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Facit 1121 Electronic Desktop Calculator

Updated 1/2/2022

This rather uncommon machine may bear a bit of familiarity for those who frequent the Old Calculator Museum. For those who might not catch the resemblance, the Facit 1121 looks very similar to the Sharp Compet 20, a calculator that has been a part of the museum's collection for quite a long time.

The Facit 1121 was not really made by Facit, but instead marketed by Facit under license from Hayakawa Electric (Sharp). The 1121 was designed and manufactured by Hayakawa Electric in Japan, and badged with the Facit nameplate and model/serial number tag for sale under the Facit brand. The only obvious in appearance between the Sharp Compet 20 and the Facit 1121 is a slight difference in the keyboard layout, along with key cap and cabinet coloring differences. Upon closer inspection, it becomes apparent that the Facit machine has two additional digits of display.

Japanese Advertisement for the Facit 1121
June, 1966
Thanks to Mr. Takaharu Yoshida for providing this scan.

Soon after introduction of its first electronic calculator in June of 1964, (the Compet 10) Hayakawa Electric (which changed its name to Sharp Corporation in 1970 after many decades of use of the brand-name "Sharp") began searching for ways to broaden the market presence for its calculator outside of Japan. While there were some reliability issues with the Compet 10 that made world-wide sales of the machine difficult, Hayakawa Electric had developed a new calculator design based on new Japanese-made Silicon transistors that were much more stable than the Germanium-based transistors used in the Compet 10. The new design was introduced as the Sharp Compet 20 in September of 1965, and once it hit the market, it was a resounding success, which further drove Hayakawa Electric to establish a marketing, sales, and service presence outside of of its homeland. During the course of 1965, limited experimental export agreements were made with distributors in Australia, England and Italy for sales of the Compet 20 under the Sharp brand-name. As it became apparent that these trial exports were successful, negotiations with Facit, a well-known and respected Swedish manufacturer of fine mechanical and electromechanical calculators, concluded in a contract for Hayakawa Electric to manufacture electronic calculators for Facit to sell and service in Facit's markets. A variant of the Compet 20 was developed that became the Facit 1121. An initial order of 1,500 1121's was inked, and production began in late 1965, with sales of the Facit 1121 beginning in the 1st quarter of 1966. Facit had a strong marketing, sales, and service network across the globe, indirectly giving Hayakawa Electric access to a much larger potential market for its new calculator. Since Facit already had a presence for its mechanical machines in Japan, the 1121 was also marketed and sold in Japan, literally competing with Hayakawa Electric in its home market. The benefits gained for distribution of its machines through the Facit connection more than outweighed the small amount of homeland competition that Facit's presence in Japan created.

In the mid-1966 time-frame, Hayakawa Electric broadened their own exports of Sharp-branded machines into France, The Netherlands, Belgium, and West Germany. Also during 1966, Hayakawa Electric began creating its own sales and service presence in North America, giving the Sharp brand name world-wide market presence. Facit already had some distributors in North America for its mechanical calculators, and the new Facit 1121 ended up being sold in North America. In fact, the Facit 1121 was available for sale in the North American market before the Sharp Compet 20.

Card-cage of the Facit 1121

Despite the visual similarity between the Compet 20 and the 1121, there are definitely differences, albeit subtle. The Compet 20 has a display of 14 digits (plus sign), while the Facit 1121 displays 16 digits (plus sign). Also, the Facit 1121 lacks the [000] key of the Compet 20. Other than these differences, the machines operate identically.

The "A.P." and "Program" Boards

The logic design is Diode-Transistor logic, with pluggable circuit boards that use discrete Silicon transistor technology along with huge number of discrete diodes, resistors, and capacitors. The Facit 1121 has one additional backplane slot versus the Compet 20, for a total of 21 total circuit boards, with 16 digit boards, one board for the sign digit, a keyboard interface board, and another board that provides for overflow detection and other miscellaneous logic. Lastly, two larger boards sandwiched together are located across the back of the machine. These two boards contain the sequencing and control logic of the calculator. The Sharp-designed machines of this generation utilize a hard coded microcode design that reduces the operation of the machine to a variable sequence of micro-operations that execute almost like a computer program, stepping the machine through low-level operations that combine to provide the means by which the calculator performs its functions. The sequence control boards in the Facit 1121 are virtually identical to those in the Compet 20, with the only differences appearing to be a number of jumpers that are strapped differently between the two board sets. It appears that the control boards were designed to be a general purpose control system for a series of similar calculators, with minor wiring changes between them allowing for the different features (for example, number of digits of capacity). Both Sharp and Facit had a number of different models (Sharp Compet 21 & Compet 30; Facit 1122) of this general design, so it would make sense for the design to be adaptable to different functionality with minimal circuit changes. Each digit board has a 4-bit register composed of flip flops (arranged as a shift register) that holds one digit of the X register, which is the entry and display register. Along with the digit storage, there is logic forming a BCD (Binary- Coded Decimal) to 1-of-10 decoder, along with driver transistors to drive the Nixie tube attached to the digit board. The remainder of the logic on the digit boards contains additional circuitry, such as other working registers, machine state flip flops, data gating circuitry, decimal point management and other glue functions. The size and general design of all of the circuit boards in the Facit 1121 are identical to those in the Compet 20. The circuit boards in the 1121 also carry the "HEC" designation (Hayakawa Electric Corporation) etched into the circuit board, same as the boards in the Compet 20. In fact, some of the boards in the Facit 1121 are exactly the same as the boards in the Compet 20, and are interchangeable between the two machines with no issues.

The ID Tag on the one of the museum's Facit 1121 calculators

Mechanically the Facit 1121 is very similar to the Compet 20. Even though the 1121 has an additional two digits of capacity, the interior and exterior dimensions of the two machines are the same, indicating that the Compet 20 mechanical design was engineered to handle enough circuit boards to provide up to sixteen digits of capacity. The base is made from a thick-walled metal casting, as is the keyboard bezel. The top cabinet is made from a sturdy thick-walled (likely injection molded) plastic. The backplane, with the exception of the additional edge connector, is mechanically identical, even so far as the power supply bus wiring, and point-to-point backplane wiring style. The power supply is physically very similar, although electronically it seems that some modifications have been made to the Facit 1121 power supply to allow it to provide more current than the supply in the Compet 20. The card cage and Nixie tube retaining systems are identical, as well as the keyboard bezel, cabinet base, and upper cabinet. Most of the mechanical parts can be interchanged between the machines with no problem.

Keyboard Detail on Facit 1121.
Note lit indicator in [X] key showing pending multiplication operation.

From the operator's viewpoint, the 1121 and the Compet 20 operate the same. The white [+] key is indexed after a number is entered to add the entered number to the existing number in the accumulator and displays the result. The red [-] key subtracts the entered number from the accumulator and displays the result. Multiplication is performed by the operator entering the multiplicand, pressing the [X] key (which lights a red indicator embedded in the key to indicate that a multiplication operation is pending), entering the multiplier, and pressing the [=] key to calculate the result. The indicator in the [X] key remains lighted indicating that the multiplier is still stored in the calculator as a constant which can be re-used by indexing another multiplicand and pressing [=]. The [X-] key operates the same as the [X] key, but it negates the multiplier before performing the multiplication. Division is performed by entering the dividend, pressing the [:] key (the ":" symbol is used on Europe in place of the "÷" symbol, despite this machine having been sold in the US market) resulting in the indicator in the [:] key illuminating, and the display clearing for entry of the divisor. The divisor is entered, and the [=] key indexed to calculate the result. The [CLE] key is used as a Clear Entry key; it clears the display to allow a number to be re-entered from scratch. The backarrow key erases the last digit entered to fix incorrect entry of a digit already input. The [CL] key is the master clear for the machine, resetting all of the registers, releasing the overflow indication, clearing any pending multiplication and division constant (and turning off the indicator in the [X] or [:] keys) and sets machine state to idle. There is a green indicator in the [CL] key that illuminates if an arithmetic overflow occurs, although when an overflow does occur, the keyboard is not locked out and calculations may continue, creating a potential for an unobservant operator to carry on calculations after an overflow has occurred. The [RC] key swaps the two working registers, X and W, useful for reversing the order of operands in division problems.

Facit 1121 Backplane Wiring

As with the Compet 20, the [M] key, a push-on/push-off switch, provides for selection of automatic or manual decimal point positioning. With the [M] key in the up position, the calculator automatically determines the location of the decimal point to the best of its abilities. Locking the [M] key in the down position allows the operator to manually control the decimal point location by entering the first number of any problem with the desired number of digits behind the decimal point. If the user desires four digits behind the decimal point in the result, the first number in the problem would be entered with that four digits behind the decimal, for example, entering "6.0000" would set the decimal location to four digits behind the decimal point.

The Facit 1121 does not have any rounding capabilities, nor does it have any leading or trailing zero suppression on the display. The NEC-manufactured CD-65 (same as used in the Compet 20, and Sharp Compet 15) Nixie tubes indicate 0 through 9 and include a right-hand decimal point. The tubes offer a large 5/8ths-inch tall digit which makes the display very readable. A special Nixie tube at the left end of the display indicates a "+" or "-" sign to indicate the numerical sign of the number in the display. There is a slight bug in the sign logic, in that performing 2 + 3 - 5 (entered as [2] [+] [3] [+] [5] [-]) gives a result of "-0000000000000000.", which is technically impossible, as there is no such number as negative zero. The bug isn't fatal, though, as adding 1 to it results in the correct answer of "+0000000000000001.".

Display at Power-Up

When first powered up, the Facit 1121 comes up with garbage in the display, typically all digits lit with 7 and 9 in each tube lit at the same time (see photo). Pressing the [CL] key resets the machine and posts "+0000000000000000." in the display. It appears that this 7 and 9 lit at the same time indicates that the number 15 (all bits set to '1') is present in each digit of the display register. This is an invalid Binary-Coded Decimal number (BCD only represents zero through nine), which the one-of-ten decoder logic translates into two outputs (7 and 9) being on at the same time. The flip flops all being set '1' to when powered is likely of the flip flop design having a slight power-on bias toward setting to '1' rather than '0'. The 7 and 9 being lit at the same time is an artifact of the decoding logic (a series of diode gates that detect each of the valid states of the digit register from zero through nine) being fed a state that it isn't designed decode. For a four-bit register holding a Binary-Coded Decimal number, the states 0000 through 1001 are valid states for the digits zero through nine, and the states 1010(10), 1011(11), 1100(12), 1101(13), 1110(14), and 1111(15) are invalid states that the decoding logic is not designed to decode.

Overflow is detected reliably by the machine, and when it occurs, a green indicator in the [CL] key lights up. Overflow indication occurs both on entry overflow, as well as overflow created by a math operation that exceeds the capacity of the machine. The overflow indication can be reset by pressing the [CL] key. Division by zero causes the machine to hang, with no activity on the display other than the decimal point disappearing. The overflow indicator does not light when this operation is attempted. Trying to enter digits or operations while the machine is in this state cause no response. The only way to clear this state is to press the [CL] key, or power-cycle the calculator.

The museum has two Facit 1121's, one that is almost fully operational (it has some decimal point positioning logic issues), and another that needs some significant work. Their serial numbers differ by 964, and the dating of the components in the machines put the earlier machine as being built in late 1966, with the later in the early part of 1967. Despite the short difference in time of production, there is one difference detectable between the two machines. The earlier machine powers up immediately upon pressing the power switch, while the later machine has a time-delay relay that causes a delay of about 2 seconds after the power switch is pressed before the machine comes to life with a 'click' from the relay. This change was likely due to some kind of power supply in-rush current issue, whereby adding the relay allowed the power supply voltages to fully stabilize before the logic supplies were connected to the circuitry of the machine through the time delay relay's contacts.

The 1121 is quite fast performing calculations, although just a tiny bit slower than the Compet 20 due to the two additional digits of capacity, which require additional cycles for each arithmetic operation. 99999999 X 99999999 (the most complex multiplication the machine can perform without overflow) takes about 300 milliseconds. Fifteen 9's divided by 1 takes about 600 milliseconds. As with the Compet 20, the full capacity of the machine cannot be utilized in the dividend during division operations because the most significant digit of the working register is used during division as a counter for keeping track of how many subtractions of the divisor have been performed.

Sincere thanks to Mr. Takaharu Yoshida for the scan of the Japanese advertisement for the Facit 1121

The Old Calculator Museum wishes to thank Ms. Grace Ulbricht for donation of Facit 1121 Serial Number 103320 to the museum

Sincere thanks to Mr. Arthur Beach for donation of Facit 1121 Serial Number 102356 to the Old Calculator Museum

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

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