The Casio 121
Photo Courtesy of Mr. Serge Devidts, Calcuseum

The "A" in both 121-A and AS-A are hints that there might have been a "Model 121" or "Model AS" that preceded the 121-A/AS-A. In fact, that is true -- there was a Casio model 121, but it only marketed as the 121, and never as the model AS, further confusing the situation with regard to the dual model numbers of the 121-A/AS-A and 121-B/AS-B. The Model 121 was the beginning of a long line of Casio 121-series calculators. The original machine in the series, the Model 121 (See the Old Calculator Museum's Commodore 1121 exhibit for an OEM version of the Casio 121 sold by Commodore), was a larger, had a more conventional (e.g., display located above keyboard) form-factor and was a more fully-featured calculator that utilized first-generation Japanese MOS Integrated Circuits combined with a large number of discrete components. The 121, and it's somewhat downgraded stable-mate, the Casio 120, were introduced in 1967, and were popular sellers. The 121, by virtue of its less advanced technology, was quite a bit larger, heavier, and significantly more expensive than the new 121-A/AS-A, and utilized the more conventional keyboard-below-display form-factor.

At the time that the 121-A/AS-A was being designed, desktop electronic calculators made by Casio's competitors were significantly more expensive, and generally not as portable as this calculator. Even though the 121-A/AS-A is still an mains-powered desktop device, the device is small and light enough (only 6 1/2 pounds) to easily carry around from place to place where calculations are required -- a feature which made the machine very popular. The 121-A/AS-A was designed to be able to fit inside a traditional briefcase, though, one would have to exercise care carrying the calculator in a briefcase without some kind of padding to prevent it from slamming around inside. To keep the price and physical size of the machine down, Casio made some compromises with the functionality and features of the calculator. These compromises included the lack of negative number handling, as well as the inability to input fractional numbers due to the lack of a decimal point key, and also limited ability to calculate with fractional numbers. These limitations, though somewhat profound compared to other calculators on the market at the time, were not an impediment to sales, as the Casio 121A/AS-A and those marketed by OEM customers of Casio were hot sellers, setting a new low-price benchmark for an electronic calculator.

Casio 121-A Circuit Board Showing "Model AS-A" Silk-screen

To further the confusion concerning the different model names of this calculator, the circuit boards inside the exhibited Casio 121-A calculator actually have "MODEL AS-A" silk-screened on them, even though the calculator has "Casio 121-A" clearly stated on the badge on the front panel, and has the model name of "121-A" on the model/serial number tag.

An Advertisement for the Remington version of the Casio 121-A marketed as the Remington Rand AS-A

Remington Rand in Australia was the first company outside of Japan to import Casio-made calculators under an OEM (Original Equipment Manufacturer) agreement. Remington Rand initially imported Casio-made electronic electronic calculators beginning with the Casio 101 in September of 1966. Remington Rand marketed the Casio 121-A as the Remington/Casio AS-A.

The Commodore version of the Casio 121-A/AS-A known as the Commodore 512

Commodore, a Canadian firm that became known in North America as an early importer of low-cost Japanese electronics, distributing and servicing the electronics in the North American market. Commodore later became famous for the development of a low-cost home computer (long before the IBM-PC) known as the Commodore 64. The Commodore 64 is computer that readers of this exhibit may have used as their first home computer. Sometime in the latter part of the 1960's, Commodore engaged in an OEM partnership with Casio. Casio did all of the design and manufacturing, and Commodore simply placed their own model name badge and model/serial number tag on the Casio-made calculators, and sold and supported them under the Commodore brand within North America. The Casio 121-A/AS-A was marketed by Commodore as the Commodore 512, which is identical in all aspects to the Casio 121-A/AS-A, but marketed, sold and supported by Commodore in North America.

Near the end of Casio's production line for Casio AS-A calculators, Circa 1970

The exhibited Casio 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 1A code the ICs with this date code were made in the first month (A) of 1971. The 121-A/AS-A's design was finalized in mid-1969, and it appears that full production began in the summer of 1969. Production of the 121-A/AS-A likely continued into early 1972, with sales extending into 1973, with the price for the calculator discounted over time to account for the tremendous competition in the electronic calculator marketplace in the early 1970s.

Internal view of Casio 121-A

The Casio 121-A/AS-A has two main circuit boards interconnected by 38 individual wire jumpers. The circuit boards are made of laminated Phenolic paper, with silk-screened white annotations. There is no solder-mask present on the boards. The circuit boards contain components and traces on one side, and traces on the reverse side. There are some jumper wires on the circuit boards to make connections between components that were unable to be made via copper etch due to space limitations. The circuit boards have feed-through holes that are plated through to provide connectivity between traces on the top and bottom sides of the boards.

The "INDICATOR & MAIN" (top) Ciruit Board

The top circuit board, designated "INDICATOR & MAIN" contains the Nixie display tubes and their decoder-driver circuitry; the display multiplexing shift register; the 50KHz master clock-generation and divider circuitry; and the keyboard conditioning and encoding circuitry. The bottom circuit board, designated "SUB & REGISTER", contains the calculating logic; with the adder, carry logic, the three working registers of the calculator, and state sequencing logic.

Discrete Transistor Nixie Tube Drivers in Casio 121-A/AS-A

The display is made up of 12 individual Hitachi CD-71 Nixie tubes put together in a metal frame that provides mechanical stability and shock isolation for the rather delicate tubes. At the bottom of the metal framework is a circuit board that provides the connections for the Nixie tubes. The Nixie circuit board connects to the INDICATOR & MAIN circuit board with jumper wires. The Nixie tube support frame mounts to the INDICATOR & MAIN circuit board with screws. The CD-71 Nixie tubes contain the digits zero through nine, and have a right-hand decimal point. The digits are approximately 1/2" in height (13mm), and the tubes operate at a nominal voltage of 200V.

The "SUB & REGISTER" (bottom) circuit board in the AS-A/121-A board set

All together, the logic of the calculator is made up of only 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 µPD108 shift registers (used as the working registers of the machine), a Hitachi HD3112 serial binary/BCD adder/subtracter (performing arithmetic operations), and a NEC µPD116 BCD to 1-of-10 decoder for converting the 4-bit binary (2'-4-2-1) internal representation of digits to the 1-of-10 signals needed to drive the Nixie tubes.

Casio 121-A/AS-A Keyboard Layout

The 121-A provides the four basic math functions. The [=+] and [=-] (a rather unusual designation, usually these functions are labeled [+=] and [-=]) keys provide addition and subtraction functions, and operate adding machine-style. Multiply and divide functions are designated by the familiar keys, [X] and [÷]. The [C] key (located where one might seek a decimal point key) clears the machine and readies it for a new calculation. Multiplication and division operate as expected with the [=+] key causing the calculation to occur.

Counting Display on Multiplication Overflow

The 121-A/AS-A does not deal well with multiplication calculations that exceed the capacity of the machine. Multiplying 999999999999 X 999999999999 (the worst-case situation) 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.

Upper Half of the 121-A Cabinet - Power Wiring, Keyboard

As mentioned, the 121-A/AS-A does not directly support negative numbers. Negative results are indicated by displaying the tens complement of the negative number. For example, -1 is displayed as 999999999999. Users performing calculations must be wary of this, as there is no indication given when such "underflow" occurs. Chain multiplication and division must be performed with intermediate presses of the [=+] key for each division. Even when that is done, chain division doesn't work very well with fractional numbers, 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 carried out, 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 quite 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 don't always result in answers that make sense. 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; entering 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 actually used as the counter for division operations, meaning that only eleven digits can be operated on in a division operation. This tactic saves on additional logic at the expense of being able to exercise the full capacity of the machine for division operations. The closest speed benchmark I can get is eleven nines divided by one, which takes about 300 milliseconds to complete.