Mr. Iwase and Mr. Kamiya of Sharp Corporation for information on introduction dates of early Sharp calculators.

Text and images Copyright ©1997-2011, Rick Bensene.