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Casio 121-F Desktop Calculator

Updated 1/13/2000

The Casio 121-F is a continuation of a long line of Casio's 121-series of calculators. The series started in the late 1960's, with the Casio 121-A, a rather primitive 12-digit, four function desktop calculator using Nixie tube displays and utilizing small and medium scale integrated circuit logic. The series was later updated with the debut of the Casio 121-B/AS-B (also marketed as the Sperry Remington Lektronic).

Casio 121-K
Image Courtesy Nigel Tout

Slightly later, and out of the letter-postfix sequence, was the Casio 121-K, a desktop Nixie-tube display calculator using new (at the time, in the mid-part of 1970) Large Scale IC's from Fairchild. All of the Casio 121-series calculators were variations on the theme of a basic 12-digit office calculator, with the main differences being in technology and packaging. The series progressed at least through the model 121-U.

Casio 121-L

Casio Root 121-S

Casio 121-U
Above photos show some of the other variants of the Casio 121 (Images Courtesy Serge Devits)

For example, the 121-F exhibited here is very similar to the 121-U (see above), with the square root function key of the 121-F replaced by a "X<->Y" (swap operand) key on the 121-U. It appears that the postfix letter designation doesn't necessary reflect the chronology of the series, as the Casio 121-U has IC's dated in the 1973 time-frame, a the 121-L has IC's from 1973, and the 121-F in the museum was made the middle of 1975 based on the date codes on the IC's.

Casio 121-F Keyboard Detail

The 121-F is a six-function, twelve digit desktop calculator with memory. Along with the standard four math functions, the calculator provides square root and percent functions. The calculator offers fully floating, or fixed decimal point settings at 0, 1, 2, 3, 4, or 6 digits behind the decimal point. The decimal mode of the machine is set with slide switches on the keyboard panel. When operating in fixed decimal point mode, the calculator can be set to truncate or round up to the specified number of digits behind the decimal point, also controlled by a slide switch on the keyboard panel. The 121-F provides a constant capability, enabled by a slide switch on the keyboard panel, that operates for multiplication and division. Arithmetic entry is used -- the most common method for calculators used in business office settings. The 121-F provides a single memory accumulator, with "M+", and "M-" keys to add/subtract the number in the display to/from the memory register. When the memory register has non-zero content, an "M" lights up in the left-most position of the display. The memory register can be cleared independently by pressing the "MC" key, and recalled to the display by pressing the "MR" key. Negative numbers are indicated by a "-" lighting up in the left-most position on the display. The calculator detects overflow and error conditions (with the exception of taking the square root of a negative number) by locking the keyboard and lighting a circular symbol in the left-most position on the display. Error/overflow conditions are cleared by pressing the "AC" (all clear) key, and input errors can be corrected by pressing the "C" key. If the square root of a negative number is taken (which has an indeterminate result), the calculator erroneously returns a negative result with no error indication.

Display Detail

The 121-F uses a thirteen-position integrated vacuum-fluorescent display tube. The left-most position in the display contains three special symbols, "M", "-", and a "dot", used as indicated in the text above. The remaining twelve positions are used for displaying numbers, and use a somewhat unusual eight-segment arrangement to form the numerals. Each digit position contains a 'tick' above and to the left of the digit segments, used as a "comma" indication for grouping large numbers into three digit groups for easier reading, and a right-hand decimal point. Leading zeroes are suppressed, with numbers displayed right-justified in the display.

Internal View of the Casio 121-F

The brains of the machine consists of a NEC uPD286 single-chip calculator LSI. A quartet of Toshiba TM4352 IC's work as glue components and appear to also be involved in display driving functions. The power supply consists of two parts, a linear, transistor- regulated supply for the logic, and a small integrated switching power supply to create the high-voltage needed to drive the vacuum-fluorescent display. All of the components with the exception of the power supply transformer and fuse are mounted on a single phenolic circuit board, with an unusual connector providing connection to the keyboard assembly. The keyboard is made up of magnetic reed switches, resulting in a reliable and high-quality keyboard. The key caps are made of plastic, with molded-in nomenclature.

The 121-F is fast, providing nearly instantaneous solutions, with only a barely perceptible flicker of the display while calculating.

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

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