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Sharp Compet 22 Electronic Desktop Calculator

Updated 6/11/2021

The Sharp Compet 22 is the "big brother" to the groundbreaking Sharp Compet 16. The Compet 16 was the first MOS (Metal Oxide Semiconductor) Integrated Circuit-based calculator on the market. Other calculators of the time were based on bipolar small-scale integrated circuits, or even all-transistor construction. The Compet 22 was introduced a while after the Compet 16, to provide additional math capacity for customers that needed to operate on large numbers. Facit marketed a slightly repackaged version of the Sharp Compet 22 (through an OEM agreement with Sharp) as the Facit 1124. Sharp also offered the Compet 23 which is identical to the Compet 22 except that it omits the "Memorizer" connector on the rear panel of the machine (see below).

Comparing the Compet 16S(left) and Compet 22C(right)

The Compet 22 is very similar to the Compet 16 in terms of function and appearance. The size and weight of the machines are the same, and the keyboard layout is identical. The main difference between the Compet 16 and Compet 22 is the addition of two extra digits of capacity, with the Compet 22 offering a full 14 digits of capacity versus the 12-digit computing capacity of the Compet 16. There are also some minor cabinet differences between the two machines, but these are not model-specific. It appears that somewhere during production, a design change was made to the cabinetry that added an extension to the cabinet providing more of a hood over the display to help eliminate glare from overhead lighting. This change is apparent when comparing the later-production Compet 16S to the older-production Compet 22C.

A later production Compet 23C
Image Courtesy Ramon On

The Compet 22 exhibited here was actually produced before the Compet 16 exhibited in the museum. The reason for this is that there were quite a few different versions of each model made. The original Compet 16 was the model CS-16A. The Compet 16 exhibited in the museum is a CS-16S, with the postfix alphabetic character indicating the revision. The Compet 16 exhibited is a late version of the Compet 16, while the Compet 22 exhibited here is a model CS-22C, a fairly early version of the Compet 22. This particular Compet 22 was built in the February of 1968, while the Compet 16 exhibited in the museum was built almost a year later, in January of 1969.

Due to design changes during the lifetime of this first-generation of Sharp's MOS IC-based calculators, some subtle differences in operation are noted between them. Most notably, the Compet 22 uses an annunciator that lights up "E" when an overflow condition exists, however, the keyboard is not inhibited and calculations can continue even though an error condition exists. The later-design Compet 16S remedied this problem by eliminating the "E" annunciator, and adding circuitry to automatically clear the calculator, lighting all the decimal points, and ignoring entries from the keyboard until the [C] key is pressed to reset the machine. The Compet 22, by virtue of it's earlier design, also does not properly error out on division by zero. The Compet 22 gives no error condition, and 'locks up' (apparently looping forever trying to solve an insoluble problem), responding to no key-presses except the [C] key. When the Compet 16 is commanded to divide by zero, it properly indicates an error condition, refusing to carry out the futile operation. Lastly, a small difference between the Compet 22 and Compet 16 is that the annunciators for memory, error indication, and sign are located at the left end of the display panel on the Compet 22; and the memory and sign annunciators are located at the right end of the display on the Compet 16.

Profile View of Sharp Compet 22

This particular Compet 22 was manufactured in February of 1968 based on information from the serial number of the machine. It appears that the Compet 22 (Model CS-22A) was introduced sometime in late 1967. A number of revisions were made (indicated by the alphabetic postfix after the model number) during the life of the model. By the time the machine in this exhibit was manufactured, the revision level had reached "C". By February of 1969, the machine retailed for $1175, $180 more than a Compet 16 of the same time period.

The CS-22 is a 14-digit, fixed decimal, four function electronic calculator with a single accumulating memory register. The calculator provides a constant function for multiplication and division, with a push-on/push-off keyboard key labeled [K] to enable the function. The Compet 22 uses arithmetic logic for addition/subtraction, and algebraic input for multiplication and division.

Like the Compet 16, the Compet 22 uses fixed-decimal point logic. The location of the decimal point is set via a rotary switch on the keyboard panel, Selections for 0, 2, 3, 4, and 6 digits behind the decimal point are provided, with two different settings available for each selection (Denoted by red and black numbers showing through the decimal point selection window). A black setting causes truncation at the selected decimal point location, and the other (red) setting causes results to be rounded off to the selected decimal position. Like the Compet 16, There's an odd blank selection 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. It isn't clear why this setting exists.

The Compet 22 allows entry of any number of digits behind the decimal point, but flags an error indication when the number of digits exceeds the decimal point selection. The error indication does not lock out entry of any further digits.

Compet 22 Sign, Memory, and Overflow Status Indicators at Left end of Display

The machine uses the dominant display technology of the time, Nixie tubes. There are fourteen individual Hitachi-made tubes with 1/2-inch tall digits providing the display to the user. Decimal points are positioned within the Nixie tube to the right and slightly below the digit electrodes. Three neon indicators are located to the left 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); another that lights "M" when the memory accumulator has non-zero content, and the last, which lights an "E" when an error condition is detected. The Nixie tubes are multiplexed and use discrete transistor driver circuitry.

The Compet 22's Keyboard Layout

The keyboard of the Compet 22 is identical to that of the Compet 16. The left-most group of keys contains the constant [K] key; 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 except the memory register, and extinguishes the "E" annunciator if it is lit. 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.

Lighted Multiply and Divide Keys

The math function keys make up the next group, with lighted [X] and [÷] keys. These keys have a molded-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 "E" annunciator is lit, but it appears that the machine goes ahead and carries out the operation, resulting in a 'rollover' of the memory register. For example, performing 99999999999999, [M+], 11, [M+] will result in the "E" indication coming on. Then pressing the [MR] key at this point will recall a result of -99999999999990.

Model/Serial Number Tag

The Compet 22 does not have an automatic power-on-clear function. The power-on clear function that clears the entry/result register at power-on (yet doesn't clear the memory accumulator) appears to have been added as part of the later revisions of the Compet 16 design, and wasn't yet designed in on the earlier Compet 22C. When the Compet 22 is first turned on, the display is filled with gibberish consisting of random digits, some with multiple numerals lit inside the same tube. Pressing the [C] key after power-up will initialize the calculator, except the memory register still contains garbage. Therefore, it is necessary to press both the [C] and [CM] keys after the Compet 22 is powered up to assure that all registers are clear and ready for normal operation.

Inside the Sharp Compet 22

It's clear from looking inside the Compet 22 that it's design is somewhat older than that of the Compet 16 "S" (Model CS-16S) version. The Compet 22C uses a lot more discrete components, especially diodes, where the later Compet 16S seems to have substituted more IC's for the discrete components. Both machines have their circuitry populated on two circuit boards. It is interesting to note that the user manual for the Compet 22 boldly states on its front cover that it is a "Sharp ALL IC Compet". The author would debate that claim due to the large count of discrete component that make up the logic of the calculator.

A 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. As with all Sharp electronic calculators beginning with the Compet 20, the machine uses a microprogrammed architecture for sequence control, utilizing a diode ROM array to store the micro-sequence bit patterns.

The Main Circuit Board
Diode/resistor array at right makes up microcode ROM

The integrated circuits in the Compet 22 are manufactured by Hitachi. The devices are all members of Hitachi's 1st generation MOS HD-70x-series integrated circuits in TO-100(10-pin) or TO-101(12-pin) can-type packages. A total of 83 IC devices, combined with a large component count of discrete transistors and diodes make up the active devices in the calculator.

The circuit boards in the Compet 22 are of the same design as those in the Compet 16. The boards are made of phenolic, with etched copper traces on both sides of the boards, with plated- through feed-throughs connecting traces on each side. The boards are flow- soldered. The main circuit board edge connector fingers are gold-plated for durability and superior electrical connectivity. The main circuit board plugs into two edge connectors that connect to the power supply, keyboard, and display board by a point-to-point wiring harness.

The greenish-colored "Memorizer" Connector

The Compet 22 sports a special connector at the back of the machine that is connected into the keyboard circuitry. This connector can be accessed by removing the model/serial number tag screws. This connector was provided for connecting an external device called a "Memorizer" (Sharp Model CSA-12) that allowed sequences of keystrokes to be "learned" and played back to allow automated operation of the calculator for linear (e.g., non-branching) programs.

The Model CSA-12 "Memorizer 60" Programming Accessory

The Memorizer device is similar in size to the Compet 22, with a few keys for controlling the operation of the device as well as a two digit Nixie tube display (for showing the step number), and a panel of indicators that display the operation stored at the indicated step. The CSA-12 shown above has two program storage areas combining for a total of 60 program steps. The program is selected by a slide switch on the front panel of the machine.

Rear panel with Display? Connector and the plug that goes into the it

A feature unique to the Compet 22 that does not appear on the Compet 16 is the addition of a connector on the back panel of the machine next to the model/serial number tag. This connector has a special plug plugged into it. If this plug is not in place when the calculator is powered up, the display comes up blank. The connector into which this plug is placed has a small (three or four wires) wiring harness that plugs into an edge-card connector on the display board. It appears that perhaps this connector allows some kind of external display (or perhaps a printer?) to be connected to the Compet 22.

The Compet 22 uses exactly the same zener diode/transistor-regulated linear power supply design as the Compet 16.

The Compet 22 has the same operational quirks as the Compet 16. Both machines share 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 shared with the Compet 16 is that the Compet 22 can't handle division problems where the dividend contains more than 13 digits. For example, dividing 88888888888888 by 1, with the decimal point selection set at zero results in an error indication, and a nonsense result in the display. The reason for this is that the uppermost digit in the working register is used as a counter during the division process.

The Compet 22 is only slightly slower than the Compet 16, likely due to the additional time it takes to process the extra two digits. As with most calculators, the Compet 16 and 22 (and their little brother, the Compet 17) use a bit-serial logic architecture. This means that each additional digit of capacity requires additional time as the digits circulate through the arithmetic logic. Addition and subtraction complete so quickly that no real difference can be observed in the speed of the Compet 22 versus the Compet 16. Multiplication and division, being more complex operations, take just a bit longer on the Compet 22. Multiplication of 9999999 by itself takes about 1/2 second to perform. Thirteen 9's divided by 1 (with decimal point set to 0) takes about 3/4-second to perform. Performing identical operations on the Compet 16 and Compet 22 deliver results in in just slightly more time on the Compet 22, but in general, the difference is not really noticeable. The clock frequency of both machines is the same at approximately 50KHz. Like the Compet 16, the Nixie tubes are not blanked during calculation and dance wonderfully as calculations are performed.

Text and images Copyright ©1997-2023, Rick Bensene.

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