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Casio 121-A Desktop Calculator

Updated 7/27/2018

The Casio 121-A is an early small and medium-scale IC-based calculator, sacrificing capability for small size, and comparatively low-cost compared to other calculators of its day. The 121-A is also near the beginning of a long line of Casio 121-series calculators. It appears that before the 121-A, Casio marketed the Model 121 (See the Commodore 1121 exhibit for an OEM version of the Casio 121 sold by Commodore), an early integrated circuit-based desktop calculator that started the 121 line. At the time that the 121-A was designed (1969), most desktop calculators were significantly more expensive, and generally not quite as portable as this machine. Even though it is still an AC powered desktop device, the 121-A is small and light enough to easily carry around from place to place where calculations are required. To keep the price and physical size of the machine down, some compromises were made in the functionality of the machine.

Casio AS-A
Image Courtesy Museo Nazionale degli Strumenti per il Calcolo (National Museum of Computing Instruments), Pisa, Italy.
Dr. Christina Paoli

The 121-A is identical to another Casio calculator designated as the Casio AS-A. While all outward appearances indicate that the machine exhibited here is the 121-A, the circuit boards in the machine actually have "MODEL AS-A" silkscreened on them. Why Casio marketed identical machines with different model designations is unclear. Perhaps it has something to do with the various markets (Asian, European, and North American) that the machine was sold into. Marketing studies have shown that the differing cultures of the various markets can be a factor when deciding model nomenclature for products, with certain model naming schemes working well in one market, while not in another. It would be pure conjecture to consider this as the reason for the differing model nomenclature for the 121-A/AS-A, but it seems a distinct possibility.

Commodore 512
Image Courtesy Rob Fuller

Sometime in the late 1960's, Casio and Commodore engaged in an OEM (Original Equipment Manufacturer) partnership. Casio did all of the design and manufacturing, and Commodore simply placed their nameplate on the machines, and sold them as their own. An example of this relationship is the Commodore 512, which is identical in all aspects to the Casio 121-A/AS-A, but sold under the Commodore brand name.

Along with Commodore, Casio also had an OEM relationship with Sperry Remington. It isn't clear if there is a Sperry version of the 121-A/AS-A, however, the follow-on machine to the 121-A, the Casio 121-B/AS-B (which replaced the discrete transistor display driver circuitry with integrated circuit drivers, and added a memory accumulator register) was also marketed by Sperry as the Sperry Remington EDC-1201GT "Lektronic".

This particular 121-A appears to have been built in early 1971, based on the '1A' date codes which are the latest codes on any of the IC's in the machine. The 121-A/AS-A's design was finalized in mid-1969, and it appers that production began somewhere between the late part of 1969 to early 1970. Production of the 121-A/AS-A likely continued into early 1972, with sales extending into 1973.

Internal view of Casio 121-A

The machine is built on two main circuit boards interconnected by a great many individual wire jumpers. The top circuit board contains the Nixie display tubes, and their driver circuitry, along with keyboard encoding. The bottom board contains the calculating logic, with the adder, register storage, and control circuitry.

Discrete Nixie Drivers in Casio 121-A

The display is made up of 12 individual Hitachi CD-71 Nixie tubes put together in a frame that provides mechanical stability and shock isolation for the rather delicate tubes.

The "SUB & REGISTER" board in the AS-A/121-A board set

Most of the calculating brains of the machine are located on the 'SUB & REGISTER' board (from a silkscreened notation on the board), sandwiched below the display board. The logic of the machine is made up of 35 Small- and Medium-Scale Integration IC's, mostly from Hitachi's HD3100-Series of PMOS integrated circuit devices. The small-scale IC devices consist of logic gates and flip-flops used for control and sequencing functions. The medium-scale integrated circuits in the 121-A consist of a number of NEC uPD108 shift registers (used as the working registers of the machine), a Hitachi HD3112 integrated adder (performing arithmetic operations), and a NEC uPD116 BCD to 1-of-10 decoder for converting the BCD (Binary-Coded Decimal) internal representation of numbers to the 1-of-10 signals needed to drive the Nixie tubes.

Keyboard Layout

The 121-A provides only the four basic math functions. The [=+] and [=-] (a rather unusual designation, usually these functions are labeled [+=] and [-=]) keys provide addition and subtraction functions, and work adding machine-style. The [C] key clears the machine and readies it for a new calculation.

'Weird' Counting Display on Multiplication Overflows

The basic function and operation of the Casio 121-A and Remington Lektronic are virtually identical with minor exceptions. The Lektronic seems to be better at dealing with multiplication calculations which exceed the capacity of the machine, while the 121-A throws fits. Multiplying 999999999999 X 999999999999 on the Lektronic results in 3939393940 (which is patently incorrect by any interpretation [correct answer is 999999999998000000000001]), while doing the same on the 121-A results in a madly counting display, with multiple digits in some Nixie tubes lit at once. Pressing the [C] key recovers from this strange state. Both machines deal with negative numbers the same way... by displaying the tens-compliment of the negative number, e.g. -1 is displayed as 999999999999. Chain division is not intuitively obvious, for example pressing "10", ÷ "2", followed by again by ÷ (with the intent to perform 10 ÷ 2 ÷ 4) results in the same symptom as dividing by zero. Chain division must be done with intermediate presses of the [=+] key for each division. Even when that is done, chain division doesn't work very well, as each time the divide key is pressed, the decimal point is reset to the rightmost digit position, losing track of where the decimal point was. For example, performing 25 ÷ 2, ÷ by 2, results in 12.5 as expected for the first division, but when the second divide is pressed, the display changes to 625000000000., a grossly incorrect answer (at least in terms of magnitude). The multiply function key also resets the decimal point location, making mixed chain calculations lose track of the decimal point position. Division by zero results in the machine getting rather confused. All of the decimal points light up dimly, and the machine acts strangely when keys are pressed when it is in this state. Clearing the machine with the [C] key remedies this condition. Divisions which result in quotients which have fractional portions require some thought to decipher the rather strange decimal point placement that the machine sometimes presents. For example, the simple division of 1 by 3 results in 333333333333. -- not quite the answer expected. The 121-A does not have any detection of input overflow -- typing in numbers in excess of the twelve digits of capacity of the machine result in the high-order numbers just shifting off the left end of the display with no warning or other indication of a problem. As with many early electronic calculators, the 121-A doesn't properly deal with large dividends properly. The machine gives an incorrect result (999999999990) if the "all-nines divided by 1" problem is fed to it. This is because one digit of the working register is acatually used as the counter for division operations, meaning that only eleven digits can be operated on in a division operation. The closest speed benchmark I can get is eleven nines divided by one, which takes about 1/3 second to complete.

Text and images Copyright ©1997-2019, Rick Bensene.