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Facit Model 1124 Desk Calculator

Facit 1124 external view Facit Model 1124, S/N 403253
Functions: ASMD, 1 memory
Technology: Bipolar DTL (Mitsubishi, 151 chips)
Display: 14 digit, Nixie tubes
Dimensions: 275W x 325D x 120H, weight 5.16kg
Manufactured: Sharp, Japan, 1968



The Facit organisation built an extensive range of mechanical calculators at Atvidaberg in Sweden from the early 1920s to the 1970s. In 1966 they introduced the first of a range of electronic calculators manufactured by Sharp in Japan. The design and function of the Facit-branded machines is generally similar to the Sharp "Compet" machines of the time.

This Facit 1124 from 1968 is a basic four-function desk calculator with a 14-digit Nixie-tube display and one user-accessible memory. A version without the user memory was sold as the Facit 1123.

The machine is typical of the transition from the previous discrete-transistor technology to the first generation of general-purpose SSI integrated circuits. The logic circuitry uses over 150 chips from an early Mitsubishi DTL family, while the display board still uses discrete-transistor drivers for the high-voltage Nixie tubes.


Internal View Internal view

The machine is built on a substantial pressed-metal chassis. There are two logic boards mounted face-down in the bottom section, with the keyboard and display board above. The power supply is mounted across the back.

The rubber feet are attached directly to the chassis and extend through holes in the lower section of the 2-part plastic case. The case fits around the mechanism, but does not support the chassis, keyboard, or any of the components.

Chassis Chassis

This view shows the card guides and backplane connectors with the boards removed.

The keyboard connects to the backplane via the 45-way plug-and-socket connector and hand-wired cable. The power switch attaches to the keyboard frame and is easily removed from above.

There is a similar 45-way socket mounted under the left-hand rear corner of the machine for connection of an external remote control or keystroke programming unit.

The power supply at the rear produces regulated 5V DC for the logic circuits and about 200V DC for the Nixie tube display. The logic ground is isolated from the chassis.

The logic boards Logic boards

The two double-sided logic boards each measure 250x230mm. The boards have a Sharp logo etched into in the copper pattern, and have a metal bar across the front to prevent flexing. The two boards carry a total of 135 ICs on a 10x8 grid. There are 16 more chips on the display board, for a grand total of 151.

All of the IC chips are from the general-purpose Mitsubishi 5300 family of bipolar DTL logic, and consist mostly of SSI gates and flip-flops. Storage for the working registers is provided by three banks of seven 8-bit shift registers.

Display board Display board

The display board measures 250x115mm and carries 14 Hitachi CD71L display tubes. The tubes are 12mm diameter and 30mm high, mounted on 14mm centres. The numerals are 12mm high with a decimal point at the lower right. The display tubes are multiplexed and driven from a total of 31 discrete transistors and their associated R-C metworks. The date codes on the tubes and driver transistors range from July to September of 1968.

The master clock generator for the machine is at the lower left of the display board. Three plastic-pack transistors generate the single-phase clock signal at 50kHz.

Keyboard assembly (underneath) Keyboard assembly

The keyboard assembly is mounted to the top of the chassis and connects via the plug-and-socket to the cable from the backplane.

The key switches are arranged in four separate sections, which are mounted on a substantial metal plate. The keyboard uses glass reed switches, operated by moving magnets on the ends of the keystems. The main numeric keypad has a mechanical interlock to prevent more than one key being pressed at a time. The rotary switch selects 0, 2, 4, or 6 decimal places.




One of the charming features of early electronic calculators is the variety found in the functions and labelling of the keyboard, as the designers struggled to develop a consistent user interface for the new technology.

The most conspicuous features of the Facit 1124 keyboard are the large Multiply and Divide keys, with symbols that illuminate to show when an operation is pending. These tend to mask the fact that there is no key marked for Plus or Add!

The RC key "Re-Calls" the value from the internal buffer, and MC re-Calls the contents of the memory. Otherwise, the operation is quite conventional.


Resources for further information


Original text and images Copyright © John Wolff 2009.
Use at own risk; beware of errors; suggestions for improvement welcome.
Last Updated: 15 July 2009

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